What am I missing?
The assumption is that when you want to create a vmap of a DMA-buf
buffer the buffer needs to be pinned somehow.
E.g. without dynamic dma-buf handling you would need to have an active
attachment. With dynamic handling the requirements could be lowered to
using the pin()/unpin() callbacks.
You also can't lock/unlock inside your vmap callback because you don't
have any guarantee that the pointer stays valid as soon as your drop
your lock.
BTW: What is vmap() still used for?
Regards,
Christian.
Am 20.11.19 um 12:47 schrieb Daniel Vetter:
Hi all,
I've been looking at dma_buf_v(un)map, with a goal to standardize
locking for at least dynamic dma-buf exporters/importers, most likely
by requiring dma_resv_lock. And I got questions around how this is
supposed to work, since a big chunk of drivers seem to entirely lack
locking around ttm_bo_kmap. Two big ones:
- ttm_bo_kmap looks at bo->mem to figure out what/where to kmap to get
at that buffer. bo->mem is supposed to be protected with
dma_resv_lock, but at least amgpu/nouveau/radeon/qxl don't grab that
in their prime vmap function.
- between the vmap and vunmap something needs to make sure the backing
storage doesn't move around. I didn't find that either anywhere,
ttm_bo_kmap simply seems to set up the mapping, leaving locking and
refcounting to callers.
- vram helpers have at least locking, but I'm still missing the
refcounting. vmwgfx doesn't bother with vmap.
What am I missing?
Thanks, Daniel
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