locking&resource refcounting for ttm_bo_kmap/dma_buf_vmap

[Daniel Vetter]

On Wed, Nov 20, 2019 at 1:02 PM Christian König
<christian.koenig at amd.com> wrote:
>
> > 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.

Yeah right now everyone seems to have an attachment, and that's how we
get away with all this. But the interface isn't supposed to work like
that, dma_buf_vmap/unmap is supposed to be a stand-alone thing (you
can call it directly on the struct dma_buf, no need for an
attachment). Also I don't think non-dynamic drivers should ever call
pin/unpin, not their job, holding onto a mapping should be enough to
get things pinned.

> 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.

Well that's why I asked where the pin/unpin pair is. If you lock&pin,
then you do know that the pointer will stay around. But looks like the
original patch from Dave for ttm based drivers played fast&loose here
with what should be done.

> BTW: What is vmap() still used for?

udl, bunch of other things (e.g. bunch of tiny drivers). Not much, but
not stuff we can drop.
-Daniel

>
> 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
>


-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch