linux-kernel - Re: [PATCH v7 2/2] drm/gem: Don't map imported GEMs

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Date:   Thu, 30 Jun 2022 22:26:37 +0200
From:   Thomas Hellström (Intel) 
        <thomas_os@...pmail.org>
To:     Dmitry Osipenko <dmitry.osipenko@...labora.com>,
        David Airlie <airlied@...ux.ie>,
        Gerd Hoffmann <kraxel@...hat.com>,
        Gurchetan Singh <gurchetansingh@...omium.org>,
        Chia-I Wu <olvaffe@...il.com>, Daniel Vetter <daniel@...ll.ch>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        Emil Velikov <emil.l.velikov@...il.com>,
        Christian König <christian.koenig@....com>
Cc:     dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        Dmitry Osipenko <digetx@...il.com>,
        linux-tegra@...r.kernel.org, kernel@...labora.com,
        virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH v7 2/2] drm/gem: Don't map imported GEMs


On 6/30/22 22:22, Dmitry Osipenko wrote:
> Hello Thomas,
>
> On 6/30/22 23:15, Thomas Hellström (Intel) wrote:
>> Hi, Dmitry,
>>
>> On 6/30/22 22:04, Dmitry Osipenko wrote:
>>> Drivers that use drm_gem_mmap() and drm_gem_mmap_obj() helpers don't
>>> handle imported dma-bufs properly, which results in mapping of something
>>> else than the imported dma-buf. On NVIDIA Tegra we get a hard lockup when
>>> userspace writes to the memory mapping of a dma-buf that was imported
>>> into
>>> Tegra's DRM GEM.
>>>
>>> Majority of DRM drivers prohibit mapping of the imported GEM objects.
>>> Mapping of imported GEMs require special care from userspace since it
>>> should sync dma-buf because mapping coherency of the exporter device may
>>> not match the DRM device. Let's prohibit the mapping for all DRM drivers
>>> for consistency.
>>>
>>> Cc: stable@...r.kernel.org
>>> Suggested-by: Thomas Hellström <thomas.hellstrom@...ux.intel.com>
>>> Signed-off-by: Dmitry Osipenko <dmitry.osipenko@...labora.com>
>> This might break drivers whose obj->funcs->mmap() callback already
>> handles this case, and has userspace that does the right thing.
> The drm-shmem helper should be the only that maps imported GEMs
> properly, but drivers that use drm-shmem already prohibit to map
> imported GEMs. Okay, I'll try to re-check once again to be sure.

OK. If you aren't 100.1% sure, then please drop the Cc: stable tag and 
let the patch ride out at least an -rc series, because breaking a stable 
kernel is something we woudln't want to do.

Thanks,

Thomas



>
>> I think the disabling must be checked on a per-driver basis to avoid
>> that; in particular drivers that already call dma_buf_mmap() should be
>> able to continue doing this.
>>
>> Also the Fixes: review comment remains,
> This should've been broken forever, don't think that we have a fixes tag
> here. I looked thought all my previous patches and added the Fixes
> wherever was possible. If I really missed something, then please let me
> know.
>