linux-kernel - Re: [PATCH v8 2/2] drm/gem: Don't map imported GEMs

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Date:   Tue, 16 Aug 2022 22:55:52 +0300
From:   Dmitry Osipenko <dmitry.osipenko@...labora.com>
To:     Christian König <ckoenig.leichtzumerken@...il.com>,
        Rob Clark <robdclark@...il.com>,
        Christian König <christian.koenig@....com>
Cc:     David Airlie <airlied@...ux.ie>, Gerd Hoffmann <kraxel@...hat.com>,
        Gurchetan Singh <gurchetansingh@...omium.org>,
        Chia-I Wu <olvaffe@...il.com>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        Emil Velikov <emil.l.velikov@...il.com>,
        Thomas Hellström <thomas_os@...pmail.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        "open list:VIRTIO GPU DRIVER" 
        <virtualization@...ts.linux-foundation.org>,
        linux-tegra@...r.kernel.org, Dmitry Osipenko <digetx@...il.com>,
        kernel@...labora.com, Daniel Vetter <daniel@...ll.ch>
Subject: Re: [PATCH v8 2/2] drm/gem: Don't map imported GEMs

On 8/16/22 15:03, Christian König wrote:
> Am 16.08.22 um 13:44 schrieb Dmitry Osipenko:
>> [SNIP]
>>> The other complication I noticed is that we don't seem to keep around
>>> the fd after importing to a GEM handle.  And I could imagine that
>>> doing so could cause issues with too many fd's.  So I guess the best
>>> thing is to keep the status quo and let drivers that cannot mmap
>>> imported buffers just fail mmap?
>> That actually should be all the drivers excluding those that use
>> DRM-SHMEM because only DRM-SHMEM uses dma_buf_mmap(), that's why it
>> works for Panfrost. I'm pretty sure mmaping of imported GEMs doesn't
>> work for the MSM driver, isn't it?
>>
>> Intel and AMD drivers don't allow to map the imported dma-bufs. Both
>> refuse to do the mapping.
>>
>> Although, AMDGPU "succeeds" to do the mapping using
>> AMDGPU_GEM_DOMAIN_GTT, but then touching the mapping causes bus fault,
>> hence mapping actually fails. I think it might be the AMDGPU
>> driver/libdrm bug, haven't checked yet.
> 
> That's then certainly broken somehow. Amdgpu should nerve ever have
> allowed to mmap() imported DMA-bufs and the last time I check it didn't.

I'll take a closer look. So far I can only tell that it's a kernel
driver issue because once I re-applied this "Don't map imported GEMs"
patch, AMDGPU began to refuse mapping AMDGPU_GEM_DOMAIN_GTT.

>> So we're back to the point that neither of DRM drivers need to map
>> imported dma-bufs and this was never tested. In this case this patch is
>> valid, IMO.

Actually, I'm now looking at Etnaviv and Nouveau and seems they should
map imported dma-buf properly. I know that people ran Android on
Etnaviv. So maybe devices with a separated GPU/display need to map
imported display BO for Android support. Wish somebody who ran Android
on one of these devices using upstream drivers could give a definitive
answer. I may try to test Nouveau later on.

-- 
Best regards,
Dmitry