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

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Message-ID: <20200227132137.irruicvlkxpdo3so@sirius.home.kraxel.org>
Date:   Thu, 27 Feb 2020 14:21:37 +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>,
        gurchetansingh@...omium.org, tzimmermann@...e.de, yuq825@...il.com,
        noralf@...nnes.org, robh@...nel.org
Subject: Re: [PATCH v5 1/3] drm/shmem: add support for per object caching
 flags.

  Hi,

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

Done.

>> [ context: why shmem helpers use pgprot_writecombine + pgprot_decrypted?
>>            we get conflicting mappings because of that, linear kernel
>>            map vs. gem object vmap/mmap ]

> Do we have any idea what drivers are actually using
> write-combine and decrypted?

drivers/gpu/drm# find -name Kconfig* -print | xargs grep -l DRM_GEM_SHMEM_HELPER
./lima/Kconfig
./tiny/Kconfig
./cirrus/Kconfig
./Kconfig
./panfrost/Kconfig
./udl/Kconfig
./v3d/Kconfig
./virtio/Kconfig

virtio needs cached.
cirrus+udl should be ok with cached too.

Not clue about the others (lima, tiny, panfrost, v3d).  Maybe they use
write-combine just because this is what they got by default from
drm_gem_mmap_obj().  Maybe they actually need that.  Trying to Cc:
maintainters (and drop stable@).

On decrypted: I guess that is only relevant for virtual machines, i.e.
virtio-gpu and cirrus?

virtio-gpu needs it, otherwise the host can't show the virtual display.
cirrus bounces everything via blits to vram, so it should be ok without
decrypted.  I guess that implies we should make decrypted configurable.

cheers,
  Gerd