On Thu, Sep 11, 2025 at 12:01 AM Maxime Ripard mripard@kernel.org wrote:
Hi TJ,
On Wed, Sep 10, 2025 at 01:44:45PM -0700, T.J. Mercier wrote:
On Wed, Sep 10, 2025 at 12:33 AM Maxime Ripard mripard@kernel.org wrote:
On Tue, Aug 26, 2025 at 09:36:03AM +0200, Maxime Ripard wrote:
Hi,
On Mon, Jul 21, 2025 at 01:17:29PM +0200, Maxime Ripard wrote:
Here's another attempt at supporting user-space allocations from a specific carved-out reserved memory region.
The initial problem we were discussing was that I'm currently working on a platform which has a memory layout with ECC enabled. However, enabling the ECC has a number of drawbacks on that platform: lower performance, increased memory usage, etc. So for things like framebuffers, the trade-off isn't great and thus there's a memory region with ECC disabled to allocate from for such use cases.
After a suggestion from John, I chose to first start using heap allocations flags to allow for userspace to ask for a particular ECC setup. This is then backed by a new heap type that runs from reserved memory chunks flagged as such, and the existing DT properties to specify the ECC properties.
After further discussion, it was considered that flags were not the right solution, and relying on the names of the heaps would be enough to let userspace know the kind of buffer it deals with.
Thus, even though the uAPI part of it had been dropped in this second version, we still needed a driver to create heaps out of carved-out memory regions. In addition to the original usecase, a similar driver can be found in BSPs from most vendors, so I believe it would be a useful addition to the kernel.
Some extra discussion with Rob Herring [1] came to the conclusion that some specific compatible for this is not great either, and as such an new driver probably isn't called for either.
Some other discussions we had with John [2] also dropped some hints that multiple CMA heaps might be a good idea, and some vendors seem to do that too.
So here's another attempt that doesn't affect the device tree at all and will just create a heap for every CMA reserved memory region.
It also falls nicely into the current plan we have to support cgroups in DRM/KMS and v4l2, which is an additional benefit.
Let me know what you think, Maxime
Any chance we can get this merged?
Guys, can we move forward on this?
Maxime
Hi Maxime,
Sorry I've been MIA the last couple of months.
The docs for the "reusable" property say, "device driver(s) owning the region need to be able to reclaim it back", but how can a driver reclaim memory backing a dmabuf, since pages allocated for a dmabuf aren't necessarily movable. Couldn't a user allocate all of it, and refuse to close those dmabufs?
I guess, but how is that any different than what we're doing on the default allocator already?
Yeah fair, it's not. I'm thinking that makes determining a size for a reusable driver-specified region that's always exposed to userspace a bit fuzzy. The requirements for the driver can probably be known, but for potentially unrelated allocations from userspace? The default ownership / file permissions for the heap would have to be changed to allow those non-reclaimable allocations, so maybe that's enough of an opt-in for such regions.
It also has to be reusable, and will not be able to reclaim any memory allocated through the heap.
I backported this to 6.6 and ran it on a Pixel. While there are already similar out-of-tree dmabuf heap drivers that expose heaps for these reserved regions, they do more than just cma_alloc (multiple flavors of buffer securing, use case specific alignment and padding, and slightly different allocation strategies) so I don't think this series would allow us to completely drop the custom heap code, but it's a nice start.
Thanks for testing, and I totally expect more heaps coming for things like protected memory, but it should indeed reduce the number of heap drivers needed going forward.
Does the cgroup part come in because the plan is to add charging in cma_heap.c?
Yes, and the system heap as well.
Maxime
Thanks,
Reviewed-by: T.J. Mercier tjmercier@google.com