[Linaro-mm-sig] Memory region attribute bits and multiple mappings
Tom.Cooksey at arm.com
Thu Apr 21 10:33:35 UTC 2011
At least with Mali, we use user-space sub-allocators. So we'll acquire hunks of memory from the kernel, map it into the GPU context's virtual address space and let the user-space driver divide it up into however many buffers it needs. So I guess we could still acquire the hunks of memory from whatever kernel-space allocator we want, but fundamentally, kernel-space will never see what individual buffers that memory is divided up into. I'm also struggling to understand how we could securely share a buffer after it has been allocated given we'd have to share entire pages. It's quite likely a buffer we want to share will have data in pages used by other buffers which we don't want to share? Fundamentally, we need to know at allocation time if a buffer is intended to be shared with another device or process so we can make sure the allocation gets its own pages.
Thinking about the uncached issue some more, I think I've convinced myself that it should be a requirement. We'll have to support older (pre-A9) cores for some time where using uncached memory provides better performance (which seems to be the consensus opinion here?).
From: rschultz at google.com [mailto:rschultz at google.com] On Behalf Of Rebecca Schultz Zavin
Sent: 20 April 2011 22:53
To: Dave Airlie
Cc: Tom Cooksey; Arnd Bergmann; linaro-mm-sig at lists.linaro.org
Subject: Re: [Linaro-mm-sig] Memory region attribute bits and multiple mappings
I've been buried all day, but here's my response to a bunch of the discussion points today:
I definitely don't think it makes sense to solve part of the problem and then leave the gpu's to require their own allocator for their common case. Why solve the allocator problem twice? Also, we want to be able to support passing of these buffers between the camera, gpu, video decoder etc. It'd be much easier if they were from a common source.
The android team's graphics folks, the their counterparts at intel, imagination tech, nvidia, qualcomm and arm have all told me that they need a method for mapping buffers uncached to userspace. The common case for this is to write vertexes, textures etc to these buffers once and never touch them again. This may happen several (or even several 10s or more) of times per frame. My experience with cache flushes on ARM architectures matches Marek's. Typically write combine makes streaming writes really really fast, and on several SOC's we've found it cheaper to flush the whole cache than to flush by line. Clearly this impacts the rest of system performance, not to mention the fact that a couple of large textures and you've totally blown your caches for the rest of the system.
Once we've solved the multiple mapping problem, it becomes quite easy to support both cached AND uncached accesses from the cpu, so I think that covers the cases where it's actually desirable to have a cached mapping -- software rendering, processing frames from a camera etc.
I think the issue of contiguous memory allocation and cache attributes are totally separate, except that in cases where large contiguous regions are necessary -- the problem qualcom's pmem and friends were written to solve -- you pretty much end up needing to put aside a pool of buffers at boot anyway in order to guarantee the availability of large order allocations. Once you've done that, the attributes problem goes away since your memory is not in the direct map.
On Wed, Apr 20, 2011 at 1:34 PM, Dave Airlie <airlied at gmail.com<mailto:airlied at gmail.com>> wrote:
> [TC] I'm not sure I completely agree with this being a use case. From my
> understanding, the problem we're trying to solve here isn't a generic
> graphics memory manager but rather a memory manager to facilitate
> cross-device sharing of 2D image buffers. GPU drivers will still have their
> own allocators for textures which will probably be in a tiled or other
> proprietary format no other device can understand anyway. The use case where
> we (GPU driver vendors) might want uncached memory is for one-time texture
> upload. If we have a texture we know we're only going to write to once (with
> the CPU), there is no benefit in using cached memory. In fact, there's a
> potential performance drop if you used cached memory because the texture
> upload will cause useful cache lines to be evicted and replaced with useless
> lines for the texture. However, I don't see any use case where we'd then
> want to share that CPU-uploaded texture with another device, in which case
> we would use our own (uncached) allocator, not this "cross-device"
> allocator. There's also a school of thought (so I'm told) that for one-time
> texture upload you still want to use cached memory because more modern cores
> have smaller write buffers (so you want to use the cache for better
> combining of writes) and are good at detecting large sequential writes and
> thus don't use the whole cache for those anyway. So, other than one-time
> texture upload, are there other graphics use cases you know of where it
> might be more optimal to use uncached memory? What about video decoder
The memory mangaer should be used for all internal GPU memory
management as well if desired.
If we have any hope of ever making open source ARM GPU drivers get
upstream they can't all just
go reinventing the wheel. They need to be based on a common layer.
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