Hi Jared,
Thanks for working on this
On Tue, Apr 22, 2025 at 12:19:39PM -0700, Jared Kangas wrote:
The CMA heap's name in devtmpfs can vary depending on how the heap is defined. Its name defaults to "reserved", but if a CMA area is defined in the devicetree, the heap takes on the devicetree node's name, such as "default-pool" or "linux,cma". To simplify naming, just name it "default_cma", and keep a legacy node in place backed by the same underlying structure for backwards compatibility.
Signed-off-by: Jared Kangas jkangas@redhat.com
Documentation/userspace-api/dma-buf-heaps.rst | 11 +++++++---- drivers/dma-buf/heaps/Kconfig | 10 ++++++++++ drivers/dma-buf/heaps/cma_heap.c | 14 +++++++++++++- 3 files changed, 30 insertions(+), 5 deletions(-)
diff --git a/Documentation/userspace-api/dma-buf-heaps.rst b/Documentation/userspace-api/dma-buf-heaps.rst index 535f49047ce64..577de813ba461 100644 --- a/Documentation/userspace-api/dma-buf-heaps.rst +++ b/Documentation/userspace-api/dma-buf-heaps.rst @@ -19,7 +19,10 @@ following heaps:
- The ``cma`` heap allocates physically contiguous, cacheable, buffers. Only present if a CMA region is present. Such a region is usually created either through the kernel commandline through the
- `cma` parameter, a memory region Device-Tree node with the
- `linux,cma-default` property set, or through the `CMA_SIZE_MBYTES` or
- `CMA_SIZE_PERCENTAGE` Kconfig options. Depending on the platform, it
- might be called ``reserved``, ``linux,cma``, or ``default-pool``.
- ``cma`` parameter, a memory region Device-Tree node with the
- ``linux,cma-default`` property set, or through the ``CMA_SIZE_MBYTES`` or
- ``CMA_SIZE_PERCENTAGE`` Kconfig options. The heap's name in devtmpfs is
- ``default_cma``. For backwards compatibility, when the
- ``DMABUF_HEAPS_CMA_LEGACY`` Kconfig option is set, a duplicate node is
- created following legacy naming conventions; the legacy name might be
- ``reserved``, ``linux,cma``, or ``default-pool``.
It looks like, in addition to documenting the new naming, you also changed all the backticks to double backticks. Why did you do so? It seems mostly unrelated to that patch, so it would be better in a separate patch.
diff --git a/drivers/dma-buf/heaps/Kconfig b/drivers/dma-buf/heaps/Kconfig index a5eef06c42264..83f3770fa820a 100644 --- a/drivers/dma-buf/heaps/Kconfig +++ b/drivers/dma-buf/heaps/Kconfig @@ -12,3 +12,13 @@ config DMABUF_HEAPS_CMA Choose this option to enable dma-buf CMA heap. This heap is backed by the Contiguous Memory Allocator (CMA). If your system has these regions, you should say Y here.
+config DMABUF_HEAPS_CMA_LEGACY
- bool "DMA-BUF CMA Heap"
- default y
- depends on DMABUF_HEAPS_CMA
- help
Add a duplicate CMA-backed dma-buf heap with legacy naming derivedfrom the CMA area's devicetree node, or "reserved" if the area is notdefined in the devicetree. This uses the same underlying allocator asCONFIG_DMABUF_HEAPS_CMA.diff --git a/drivers/dma-buf/heaps/cma_heap.c b/drivers/dma-buf/heaps/cma_heap.c index e998d8ccd1dc6..cd742c961190d 100644 --- a/drivers/dma-buf/heaps/cma_heap.c +++ b/drivers/dma-buf/heaps/cma_heap.c @@ -22,6 +22,7 @@ #include <linux/slab.h> #include <linux/vmalloc.h> +#define DEFAULT_CMA_NAME "default_cma"
I appreciate this is kind of bikeshed-color territory, but I think "cma" would be a better option here. There's nothing "default" about it.
struct cma_heap { struct dma_heap *heap; @@ -394,15 +395,26 @@ static int __init __add_cma_heap(struct cma *cma, const char *name) static int __init add_default_cma_heap(void) { struct cma *default_cma = dev_get_cma_area(NULL);
- const char *legacy_cma_name; int ret;
if (!default_cma) return 0;
- ret = __add_cma_heap(default_cma, cma_get_name(default_cma));
- ret = __add_cma_heap(default_cma, DEFAULT_CMA_NAME); if (ret) return ret;
- legacy_cma_name = cma_get_name(default_cma);
- if (IS_ENABLED(CONFIG_DMABUF_HEAPS_CMA_LEGACY) &&
strcmp(legacy_cma_name, DEFAULT_CMA_NAME)) {ret = __add_cma_heap(default_cma, legacy_cma_name);if (ret)pr_warn("cma_heap: failed to add legacy heap: %pe\n",ERR_PTR(-ret));- }
It would also simplify this part, since you would always create the legacy heap.
