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Message-ID: <0335814b-dc11-4a54-9f24-bb4e9dbef35c@amd.com>
Date: Mon, 4 Jul 2022 14:33:19 +0200
From: Christian König <christian.koenig@....com>
To: Dmitry Osipenko <dmitry.osipenko@...labora.com>,
Thomas Hellström (Intel)
<thomas_os@...pmail.org>
Cc: intel-gfx@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
dri-devel@...ts.freedesktop.org,
virtualization@...ts.linux-foundation.org,
linaro-mm-sig@...ts.linaro.org, amd-gfx@...ts.freedesktop.org,
linux-tegra@...r.kernel.org, Dmitry Osipenko <digetx@...il.com>,
kernel@...labora.com, linux-media@...r.kernel.org,
David Airlie <airlied@...ux.ie>,
Gerd Hoffmann <kraxel@...hat.com>,
Gurchetan Singh <gurchetansingh@...omium.org>,
Daniel Vetter <daniel@...ll.ch>,
Daniel Almeida <daniel.almeida@...labora.com>,
Gert Wollny <gert.wollny@...labora.com>,
Gustavo Padovan <gustavo.padovan@...labora.com>,
Daniel Stone <daniel@...ishbar.org>,
Tomeu Vizoso <tomeu.vizoso@...labora.com>,
Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
Maxime Ripard <mripard@...nel.org>,
Thomas Zimmermann <tzimmermann@...e.de>,
Rob Herring <robh@...nel.org>,
Steven Price <steven.price@....com>,
Alyssa Rosenzweig <alyssa.rosenzweig@...labora.com>,
Rob Clark <robdclark@...il.com>,
Emil Velikov <"emil.l.veli kov"@gmail.com>,
Robin Murphy <robin.murphy@....com>,
Qiang Yu <yuq825@...il.com>,
Sumit Semwal <sumit.semwal@...aro.org>,
Christian König <christian.koenig@....com>,
"Pan, Xinhui" <Xinhui.Pan@....com>,
Thierry Reding <thierry.reding@...il.com>,
Tomasz Figa <tfiga@...omium.org>,
Mauro Carvalho Chehab <mchehab@...nel.org>,
Alex Deucher <alexander.deucher@....com>,
Jani Nikula <jani.nikula@...ux.intel.com>,
Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>,
Rodrigo Vivi <rodrigo.vivi@...el.com>,
Tvrtko Ursulin <tvrtko.ursulin@...ux.intel.com>
Subject: Re: [Linaro-mm-sig] Re: [PATCH v6 02/22] drm/gem: Move mapping of
imported dma-bufs to drm_gem_mmap_obj()
Am 30.06.22 um 01:06 schrieb Dmitry Osipenko:
> On 6/29/22 11:43, Thomas Hellström (Intel) wrote:
>> On 6/29/22 10:22, Dmitry Osipenko wrote:
>>> On 6/29/22 09:40, Thomas Hellström (Intel) wrote:
>>>> On 5/27/22 01:50, 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. For example, 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.
>>>>>
>>>>> To fix this bug, move mapping of imported dma-bufs to
>>>>> drm_gem_mmap_obj().
>>>>> Now mmaping of imported dma-bufs works properly for all DRM drivers.
>>>> Same comment about Fixes: as in patch 1,
>>>>> Cc: stable@...r.kernel.org
>>>>> Signed-off-by: Dmitry Osipenko <dmitry.osipenko@...labora.com>
>>>>> ---
>>>>> drivers/gpu/drm/drm_gem.c | 3 +++
>>>>> drivers/gpu/drm/drm_gem_shmem_helper.c | 9 ---------
>>>>> drivers/gpu/drm/tegra/gem.c | 4 ++++
>>>>> 3 files changed, 7 insertions(+), 9 deletions(-)
>>>>>
>>>>> diff --git a/drivers/gpu/drm/drm_gem.c b/drivers/gpu/drm/drm_gem.c
>>>>> index 86d670c71286..7c0b025508e4 100644
>>>>> --- a/drivers/gpu/drm/drm_gem.c
>>>>> +++ b/drivers/gpu/drm/drm_gem.c
>>>>> @@ -1038,6 +1038,9 @@ int drm_gem_mmap_obj(struct drm_gem_object *obj,
>>>>> unsigned long obj_size,
>>>>> if (obj_size < vma->vm_end - vma->vm_start)
>>>>> return -EINVAL;
>>>>> + if (obj->import_attach)
>>>>> + return dma_buf_mmap(obj->dma_buf, vma, 0);
>>>> If we start enabling mmaping of imported dma-bufs on a majority of
>>>> drivers in this way, how do we ensure that user-space is not blindly
>>>> using the object mmap without calling the needed DMA_BUF_IOCTL_SYNC
>>>> which is needed before and after cpu access of mmap'ed dma-bufs?
>>>>
>>>> I was under the impression (admittedly without looking) that the few
>>>> drivers that actually called into dma_buf_mmap() had some private
>>>> user-mode driver code in place that ensured this happened.
>>> Since it's a userspace who does the mapping, then it should be a
>>> responsibility of userspace to do all the necessary syncing.
>> Sure, but nothing prohibits user-space to ignore the syncing thinking
>> "It works anyway", testing those drivers where the syncing is a NOP. And
>> when a driver that finally needs syncing is tested it's too late to fix
>> all broken user-space.
>>
>>> I'm not
>>> sure whether anyone in userspace really needs to map imported dma-bufs
>>> in practice. Nevertheless, this use-case is broken and should be fixed
>>> by either allowing to do the mapping or prohibiting it.
>>>
>> Then I'd vote for prohibiting it, at least for now. And for the future
>> moving forward we could perhaps revisit the dma-buf need for syncing,
>> requiring those drivers that actually need it to implement emulated
>> coherent memory which can be done not too inefficiently (vmwgfx being
>> one example).
> Alright, I'll change it to prohibit the mapping. This indeed should be a
> better option.
Oh, yes please. But I would expect that some people start screaming.
Over time I've got tons of TTM patches because people illegally tried to
mmap() imported DMA-bufs in their driver.
Anyway this is probably the right thing to do and we can work on fixing
the fallout later on.
Regards,
Christian.
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