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Date:   Thu, 30 Jun 2022 02:06:01 +0300
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Subject: Re: [PATCH v6 02/22] drm/gem: Move mapping of imported dma-bufs to
 drm_gem_mmap_obj()
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To:     =?UTF-8?Q?Thomas_Hellstr=c3=b6m_=28Intel=29?= 
        <thomas_os@shipmail.org>
Cc:     intel-gfx@lists.freedesktop.org, linux-kernel@vger.kernel.org,
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        linux-tegra@vger.kernel.org, Dmitry Osipenko <digetx@gmail.com>,
        kernel@collabora.com, linux-media@vger.kernel.org,
        David Airlie <airlied@linux.ie>,
        Gerd Hoffmann <kraxel@redhat.com>,
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        =?UTF-8?Q?Christian_K=c3=b6nig?= <christian.koenig@amd.com>,
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References: <20220526235040.678984-1-dmitry.osipenko@collabora.com>
 <20220526235040.678984-3-dmitry.osipenko@collabora.com>
 <b8271f0c-d6a3-4194-1959-e112859756a3@shipmail.org>
 <c0273ac2-c87c-2612-03d4-dc52510b22f7@collabora.com>
 <b4086751-9bff-ea5e-93fc-ce2c513b129b@shipmail.org>
From:   Dmitry Osipenko <dmitry.osipenko@collabora.com>
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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@vger.kernel.org
>>>> Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.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.

-- 
Best regards,
Dmitry