Maxime
On Thu, Apr 24, 2025 at 1:34 AM Maxime Ripard mripard@kernel.org wrote:
On Tue, Apr 22, 2025 at 12:19:39PM -0700, Jared Kangas wrote:
@@ -22,6 +22,7 @@ #include <linux/slab.h> #include <linux/vmalloc.h>
+#define DEFAULT_CMA_NAME "default_cma"
I appreciate this is kind of bikeshed-color territory, but I think "cma" would be a better option here. There's nothing "default" about it.
I disagree. It very much is "default" as it's returning the dma_contiguous_default_area.
There can be multiple CMA areas, and out of tree, vendors do reserve separate areas for specific purposes, exposing multiple CMA dmabuf heaps. There have been patches to expose multiple CMA heaps, but with no upstream drivers using those purpose specific regions, we haven't taken them yet. I do hope as the drivers that utilize these purpose focused heaps go upstream, we can add that logic, so I think being specific that this is default CMA is a good idea.
thanks -john
On Thu, Apr 24, 2025 at 05:13:47PM -0700, John Stultz wrote:
On Thu, Apr 24, 2025 at 1:34 AM Maxime Ripard mripard@kernel.org wrote:
On Tue, Apr 22, 2025 at 12:19:39PM -0700, Jared Kangas wrote:
@@ -22,6 +22,7 @@ #include <linux/slab.h> #include <linux/vmalloc.h>
+#define DEFAULT_CMA_NAME "default_cma"
I appreciate this is kind of bikeshed-color territory, but I think "cma" would be a better option here. There's nothing "default" about it.
I disagree. It very much is "default" as it's returning the dma_contiguous_default_area.
My main concern here is that it's "default" as opposed to what, exactly? We have a single CMA allocator. We could have multiple buffer attributes, but then "cached_cma" would make more sense to me if we expect to have uncached CMA allocations at some point.
There can be multiple CMA areas, and out of tree, vendors do reserve separate areas for specific purposes, exposing multiple CMA dmabuf heaps.
By "CMA areas", I guess you mean carved-out memory regions? If so, how is it relevant to userspace if we use CMA or any other implementation to expose a carved-out region, and thus that we carry that implemenattion detail in the name?
There have been patches to expose multiple CMA heaps, but with no upstream drivers using those purpose specific regions, we haven't taken them yet. I do hope as the drivers that utilize these purpose focused heaps go upstream, we can add that logic, so I think being specific that this is default CMA is a good idea.
If heaps names are supposed to carry the region it exposes, then it should be default_cma_region/area. If heap names are supposed to expose the allocator (but I don't think it's a good idea), it should be cma. If they are meant to carry all that plus some policy, cached_default_cma_region should be used.
Either way, default_cma seems to me either too specific or not specific enough. And we should really document what the policy for those heaps are supposed to be.
Maxime
On Thu, Apr 24, 2025 at 11:58 PM Maxime Ripard mripard@kernel.org wrote:
On Thu, Apr 24, 2025 at 05:13:47PM -0700, John Stultz wrote:
On Thu, Apr 24, 2025 at 1:34 AM Maxime Ripard mripard@kernel.org wrote:
I appreciate this is kind of bikeshed-color territory, but I think "cma" would be a better option here. There's nothing "default" about it.
I disagree. It very much is "default" as it's returning the dma_contiguous_default_area.
My main concern here is that it's "default" as opposed to what, exactly? We have a single CMA allocator. We could have multiple buffer attributes, but then "cached_cma" would make more sense to me if we expect to have uncached CMA allocations at some point.
Well, there may be one CMA allocator, but there can be multiple CMA regions.
So in the kernel, cma_alloc() always takes the cma area as an argument. And dma_alloc_contiguous() lets you do allocations against a device, which may reference a specific cma area. Or if the device doesn't specify a region it will utilize the default region.
There can be multiple CMA areas, and out of tree, vendors do reserve separate areas for specific purposes, exposing multiple CMA dmabuf heaps.
