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.
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
