Messages in this thread | | | Date | Wed, 29 Jun 2022 10:43:45 +0200 | Subject | Re: [PATCH v6 02/22] drm/gem: Move mapping of imported dma-bufs to drm_gem_mmap_obj() | From | Thomas Hellström (Intel) <> |
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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).
/Thomas
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