Re: [PATCH v2 7/7] dma-buf: Write down some rules for vmap usage

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On Fri, Dec 04, 2020 at 09:47:08AM +0100, Christian König wrote:
> Am 04.12.20 um 09:32 schrieb Thomas Zimmermann:
> > Hi
> > 
> > Am 03.12.20 um 21:41 schrieb Daniel Vetter:
> > > On Thu, Dec 03, 2020 at 07:59:04PM +0100, Thomas Zimmermann wrote:
> > > > Hi
> > > > 
> > > > Am 03.12.20 um 16:26 schrieb Daniel Vetter:
> > > > > On Thu, Dec 03, 2020 at 03:02:59PM +0100, Thomas Zimmermann wrote:
> > > > > > Dma-buf's vmap and vunmap callbacks are undocumented and various
> > > > > > exporters currently have slightly different semantics for them. Add
> > > > > > documentation on how to implement and use these interfaces correctly.
> > > > > > 
> > > > > > v2:
> > > > > >     * document vmap semantics in struct dma_buf_ops
> > > > > >     * add TODO item for reviewing and maybe fixing dma-buf exporters
> > > > > > 
> > > > > > Signed-off-by: Thomas Zimmermann <tzimmermann@xxxxxxx>
> > > > > 
> > > > > I still don't think this works, we're breaking dma_buf_vmap
> > > > > for everyone
> > > > > else here.
> > > > 
> > > > I removed the text on the importer. These notes for callers in
> > > > the docs are
> > > > more or less a consequence of the exporter semantics.
> > > 
> > > Callers are importers, so I'm not seeing how that fixes anything.
> > > 
> > > > I thought we at least agreed on the exporter semantics in the
> > > > other thread,
> > > > didn't we?
> > > > 
> > > > What I'm trying to do is to write dome some rules for exporters,
> > > > even if not
> > > > all exporters follow them.
> > > 
> > > This is a standard interface, everyone needs to follow the same
> > > rules. And
> > > if they change, we need to make sure nothing breaks and we're not
> > > creating
> > > issues.
> > > 
> > > In the past the rule was the dma_buf_vmap was allowed to take the
> > > dma_resv_lock, and that the buffer should be pinned. Now some ttm
> > > drivers
> > > didn't ever bother with the pinning, and mostly got away with that
> > > because
> > > drm_prime helpers do the pinning by default at attach time, and most
> > > users
> > > do call dma_buf_attach.
> > > 
> > > But if you look through dma-buf docs nothing ever said you have to do a
> > > dummy attachment before you call dma_buf_vmap, that's just slightly
> > > crappy
> > > implementations that didn't blow up yet.
> > 
> > I had a patch for adding pin to radeon's implementation of vmap. [1]
> > Christian told me to not do this; instead just get the lock in the fbdev
> > code. His advise almost seems the opposite of what you're telling me
> > here.
> 
> I think what Daniel suggests here is that we need to smoothly transition the
> code from making assumptions to having a straight interface where importers
> explicitly say when stuff is locked and when stuff is pinned.
> 
> I've started this with the attach interface by adding a new dynamic approach
> to that, but you probably need to carry on here with that for vmap as well.
> 
> When that is done we can migrate every exporter over to the new dynamic
> approach.
> 
> > 
> > For the GEM VRAM helpers, that implicit pin in vmap gave me headaches.
> > Because scanouts can only be done from VRAM, which is badly suited for
> > exporting. So I ended up with an implicit pin that pins the buffer to
> > whatever domain it currently is. I got away with it because GEM VRAM BOs
> > are not sharable among devices; fbdev is the only user of that
> > functionality and only pins for short periods of time.
> > 
> > I suspect that fixing TTM-based drivers by adding an implicit pin would
> > result in a similar situation. Whatever domain it ends up pinning, some
> > functionality might not be compatible with that.
> 
> Correct, exactly that's the problem.
> 
> > 
> > > 
> > > > Given the circumstances, we should leave out this patch from the
> > > > patchset.
> > > 
> > > So the defacto rules are already a big mess, but that's not a good
> > > excuse
> > > to make it worse.
> > > 
> > > What I had in mind is that we split dma_buf_vmap up into two variants:
> > > 
> > > - The current one, which should guarantee that the buffer is pinned.
> > >    Because that's what all current callers wanted, before the fbdev code
> > >    started allowing non-pinned buffers.
> > 
> > Can we add an explicit pin operation to dma_buf_vmap() to enforce the
> > semantics?
> 
> At least I would be fine with that. For now amdgpu is the only exporter who
> implements the explicit pin/unpin semantics anyway.

