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 13:16:45 +0100
From:   Thomas Hellström (VMware) 
        <thomas_os@...pmail.org>
To:     Gerd Hoffmann <kraxel@...hat.com>
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.

On 2/27/20 11:56 AM, Gerd Hoffmann wrote:
>    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.

OK. I wasn't checking where this all came from from the start...

> 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?

OK with me. Do we have any idea what drivers are actually using 
write-combine and decrypted?

/Thomas



>
> cheers,
>    Gerd