On 29/11/2018 19:57, Tomasz Figa wrote:
On Thu, Nov 29, 2018 at 11:40 AM Jordan Crouse <jcrouse@xxxxxxxxxxxxxx> wrote:
On Thu, Nov 29, 2018 at 01:48:15PM -0500, Rob Clark wrote:
On Thu, Nov 29, 2018 at 10:54 AM Christoph Hellwig <hch@xxxxxx> wrote:
On Thu, Nov 29, 2018 at 09:42:50AM -0500, Rob Clark wrote:
Maybe the thing we need to do is just implement a blacklist of
compatible strings for devices which should skip the automatic
iommu/dma hookup. Maybe a bit ugly, but it would also solve a problem
preventing us from enabling per-process pagetables for a5xx (where we
need to control the domain/context-bank that is allocated by the dma
api).
You can detach from the dma map attachment using arm_iommu_detach_device,
which a few drm drivers do, but I don't think this is the problem.
I think even with detach, we wouldn't end up with the context-bank
that the gpu firmware was hard-coded to expect, and so it would
overwrite the incorrect page table address register. (I could be
mis-remembering that, Jordan spent more time looking at that. But it
was something along those lines.)
Right - basically the DMA domain steals context bank 0 and the GPU is hard coded
to use that context bank for pagetable switching.
I believe the Tegra guys also had a similar problem with a hard coded context
bank.
AIUI, they don't need a specific hardware context, they just need to
know which one they're actually using, which the domain abstraction hides.
Wait, if we detach the GPU/display struct device from the default
domain and attach it to a newly allocated domain, wouldn't the newly
allocated domain use the context bank we need? Note that we're already
The arm-smmu driver doesn't, but there's no fundamental reason it
couldn't. That should just need code to refcount domain users and
release hardware contexts for domains with no devices currently attached.
Robin.
doing that, except that we're doing it behind the back of the DMA
mapping subsystem, so that it keeps using the IOMMU version of the DMA
ops for the device and doing any mapping operations on the default
domain. If we ask the DMA mapping to detach, wouldn't it essentially
solve the problem?
Best regards,
Tomasz
