Re: DMA-buf and uncached system memory
From: Christian König
Date: Mon Feb 15 2021 - 04:36:13 EST
Am 15.02.21 um 10:06 schrieb Simon Ser:
On Monday, February 15th, 2021 at 9:58 AM, Christian König <christian.koenig@xxxxxxx> wrote:
we are currently working an Freesync and direct scan out from system
memory on AMD APUs in A+A laptops.
On problem we stumbled over is that our display hardware needs to scan
out from uncached system memory and we currently don't have a way to
communicate that through DMA-buf.
For our specific use case at hand we are going to implement something
driver specific, but the question is should we have something more
generic for this?
After all the system memory access pattern is a PCIe extension and as
such something generic.
Intel also needs uncached system memory if I'm not mistaken?
No idea, that's why I'm asking. Could be that this is also interesting
for I+A systems.
Where are the buffers allocated? If GBM, then it needs to allocate memory that
can be scanned out if the USE_SCANOUT flag is set or if a scanout-capable
modifier is picked.
If this is about communicating buffer constraints between different components
of the stack, there were a few proposals about it. The most recent one is [1].
Well the problem here is on a different level of the stack.
See resolution, pitch etc:.. can easily communicated in userspace
without involvement of the kernel. The worst thing which can happen is
that you draw garbage into your own application window.
But if you get the caching attributes in the page tables (both CPU as
well as IOMMU, device etc...) wrong then ARM for example has the
tendency to just spontaneously reboot
X86 is fortunately a bit more gracefully and you only end up with random
data corruption, but that is only marginally better.
So to sum it up that is not something which we can leave in the hands of
userspace.
I think that exporters in the DMA-buf framework should have the ability
to tell importers if the system memory snooping is necessary or not.
Userspace components can then of course tell the exporter what the
importer needs, but validation if that stuff is correct and doesn't
crash the system must happen in the kernel.
Regards,
Christian.
Simon
[1]: https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Fxdc2020.x.org%2Fevent%2F9%2Fcontributions%2F615%2F&data=04%7C01%7Cchristian.koenig%40amd.com%7Cb2824bd79bd44862b38e08d8d190f344%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637489767796900783%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&sdata=hIptfin5Xx6XlYBtGFYAAbfuNsnD6kmQEiggq9k10E8%3D&reserved=0