Re: [PATCH 2/3] drm/prime: set the dma_coherent flag for export

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On Thu, Oct 20, 2022 at 5:13 AM Christian König
<ckoenig.leichtzumerken@xxxxxxxxx> wrote:
>
> When a device driver is snooping the CPU cache during access we assume
> that all importers need to be able to snoop the CPU cache as well.
>
> Signed-off-by: Christian König <christian.koenig@xxxxxxx>
> ---
>  drivers/gpu/drm/drm_prime.c | 2 ++
>  1 file changed, 2 insertions(+)
>
> diff --git a/drivers/gpu/drm/drm_prime.c b/drivers/gpu/drm/drm_prime.c
> index 20e109a802ae..d5c70b6fe8a4 100644
> --- a/drivers/gpu/drm/drm_prime.c
> +++ b/drivers/gpu/drm/drm_prime.c
> @@ -28,6 +28,7 @@
>
>  #include <linux/export.h>
>  #include <linux/dma-buf.h>
> +#include <linux/dma-map-ops.h>
>  #include <linux/rbtree.h>
>  #include <linux/module.h>
>
> @@ -889,6 +890,7 @@ struct dma_buf *drm_gem_prime_export(struct drm_gem_object *obj,
>                 .size = obj->size,
>                 .flags = flags,
>                 .priv = obj,
> +               .coherent = dev_is_dma_coherent(dev->dev),

To set the coherent flag correctly, I think I'd need a way to override
on a per buffer basis, since coherency is a property of the gpu
pgtables (which in the msm case is an immutable property of the gem
object).  We also have some awkwardness that drm->dev isn't actually
the GPU, thanks to the kernels device model seeing a collection of
other small devices shoehorned into a single drm device to fit
userspace's view of the world.  So relying on drm->dev isn't really
going to give sensible results.

I guess msm could just bury our heads in the sand and continue to do
things the way we have been (buffers that are mapped cached-coherent
are only self-shared) but would be nice to catch if userspace tried to
import one into (for ex) v4l2..

BR,
-R

>                 .resv = obj->resv,
>         };
>
> --
> 2.25.1
>




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