Re: [RFC 2/2] drm/msm/gem: Make use of the system cache

[Arnd Bergmann]

On Wed, Nov 17, 2021 at 12:16 AM Georgi Djakov
<quic_c_gdjako@xxxxxxxxxxx> wrote:
>
> Instead of writing to WC cmdstream buffers that go all the way to the main
> memory, let's use the system cache to improve the performance.
>
> Signed-off-by: Georgi Djakov <quic_c_gdjako@xxxxxxxxxxx>
> ---
>  drivers/gpu/drm/msm/msm_gem.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/gpu/drm/msm/msm_gem.c b/drivers/gpu/drm/msm/msm_gem.c
> index 104fdfc14027..921a1c24721e 100644
> --- a/drivers/gpu/drm/msm/msm_gem.c
> +++ b/drivers/gpu/drm/msm/msm_gem.c
> @@ -214,7 +214,7 @@ void msm_gem_put_pages(struct drm_gem_object *obj)
>  static pgprot_t msm_gem_pgprot(struct msm_gem_object *msm_obj, pgprot_t prot)
>  {
>         if (msm_obj->flags & (MSM_BO_WC|MSM_BO_UNCACHED))
> -               return pgprot_writecombine(prot);
> +               return pgprot_syscached(prot);
>         return prot;
>  }

Based on the definition in patch 1, doesn't this mean that 32-bit
kernels degrade
from writecombined to uncached, making them a lot slower?

My feeling about this series is that there should be a clearer
definition of what
exactly happens on systems with and without system cache.

         Arnd