By "CMA areas", I guess you mean carved-out memory regions? If so, how is it relevant to userspace if we use CMA or any other implementation to expose a carved-out region, and thus that we carry that implemenattion detail in the name?
So, no, I don't mean carve-out regions. It's more about dealing with competition between multiple CMA users. In some cases, where there are known fixed buffer sizes, say camera buffers, it's much easier to reserve a separate specific sized region to allocate from so that you know it will always succeed and you don't need to waste much on safety margins. Having this added as a separate CMA region makes it a lot easier to account or reason about, and the kernel can still make (limited) use of the CMA space when it's idle. Then you don't have to worry about some other device having a short term cma allocation that pushes back the alignment for your large allocation, possibly impacting some other devices larger allocations.
And unlike with just using a carveout, you don't end up just wasting all that space when it is unused.
So userland may want to allocate contiguous memory, but it may also be relevant to userland to be able to allocate contiguous memory from a purpose specific pool.
And while not used in Android, you could imagine having separate purpose reserved cma heaps with different permissions on the heap devnodes, allowing less trusted applications to allocate cma from a small pool without having the potential to DoS the system.
There have been patches to expose multiple CMA heaps, but with no upstream drivers using those purpose specific regions, we haven't taken them yet. I do hope as the drivers that utilize these purpose focused heaps go upstream, we can add that logic, so I think being specific that this is default CMA is a good idea.
If heaps names are supposed to carry the region it exposes, then it should be default_cma_region/area. If heap names are supposed to expose the allocator (but I don't think it's a good idea), it should be cma. If they are meant to carry all that plus some policy, cached_default_cma_region should be used.
Either way, default_cma seems to me either too specific or not specific enough. And we should really document what the policy for those heaps are supposed to be.
I don't see it as such a problem. It is clear it is cma, it also is clear conceptually that it is the "default" region that the kernel uses when devices aren't specific. But I wouldn't object to cma_default_region/area as a name either, but I don't see it as particularly improved over cma_default.
To your larger point about policy, I do get the tension that you want to be able to programmatically derive or evaluate heap names, so that applications can consistently derive a pathname to get what they want. But I also think that there is so much variety in both the devices and uses that there is no way that all use cases and all devices can be satisfied with such a static or even programmatic mapping. From my perspective, there just is going to have to be some device specific glue logic that maps use->heap name. Same reason we have fstab and the passwd file. That said, I think advocating for naming conventions is definitely useful, but I'm wary of trying to enforce too specific a schema on the names as the incompleteness theorem will bite us.
thanks -john
Hi John,
On Fri, Apr 25, 2025 at 12:39:40PM -0700, John Stultz wrote:
On Thu, Apr 24, 2025 at 11:58 PM Maxime Ripard mripard@kernel.org wrote:
On Thu, Apr 24, 2025 at 05:13:47PM -0700, John Stultz wrote:
On Thu, Apr 24, 2025 at 1:34 AM Maxime Ripard mripard@kernel.org wrote:
I appreciate this is kind of bikeshed-color territory, but I think "cma" would be a better option here. There's nothing "default" about it.
I disagree. It very much is "default" as it's returning the dma_contiguous_default_area.
My main concern here is that it's "default" as opposed to what, exactly? We have a single CMA allocator. We could have multiple buffer attributes, but then "cached_cma" would make more sense to me if we expect to have uncached CMA allocations at some point.
Well, there may be one CMA allocator, but there can be multiple CMA regions.
So in the kernel, cma_alloc() always takes the cma area as an argument. And dma_alloc_contiguous() lets you do allocations against a device, which may reference a specific cma area. Or if the device doesn't specify a region it will utilize the default region.
There can be multiple CMA areas, and out of tree, vendors do reserve separate areas for specific purposes, exposing multiple CMA dmabuf heaps.
By "CMA areas", I guess you mean carved-out memory regions? If so, how is it relevant to userspace if we use CMA or any other implementation to expose a carved-out region, and thus that we carry that implemenattion detail in the name?
So, no, I don't mean carve-out regions. It's more about dealing with competition between multiple CMA users. In some cases, where there are known fixed buffer sizes, say camera buffers, it's much easier to reserve a separate specific sized region to allocate from so that you know it will always succeed and you don't need to waste much on safety margins. Having this added as a separate CMA region makes it a lot easier to account or reason about, and the kernel can still make (limited) use of the CMA space when it's idle. Then you don't have to worry about some other device having a short term cma allocation that pushes back the alignment for your large allocation, possibly impacting some other devices larger allocations.
And unlike with just using a carveout, you don't end up just wasting all that space when it is unused.
