linux-kernel - Re: [PATCH v5 1/3] drm/shmem: add support for per object caching flags.

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Date:   Thu, 27 Feb 2020 11:56:43 +0100
From:   Gerd Hoffmann <kraxel@...hat.com>
To:     Thomas Hellström (VMware) 
        <thomas_os@...pmail.org>
Cc:     dri-devel@...ts.freedesktop.org, Guillaume.Gardet@....com,
        David Airlie <airlied@...ux.ie>,
        open list <linux-kernel@...r.kernel.org>,
        stable@...r.kernel.org, gurchetansingh@...omium.org,
        tzimmermann@...e.de
Subject: Re: [PATCH v5 1/3] drm/shmem: add support for per object caching
 flags.

  Hi,

> I think it might be safe for some integrated graphics where the driver
> maintainers can guarantee that it's safe on all particular processors used
> with that driver, but then IMO it should be moved out to those drivers.
> 
> Other drivers needing write-combine shouldn't really use shmem.
> 
> So again, to fix the regression, could we revert 0be895893607f ("drm/shmem:
> switch shmem helper to &drm_gem_object_funcs.mmap") or does that have other
> implications?

This patch isn't a regression.  The old code path has the
pgprot_writecombine() call in drm_gem_mmap_obj(), so the behavior
is the same before and afterwards.

But with the patch in place we can easily have shmem helpers do their
own thing instead of depending on whatever drm_gem_mmap_obj() is doing.
Just using cached mappings unconditionally would be perfectly fine for
virtio-gpu.

Not sure about the other users though.  I'd like to fix the virtio-gpu
regression (coming from ttm -> shmem switch) asap, and I don't feel like
changing the behavior for other drivers in 5.6-rc is a good idea.

So I'd like to push patches 1+2 to -fixes and sort everything else later
in -next.  OK?

cheers,
  Gerd