[PATCH 1/8] drm/gem: Write down some rules for vmap usage
Thomas Zimmermann
tzimmermann at suse.de
Tue Dec 1 10:27:46 UTC 2020
Hi
Am 01.12.20 um 11:00 schrieb Daniel Vetter:
> On Tue, Dec 1, 2020 at 10:40 AM Thomas Zimmermann <tzimmermann at suse.de> wrote:
>>
>> Hi
>>
>> Am 01.12.20 um 10:10 schrieb Daniel Vetter:
>>> On Tue, Dec 1, 2020 at 9:32 AM Thomas Zimmermann <tzimmermann at suse.de> 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 at suse.de>
>>>>>>> ---
>>>>>>> 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 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 at lists.freedesktop.org
>>>>> 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
>>
>
>
--
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Maxfeldstr. 5, 90409 Nürnberg, Germany
(HRB 36809, AG Nürnberg)
Geschäftsführer: Felix Imendörffer
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