Re: [PATCH v6 02/22] drm/gem: Move mapping of imported dma-bufs to drm_gem_mmap_obj()

[Dmitry Osipenko <dmitry.osipenko@collabora.com>]

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