The way I see it, it's an implementation detail and is abstracted away from userspace. That's what I meant by carved-out I guess: a region dedicated to a (set of) devices(s) that the rest of the system won't use from userspace point of view.
So userland may want to allocate contiguous memory, but it may also be relevant to userland to be able to allocate contiguous memory from a purpose specific pool.
And while not used in Android, you could imagine having separate purpose reserved cma heaps with different permissions on the heap devnodes, allowing less trusted applications to allocate cma from a small pool without having the potential to DoS the system.
Yeah... I don't think it's the right approach for that. If Android doesn't use it, and if it's the only Linux distro with 1 app / 1 user policy, then the only permissions we'll effectively have is one for the whole use, trusted and untrusted apps alike.
cgroups look like a much better path forward, and wouldn't require multiple heaps.
Anyway... It's not really important at this point I guess.
There have been patches to expose multiple CMA heaps, but with no upstream drivers using those purpose specific regions, we haven't taken them yet. I do hope as the drivers that utilize these purpose focused heaps go upstream, we can add that logic, so I think being specific that this is default CMA is a good idea.
If heaps names are supposed to carry the region it exposes, then it should be default_cma_region/area. If heap names are supposed to expose the allocator (but I don't think it's a good idea), it should be cma. If they are meant to carry all that plus some policy, cached_default_cma_region should be used.
Either way, default_cma seems to me either too specific or not specific enough. And we should really document what the policy for those heaps are supposed to be.
I don't see it as such a problem. It is clear it is cma, it also is clear conceptually that it is the "default" region that the kernel uses when devices aren't specific. But I wouldn't object to cma_default_region/area as a name either, but I don't see it as particularly improved over cma_default.
To your larger point about policy, I do get the tension that you want to be able to programmatically derive or evaluate heap names, so that applications can consistently derive a pathname to get what they want.
We've discussed it in the past, I don't really want to. But it was clear from the last discussion that you (plural) wanted to infer heap semantics from the names. I'm ok with that, but then if we want to make it work we need to have well defined names.
And it's actually what I really want to discuss here: we've discussed at length how bad the heaps name are (and not only here), but I don't think we have any documented policy on what makes a good name?
For example, I'm not sure exposing the allocator name is a good idea: it's an implementation detail and for all userspace cares about, we could change it every release if it provided the same kind of buffers.
Taking your camera buffers example before, then we could also expose a memory region id, and let the platform figure it out, or use the usecase as the name.
But if we don't document that, how can we possibly expect everyone including downstream to come up with perfect names every time. And FTR, I'm willing to write that doc down once the discussion settles.
But I also think that there is so much variety in both the devices and uses that there is no way that all use cases and all devices can be satisfied with such a static or even programmatic mapping. From my perspective, there just is going to have to be some device specific glue logic that maps use->heap name. Same reason we have fstab and the passwd file.
fstab and passwd can be generated at (first) boot time / install. fstab is also being somewhat less important with the auto-partition discovery. How would you generate that configuration file at boot?
I'm not really asking this as a theoretical question. Being able to auto-discover which heap a driver/device would allocate from is central for the cgroup work I mentioned earlier.
And I'm really not sure how distros or applications developpers are supposed to keep up with the raw volume of devices that go out every year, each and every one of them having different heap names, etc. Possibly different from one version of the firmware to another.
That said, I think advocating for naming conventions is definitely useful, but I'm wary of trying to enforce too specific a schema on the names as the incompleteness theorem will bite us.
On Mon, Apr 28, 2025 at 7:52 AM Maxime Ripard mripard@kernel.org wrote:
On Fri, Apr 25, 2025 at 12:39:40PM -0700, John Stultz wrote:
To your larger point about policy, I do get the tension that you want to be able to programmatically derive or evaluate heap names, so that applications can consistently derive a pathname to get what they want.
We've discussed it in the past, I don't really want to. But it was clear from the last discussion that you (plural) wanted to infer heap semantics from the names. I'm ok with that, but then if we want to make it work we need to have well defined names.
So my name keeps on getting attached to that, but I don't think I was involved in the LPC conversation when that got decided.
And it's actually what I really want to discuss here: we've discussed at length how bad the heaps name are (and not only here), but I don't think we have any documented policy on what makes a good name?
I very much think having a policy/guidance for better names is a good goal.
