Am 01.12.20 um 11:27 schrieb Thomas Zimmermann:
Hi
Am 01.12.20 um 11:00 schrieb Daniel Vetter:
On Tue, Dec 1, 2020 at 10:40 AM Thomas Zimmermann
<tzimmermann@xxxxxxx> wrote:
Hi
Am 01.12.20 um 10:10 schrieb Daniel Vetter:
On Tue, Dec 1, 2020 at 9:32 AM Thomas Zimmermann
<tzimmermann@xxxxxxx> wrote:
Hi
Am 30.11.20 um 16:33 schrieb Christian König:
Am 30.11.20 um 16:30 schrieb Daniel Vetter:
On Mon, Nov 30, 2020 at 01:04:26PM +0100, Thomas Zimmermann wrote:
Mapping a GEM object's buffer into kernel address space
prevents the
buffer from being evicted from VRAM, which in turn may result in
out-of-memory errors. It's therefore required to only vmap GEM
BOs for
short periods of time; unless the GEM implementation provides
additional
guarantees.
Signed-off-by: Thomas Zimmermann <tzimmermann@xxxxxxx>
---
drivers/gpu/drm/drm_prime.c | 6 ++++++
include/drm/drm_gem.h | 16 ++++++++++++++++
2 files changed, 22 insertions(+)
diff --git a/drivers/gpu/drm/drm_prime.c
b/drivers/gpu/drm/drm_prime.c
index 7db55fce35d8..9c9ece9833e0 100644
--- a/drivers/gpu/drm/drm_prime.c
+++ b/drivers/gpu/drm/drm_prime.c
@@ -669,6 +669,12 @@ EXPORT_SYMBOL(drm_gem_unmap_dma_buf);
* callback. Calls into &drm_gem_object_funcs.vmap for device
specific handling.
* The kernel virtual address is returned in map.
*
+ * To prevent the GEM object from being relocated, callers
must hold
the GEM
+ * object's reservation lock from when calling this function
until
releasing the
+ * mapping. Holding onto a mapping and the associated reservation
lock for an
+ * unbound time may result in out-of-memory errors. Calls to
drm_gem_dmabuf_vmap()
+ * should therefore be accompanied by a call to
drm_gem_dmabuf_vunmap().
+ *
* Returns 0 on success or a negative errno code otherwise.
This is a dma-buf hook, which means just documenting the rules
you'd like
to have here isn't enough. We need to roll this out at the
dma-buf level,
and enforce it.
Enforce it = assert_lock_held
Roll out = review everyone. Because this goes through dma-buf
it'll come
back through shmem helpers (and other helpers and other
subsystems) back
to any driver using vmap for gpu buffers. This includes the media
subsystem, and the media subsystem definitely doesn't cope with
just
temporarily mapping buffers. So there we need to pin them, which
I think
means we'll need 2 version of dma_buf_vmap - one that's
temporary and
requires we hold dma_resv lock, the other requires that the
buffer is
pinned.
OR start to proper use the dma_buf_pin/dma_buf_unpin functions
which I
added to cover this use case as well.
While I generally agree, here are some thoughts:
I found all generic pin functions useless, because they don't
allow for
specifying where to pin. With fbdev emulation, this means that
console
buffers might never make it to VRAM for scanout. If anything, the
policy
should be that pin always pins in HW-accessible memory.
Pin has quite a bit of overhead (more locking, buffer movement),
so it
should be the second choice after regular vmap. To make both work
together, pin probably relies on holding the reservation lock
internally.
Therefore I think we still would want some additional helpers,
such as:
pin_unlocked(), which acquires the resv lock, calls regular
pin and
then drops the resv lock. Same for unpin_unlocked()
vmap_pinned(), which enforces that the buffer has been pinned
and
then calls regalar vmap. Same for vunmap_pinned()
A typical pattern with these functions would look like this.
drm_gem_object bo;
dma_buf_map map;
init() {
pin_unlocked(bo);
vmap_pinned(bo, map);
}
worker() {
begin_cpu_access()
// access bo via map
end_cpu_access()
}
fini() {
vunmap_pinned(bo, map);
unpin_unlocked(bo);
}
init()
while (...) {
worker()
}
fini()
Is that reasonable for media drivers?
So media drivers go through dma-buf, which means we always pin into
system memory. Which I guess for vram-only display drivers makes no
sense and should be rejected, but we still need somewhat consistent
rules.
The other thing is that if you do a dma_buf_attach without dynamic
mode, dma-buf will pin things for you already. So in many cases it
Do you have a pointer to code that illustrates this well?
could be that we don't need a separate pin (but since the pin is in
the exporter, not dma-buf layer, we can't check for that). I'm also
not seeing why existing users need to split up their dma_buf_vmap into
a pin + vmap, they don't need them separately.
