Re: [PATCH RFC] drm/nouveau: Use write-combined maps for coherent on Tegra

[Aaron Kling]

On Tue, Jul 28, 2026 at 3:38 AM Lucas Stach <dev@xxxxxxxxxx> wrote:
>
> Am Samstag, dem 25.07.2026 um 18:28 -0500 schrieb Aaron Kling via B4
> Relay:
> > From: Faith Ekstrand <faith.ekstrand@xxxxxxxxxxxxx>
> >
> > On Tegra devices, uncached maps traslate to device memory, causing
> > unaligned accesses by userspace resulting in a SIGBUS. Instead, use
> > write-combined maps to ensure proper access.
> >
> This is not only an issue on Tegra. All ARM64 platforms translate
> uncached to device memory, which does not support unaligned access. So
> the issue could happen in the same way when connecting a discrete GPU
> to a ARM64 platform with PCIe support.
>
> I would argue that there is no downside to always using write combined
> memory for coherent buffers. You gain potentially faster writes while
> the only issue is that writes may get delayed visibility at the device
> side. However, I don't think anyone uses coherent buffers to modify the
> content from the CPU side while the device is operating on the buffer.
> For all regular use-case the memory barrier in FIRE_RING() will make
> sure that the buffered writes are visible to the device before the
> commands using the buffer are made visible to the device.

Should I send a new revision that sets write combined for all coherent
then? I only have Tegra devices to verify this on, so I can't attest
to how this affects discrete cards.

Aaron

> Regards,
> Lucas
>
> > Signed-off-by: Faith Ekstrand <faith.ekstrand@xxxxxxxxxxxxx>
> > Co-developed-by: Aaron Kling <webgeek1234@xxxxxxxxx>
> > Signed-off-by: Aaron Kling <webgeek1234@xxxxxxxxx>
> > ---
> > This patch is marked RFC because there is not complete confidence that
> > the change is correct. Without this change, nouveau can easily hit
> > sigbus errors in Android UI rendering via nvk or running vulkan deqp
> > tests on Tegra. Changing coherent maps to use write-combined stops the
> > crashing, but it is unknown what the performance impact is, if any.
> > ---
> >  drivers/gpu/drm/nouveau/nouveau_sgdma.c | 9 ++++++---
> >  1 file changed, 6 insertions(+), 3 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/nouveau/nouveau_sgdma.c b/drivers/gpu/drm/nouveau/nouveau_sgdma.c
> > index fa3b4ebf38a83..10f6ef1c541bd 100644
> > --- a/drivers/gpu/drm/nouveau/nouveau_sgdma.c
> > +++ b/drivers/gpu/drm/nouveau/nouveau_sgdma.c
> > @@ -72,9 +72,12 @@ nouveau_sgdma_create_ttm(struct ttm_buffer_object *bo, uint32_t page_flags)
> >       struct nouveau_sgdma_be *nvbe;
> >       enum ttm_caching caching;
> >
> > -     if (nvbo->force_coherent)
> > -             caching = ttm_uncached;
> > -     else if (drm->agp.bridge)
> > +     if (nvbo->force_coherent) {
> > +             if (drm->client.device.info.platform == NV_DEVICE_INFO_V0_SOC)
> > +                     caching = ttm_write_combined;
> > +             else
> > +                     caching = ttm_uncached;
> > +     } else if (drm->agp.bridge)
> >               caching = ttm_write_combined;
> >       else
> >               caching = ttm_cached;
> >
> > ---
> > base-commit: 1a1757b76427f6201bfe0bf1bea9f7574f332a93
> > change-id: 20260725-tegra-coherent-wc-19941d1a5a60
> >
> > Best regards,