Yup, I think that makes sense (if it works). Maybe we could use something
like:

a) dma_buf pin exists, driver is dynamic. This means dma_buf_vmap needs to
first pin, then call ->vmap. dma_buf_vmap_local otoh can directly call
->vmap since the exporter relies on either a pin or dma_resv_lock.

b) dma_buf pin not implement, driver is a legacy pile. dma_buf_vmap will
pin (somewhere at least, or rely on some implicit pin), dma_buf_vmap_local
doesn't work and should fail.

I think for less transition work fbdev helpers could first try
dma_resv_lock + dma_buf_vmap_local, if that fails, drop the dma_resv_lock
and do the pinning dma_buf_vmap. That way we don't have to convert shmem
helpers over to dma_resv locking, which should help.

And ttm drivers would do the new clean interface, so at least everyone
using dma_resv today is all fine. Intel's conversion to dma_resv lock is
in-flight, but that needs a conversion to the dynamic interface anyway,
the current code splats. And dynamic brings means explicit pin/unpin
callbacks, so should be good too.
-Daniel

> 
> Regards,
> Christian.
> 
> > 
> > Best regards
> > Thomas
> > 
> > [1] https://patchwork.freedesktop.org/patch/400054/?series=83765&rev=1
> > 
> > > 
> > > - The new one, which allows vmapping with just dma_resv locked, and
> > > should
> > >    have some caching in exporters.
> > > 
> > > Breaking code and then adding todos about that is kinda not so cool
> > > approach here imo.
> > > 
> > > Also I guess ttm_bo_vmap should have a check that either the buffer is
> > > pinned, or dma_resv_lock is held.
> > > 
> > > Cheers, Daniel
> > > 
> > > 
> > > 
> > > > 
> > > > Best regards
> > > > Thomas
> > > > 
> > > > > 
> > > > > > ---
> > > > > >    Documentation/gpu/todo.rst | 15 +++++++++++++
> > > > > >    include/drm/drm_gem.h      |  4 ++++
> > > > > >    include/linux/dma-buf.h    | 45
> > > > > > ++++++++++++++++++++++++++++++++++++++
> > > > > >    3 files changed, 64 insertions(+)
> > > > > > 
> > > > > > diff --git a/Documentation/gpu/todo.rst b/Documentation/gpu/todo.rst
> > > > > > index 009d8e6c7e3c..32bb797a84fc 100644
> > > > > > --- a/Documentation/gpu/todo.rst
> > > > > > +++ b/Documentation/gpu/todo.rst
> > > > > > @@ -505,6 +505,21 @@ Contact: Thomas Zimmermann
> > > > > > <tzimmermann@xxxxxxx>, Christian König, Daniel Vette
> > > > > >    Level: Intermediate
> > > > > > +Enforce rules for dma-buf vmap and pin ops
> > > > > > +------------------------------------------
> > > > > > +
> > > > > > +Exporter implementations of vmap and pin in struct
> > > > > > dma_buf_ops (and consequently
> > > > > > +struct drm_gem_object_funcs) use a variety of locking
> > > > > > semantics. Some rely on
> > > > > > +the caller holding the dma-buf's reservation lock, some
> > > > > > do their own locking,
> > > > > > +some don't require any locking. VRAM helpers even used
> > > > > > to pin as part of vmap.
> > > > > > +
> > > > > > +We need to review each exporter and enforce the documented rules.
> > > > > > +
> > > > > > +Contact: Christian König, Daniel Vetter, Thomas
> > > > > > Zimmermann <tzimmermann@xxxxxxx>
> > > > > > +
> > > > > > +Level: Advanced
> > > > > > +
> > > > > > +
> > > > > >    Core refactorings
> > > > > >    =================
> > > > > > diff --git a/include/drm/drm_gem.h b/include/drm/drm_gem.h
> > > > > > index 5e6daa1c982f..1864c6a721b1 100644
> > > > > > --- a/include/drm/drm_gem.h
> > > > > > +++ b/include/drm/drm_gem.h
> > > > > > @@ -138,6 +138,8 @@ struct drm_gem_object_funcs {
> > > > > >         * drm_gem_dmabuf_vmap() helper.
> > > > > >         *
> > > > > >         * This callback is optional.