It's two different operations, with pin having some possible overhead
and failure conditions. And during the last discussion, pin was say to
be for userspace-managed buffers. Kernel code should hold the
reservation lock.
For my POV, the current interfaces have no clear policy or semantics.
Looking through the different GEM implementations, each one seems to
have its own interpretation.
Yup, that's the problem really. In the past we've had vmap exclusively
for permanently pinned access, with no locking requirements. Now we're
trying to make this more dynamic, but in a somewhat ad-hoc fashion
(generic fbdev emulation charged ahead with making the fbdev
framebuffer evictable), and now it breaks at every seam. Adding more
ad-hoc semantics on top doesn't seem like a good idea.
That's why I think we should have 2 different interfaces:
- dma_buf_vmap, the one we currently have. Permanently pins the
buffer, mapping survives, no locking required.
- dma_buf_vmap_local, the new interface, the one that generic fbdev
should have used (but oh well mistakes happen), requires
dma_resv_lock, the mapping is only local to the caller
In patch 6 of this series, there's ast cursor code that acquires two
BO's reservation locks and vmaps them afterwards. That's probably how
you intend to use dma_buf_vmap_local.
However, I think it's more logically to have a vmap callback that only
does the actual vmap. This is all that exporters would have to implement.
And with that, build one helper that pins before vmap and one helper
that gets the resv lock.
I don't think that this is will work nor is it a good approach.
See the ast cursor handling for example. You need to acquire two BOs
here, not just one. And this can't be done cleanly with a single vmap call.
Regards,
Christian.
I know that it might require some work on exporting drivers. But with
your proposal, we probably need another dma-buf callback just for
vmap_local. (?) That seems even less appealing to me.
Best regards
Thomas
Trying to shovel both semantics into one interface, depending upon
which implementation we have backing the buffer, doesn't work indeed.
Also on the pin topic, I think neither interface should require
callers to explicitly pin anything. For existing users it should
happen automatically behind the scenes imo, that's what they're
expecting.
-Daniel
I think we could use what we've done for dynamic dma-buf attachment
(which also change locking rules) and just have new functions for the
new way (i.e. short term vmap protected by dma_resv lock. Maybe call
these dma_buf_vmap_local, in the spirit of the new kmap_local which
are currently under discussion. I think _local suffix is better, for
otherwise people might do something silly like
dma_resv_lock();
dma_buf_vmap_locked();
dma_resv_unlock();
/* actual access maybe even in some other thread */
dma_buf_resv_lock();
dma_buf_vunmap_unlocked();
dma_resv_unlock();
_local suffix is better at telling that the resulting pointer has very
limited use (essentially just local to the calling context, if you
don't change any locking or anything).
_local sounds good.
Best regards
Thomas
I think encouraging importers to call dma_buf_pin/unpin isn't a good
idea. Yes dynamic ones need it, but maybe we should check for that
somehow in the exporterd interface (atm only amdgpu is using it).
-Daniel
Best regards
Thomas
Cheers,
Christian.
That's what I meant with that this approach here is very
sprawling :-/
-Daniel
*/
int drm_gem_dmabuf_vmap(struct dma_buf *dma_buf, struct
dma_buf_map
*map)
diff --git a/include/drm/drm_gem.h b/include/drm/drm_gem.h
index 5e6daa1c982f..7c34cd5ec261 100644
--- a/include/drm/drm_gem.h
+++ b/include/drm/drm_gem.h
@@ -137,7 +137,21 @@ struct drm_gem_object_funcs {
* Returns a virtual address for the buffer. Used by the
* drm_gem_dmabuf_vmap() helper.
*
+ * Notes to implementors:
+ *
+ * - Implementations must expect pairs of @vmap and
@vunmap to be
+ * called frequently and should optimize for this case.
+ *
+ * - Implemenations may expect the caller to hold the GEM
object's
+ * reservation lock to protect against concurrent calls and
relocation
+ * of the GEM object.
+ *
+ * - Implementations may provide additional guarantees (e.g.,
working
+ * without holding the reservation lock).
+ *
* This callback is optional.
+ *
+ * See also drm_gem_dmabuf_vmap()
*/
int (*vmap)(struct drm_gem_object *obj, struct
dma_buf_map *map);
@@ -148,6 +162,8 @@ struct drm_gem_object_funcs {
* drm_gem_dmabuf_vunmap() helper.
*
* This callback is optional.
+ *
+ * See also @vmap.
*/
void (*vunmap)(struct drm_gem_object *obj, struct
dma_buf_map
*map);
--
2.29.2
_______________________________________________
dri-devel mailing list
dri-devel@xxxxxxxxxxxxxxxxxxxxx
https://lists.freedesktop.org/mailman/listinfo/dri-devel
--
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
_______________________________________________
dri-devel mailing list
dri-devel@xxxxxxxxxxxxxxxxxxxxx
https://lists.freedesktop.org/mailman/listinfo/dri-devel