I just want to make sure it doesn't become a strict policy that lead folks to make mistaken assumptions about a static solution being viable in userland (like folks nostalgicly using "eth0" or a fixed network device name in scripts expecting it to work on a different system)
For example, I'm not sure exposing the allocator name is a good idea: it's an implementation detail and for all userspace cares about, we could change it every release if it provided the same kind of buffers.
That is a fair point.
Taking your camera buffers example before, then we could also expose a memory region id, and let the platform figure it out, or use the usecase as the name.
But if we don't document that, how can we possibly expect everyone including downstream to come up with perfect names every time. And FTR, I'm willing to write that doc down once the discussion settles.
So again, yeah, I very much support having better guidance on the names.
I think the number of device constraints and device combinations makes a raw enumeration of things difficult.
This is why the per-device use->heap mapping "glue" seems necessary to me.
And, I do get that this runs into a similar problem with enumerating and defining "uses" (which boil down to a combination of devices-in-a-pipeline and access use patterns), but for Andorid it has so far been manageable.
Personally, I think the best idea I've heard so far to resolve this from userland was Christian's suggestion that devices expose links to compatible heaps, and then userland without a use->heap mapping could for the set of devices they plan to use in a pipeline, figure out the common heap name and use that to allocate.
However, that pushes the problem down a bit, requiring drivers (instead of userland) to know what heaps they can work with and what the names might be (which again, your goal for standardizing the heap names isn't a bad thing!). Though, this approach also runs into trouble as it opens a question of: should it only encode strict constraint satisfaction, or something more subtle, as while something might work with multiple heaps, its possible it won't be performant enough unless it picks a specific one on device A or a different one on device B. And getting that sort of device-specific details embedded into a driver isn't great either.
But I also think that there is so much variety in both the devices and uses that there is no way that all use cases and all devices can be satisfied with such a static or even programmatic mapping. From my perspective, there just is going to have to be some device specific glue logic that maps use->heap name. Same reason we have fstab and the passwd file.
fstab and passwd can be generated at (first) boot time / install. fstab is also being somewhat less important with the auto-partition discovery. How would you generate that configuration file at boot?
I'm not really asking this as a theoretical question. Being able to auto-discover which heap a driver/device would allocate from is central for the cgroup work I mentioned earlier.
And I'm really not sure how distros or applications developpers are supposed to keep up with the raw volume of devices that go out every year, each and every one of them having different heap names, etc. Possibly different from one version of the firmware to another.
For generic distros, I don't have a good answer here. Historically the focus has always been on single device usage, so having the driver do the allocation was fine, and if you were using multiple devices you could just copy the memory between the driver allocated buffers. But as we've moved to disaggregated IP blocks and device pipelines, all those potential copies wreck performance and power. I'm not sure generic distros have the concept of a device pipeline very well abstracted (obviously mesa and the wayland/X have had to deal with it, and the video and camera side is dealing with it more and more). Maybe a more established notion of use -> pipeline/device collections, is needed as a starting point? Then using Christian's suggestion, one could at least enumerate use -> heap that would be functional. And maybe device makers could then supplement explicit optimized mapping overrides for their device?
I just think leaving individual applications (or even individual frameworks like mesa) to embed assumptions about heap names -> functionality is going to be a problematic approach.
thanks -john
On Tue, Apr 29, 2025 at 09:25:00AM -0700, John Stultz wrote:
On Mon, Apr 28, 2025 at 7:52 AM Maxime Ripard mripard@kernel.org wrote:
On Fri, Apr 25, 2025 at 12:39:40PM -0700, John Stultz wrote:
To your larger point about policy, I do get the tension that you want to be able to programmatically derive or evaluate heap names, so that applications can consistently derive a pathname to get what they want.
We've discussed it in the past, I don't really want to. But it was clear from the last discussion that you (plural) wanted to infer heap semantics from the names. I'm ok with that, but then if we want to make it work we need to have well defined names.
So my name keeps on getting attached to that, but I don't think I was involved in the LPC conversation when that got decided.
Sorry then :/
That's what I recalled, but I guess the pastries got the best of me :)
Does that mean that you disagree with this point though? or just that you agree but still wanted to point out you were not part of that discussion?
And it's actually what I really want to discuss here: we've discussed at length how bad the heaps name are (and not only here), but I don't think we have any documented policy on what makes a good name?
I very much think having a policy/guidance for better names is a good goal.
I just want to make sure it doesn't become a strict policy that lead folks to make mistaken assumptions about a static solution being viable in userland (like folks nostalgicly using "eth0" or a fixed network device name in scripts expecting it to work on a different system)
I think that's one of the point where the "derive the buffer attributes" from the name interact badly though. In your example, eth0 wouldn't have had any non-discoverable guarantees. So it can have any name you want, it doesn't matter, you can always discover it through some other mean, and go from there.
