On Wed, Dec 9, 2020 at 10:32 AM Thomas Zimmermann tzimmermann@suse.de wrote:
Hi
Am 09.12.20 um 01:13 schrieb Daniel Vetter:
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@suse.de > > 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 I read in the dma-buf documentation that pin is supposed to put the BO in a domain that is suitable for scanout. Now I don't really trust this to work. Amdgpu, radeon and nouveau put it into the GTT region. Qxl appears to put it wherever it is.
Uh that sounds wrong, or at least not the full complexity. ->pin's main use right now is to freeze the dma-buf into system memory when there's a non-dynamic attachement for a dynamic exporter. There's also a dma_buf_pin call in amdgpu, and I think that only works for scanout on integrated gpu. Pinning to vram would break sharing for all existing dma-buf users.
Christian can you perhaps explain when amdgpu uses dma_buf_pin()?
My take is the ->pin callback should clarify that it should pin into system memory, for legacy (non-dynamic) dma-buf users. And dma_buf_pin() should gain some kerneldoc which then states that "This should only be used in limited use cases like scanout and not for temporary pin operations." I think the problem with this kerneldoc is simply that it tries to document the exporter and importer side of the contract in one place and makes a mess of it, plus leaves the actual importer side function undocumented. I think the kerneldoc also predates the final patch version, and we didn't update it fully.
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.
I have meanwhile made a patchset that updates helpers for cma, shmem and vram with vmap_local; and converts fbdev emulation as well. It needs a bit more testing before being posted.
Nice, even better! -Daniel
Best regards Thomas
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@suse.de, 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@suse.de >> + >> +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
-- Thomas Zimmermann Graphics Driver Developer SUSE Software Solutions Germany GmbH Maxfeldstr. 5, 90409 Nürnberg, Germany (HRB 36809, AG Nürnberg) Geschäftsführer: Felix Imendörffer
Am 09.12.20 um 11:16 schrieb Daniel Vetter:
[SNIP]
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 I read in the dma-buf documentation that pin is supposed to put the BO in a domain that is suitable for scanout. Now I don't really trust this to work. Amdgpu, radeon and nouveau put it into the GTT region. Qxl appears to put it wherever it is.
Uh that sounds wrong, or at least not the full complexity. ->pin's main use right now is to freeze the dma-buf into system memory when there's a non-dynamic attachement for a dynamic exporter. There's also a dma_buf_pin call in amdgpu, and I think that only works for scanout on integrated gpu. Pinning to vram would break sharing for all existing dma-buf users.
Christian can you perhaps explain when amdgpu uses dma_buf_pin()?
You guys are writing mails faster than I can answer :)
dma_buf_pin() should be used when the buffer can't move any more and it is preferred to bin in a location which is accessible by all devices, that usually means system memory.
My take is the ->pin callback should clarify that it should pin into system memory, for legacy (non-dynamic) dma-buf users.
I though I've wrote that on the documentation for the pin callback, but looks like I've forgotten to do this.
And dma_buf_pin() should gain some kerneldoc which then states that "This should only be used in limited use cases like scanout and not for temporary pin operations." I think the problem with this kerneldoc is simply that it tries to document the exporter and importer side of the contract in one place and makes a mess of it, plus leaves the actual importer side function undocumented. I think the kerneldoc also predates the final patch version, and we didn't update it fully.
That's a really good point and I noticed this mess before as well.
How about this: We document the exporter side on the dma-buf function table (because the exporter should implement those), and the importer side on the dma-buf functions which then uses the exporter functions.
Regards, Christian.
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.
I have meanwhile made a patchset that updates helpers for cma, shmem and vram with vmap_local; and converts fbdev emulation as well. It needs a bit more testing before being posted.
Nice, even better! -Daniel
Best regards Thomas
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://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Fpatchwork....
- 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@suse.de, 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@suse.de >>> + >>> +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 >
-- Thomas Zimmermann Graphics Driver Developer SUSE Software Solutions Germany GmbH Maxfeldstr. 5, 90409 Nürnberg, Germany (HRB 36809, AG Nürnberg) Geschäftsführer: Felix Imendörffer
linaro-mm-sig@lists.linaro.org
-
Christian König -
Daniel Vetter