> > > > > > +     *
> > > > > > +     * See also struct dma_buf_ops.vmap
> > > > > >         */
> > > > > >        int (*vmap)(struct drm_gem_object *obj, struct
> > > > > > dma_buf_map *map);
> > > > > > @@ -148,6 +150,8 @@ struct drm_gem_object_funcs {
> > > > > >         * drm_gem_dmabuf_vunmap() helper.
> > > > > >         *
> > > > > >         * This callback is optional.
> > > > > > +     *
> > > > > > +     * See also struct dma_buf_ops.vunmap
> > > > > >         */
> > > > > >        void (*vunmap)(struct drm_gem_object *obj, struct
> > > > > > dma_buf_map *map);
> > > > > > diff --git a/include/linux/dma-buf.h b/include/linux/dma-buf.h
> > > > > > index cf72699cb2bc..dc81fdc01dda 100644
> > > > > > --- a/include/linux/dma-buf.h
> > > > > > +++ b/include/linux/dma-buf.h
> > > > > > @@ -267,7 +267,52 @@ struct dma_buf_ops {
> > > > > >         */
> > > > > >        int (*mmap)(struct dma_buf *, struct vm_area_struct *vma);
> > > > > > +    /**
> > > > > > +     * @vmap:
> > > > > 
> > > > > There's already a @vmap and @vunamp kerneldoc at the top comment, that
> > > > > needs to be removed.
> > > > > -Daniel
> > > > > 
> > > > > > +     *
> > > > > > +     * Returns a virtual address for the buffer.
> > > > > > +     *
> > > > > > +     * Notes to callers:
> > > > > > +     *
> > > > > > +     * - Callers must hold the struct dma_buf.resv lock
> > > > > > before calling
> > > > > > +     *   this interface.
> > > > > > +     *
> > > > > > +     * - Callers must provide means to prevent the
> > > > > > mappings from going
> > > > > > +     *   stale, such as holding the reservation lock or providing a
> > > > > > +     *   move-notify callback to the exporter.
> > > > > > +     *
> > > > > > +     * Notes to implementors:
> > > > > > +     *
> > > > > > +     * - Implementations must expect pairs of @vmap and
> > > > > > @vunmap to be
> > > > > > +     *   called frequently and should optimize for this case.
> > > > > > +     *
> > > > > > +     * - Implementations should avoid additional operations, such as
> > > > > > +     *   pinning.
> > > > > > +     *
> > > > > > +     * - Implementations may expect the caller to hold the dma-buf's
> > > > > > +     *   reservation lock to protect against concurrent calls and
> > > > > > +     *   relocation.
> > > > > > +     *
> > > > > > +     * - Implementations may provide additional
> > > > > > guarantees, such as working
> > > > > > +     *   without holding the reservation lock.
> > > > > > +     *
> > > > > > +     * This callback is optional.
> > > > > > +     *
> > > > > > +     * Returns:
> > > > > > +     *
> > > > > > +     * 0 on success or a negative error code on failure.
> > > > > > +     */
> > > > > >        int (*vmap)(struct dma_buf *dmabuf, struct dma_buf_map *map);
> > > > > > +
> > > > > > +    /**
> > > > > > +     * @vunmap:
> > > > > > +     *
> > > > > > +     * Releases the address previously returned by @vmap.
> > > > > > +     *
> > > > > > +     * This callback is optional.
> > > > > > +     *
> > > > > > +     * See also @vmap()
> > > > > > +     */
> > > > > >        void (*vunmap)(struct dma_buf *dmabuf, struct
> > > > > > dma_buf_map *map);
> > > > > >    };
> > > > > > -- 
> > > > > > 2.29.2
> > > > > > 
> > > > > 
> > > > 
> > > > -- 
> > > > Thomas Zimmermann
> > > > Graphics Driver Developer
> > > > SUSE Software Solutions Germany GmbH
> > > > Maxfeldstr. 5, 90409 Nürnberg, Germany
> > > > (HRB 36809, AG Nürnberg)
> > > > Geschäftsführer: Felix Imendörffer
> > > > 
> > > 
> > > 
> > > 
> > > 
> > 
> 

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
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