If we say the name is how you can associate a heap and the kind of buffers you get, then we can't just use another heap name just like that. We could get buffers with a totally different semantics.
I mean, it would probably work with Android, but for any other distribution, even if we came up with a gralloc-like solution, as soon as you start updating the kernel and whatever is using the heaps separately, it's game over. And pretty much all non-Android distros do?
For example, I'm not sure exposing the allocator name is a good idea: it's an implementation detail and for all userspace cares about, we could change it every release if it provided the same kind of buffers.
That is a fair point.
Taking your camera buffers example before, then we could also expose a memory region id, and let the platform figure it out, or use the usecase as the name.
But if we don't document that, how can we possibly expect everyone including downstream to come up with perfect names every time. And FTR, I'm willing to write that doc down once the discussion settles.
So again, yeah, I very much support having better guidance on the names.
I think the number of device constraints and device combinations makes a raw enumeration of things difficult.
This is why the per-device use->heap mapping "glue" seems necessary to me.
And, I do get that this runs into a similar problem with enumerating and defining "uses" (which boil down to a combination of devices-in-a-pipeline and access use patterns), but for Andorid it has so far been manageable.
Personally, I think the best idea I've heard so far to resolve this from userland was Christian's suggestion that devices expose links to compatible heaps, and then userland without a use->heap mapping could for the set of devices they plan to use in a pipeline, figure out the common heap name and use that to allocate.
I plan to work on that, but also, it covers only what the driver cares about, ie, buffer location, etc. It doesn't really cover what userspace might care about, like whether the buffer is cachable or not. Both would work for any driver, but userspace will have to prefer one over the other if it plans to do CPU accesses.
So we'd still need some (arguably more limited) enumeration on the userspace side.
However, that pushes the problem down a bit, requiring drivers (instead of userland) to know what heaps they can work with and what the names might be (which again, your goal for standardizing the heap names isn't a bad thing!). Though, this approach also runs into trouble as it opens a question of: should it only encode strict constraint satisfaction, or something more subtle, as while something might work with multiple heaps, its possible it won't be performant enough unless it picks a specific one on device A or a different one on device B. And getting that sort of device-specific details embedded into a driver isn't great either.
Yeah :/
But I also think that there is so much variety in both the devices and uses that there is no way that all use cases and all devices can be satisfied with such a static or even programmatic mapping. From my perspective, there just is going to have to be some device specific glue logic that maps use->heap name. Same reason we have fstab and the passwd file.
fstab and passwd can be generated at (first) boot time / install. fstab is also being somewhat less important with the auto-partition discovery. How would you generate that configuration file at boot?
I'm not really asking this as a theoretical question. Being able to auto-discover which heap a driver/device would allocate from is central for the cgroup work I mentioned earlier.
And I'm really not sure how distros or applications developpers are supposed to keep up with the raw volume of devices that go out every year, each and every one of them having different heap names, etc. Possibly different from one version of the firmware to another.
For generic distros, I don't have a good answer here. Historically the focus has always been on single device usage, so having the driver do the allocation was fine, and if you were using multiple devices you could just copy the memory between the driver allocated buffers. But as we've moved to disaggregated IP blocks and device pipelines, all those potential copies wreck performance and power. I'm not sure generic distros have the concept of a device pipeline very well abstracted (obviously mesa and the wayland/X have had to deal with it, and the video and camera side is dealing with it more and more). Maybe a more established notion of use -> pipeline/device collections, is needed as a starting point? Then using Christian's suggestion, one could at least enumerate use -> heap that would be functional. And maybe device makers could then supplement explicit optimized mapping overrides for their device?
I just think leaving individual applications (or even individual frameworks like mesa) to embed assumptions about heap names -> functionality is going to be a problematic approach.
I totally agree on the conclusion, but I still don't see how having a central component in charge of that will make things better. It just won't scale to the thousands of devices out there.
And that's great improvements for the future, but heaps have use-cases today: the CMA heap is the only way to get a physically contiguous cacheable buffer in userspace at the moment for example.
libcamera uses it for its software ISP implementation for example.
So, while working on improving things in the future is a reasonable goal, we also need to improve things for the current users right now. And there's definitely users for it outside of Android.
Which brings us back to the question: What would be a good name? Do we want to expose a platform specific region name, possibly with a suffix or prefix to define whether it's cached or not?
Maxime
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