From: Thomas Zimmermann <tzimmermann@suse.de>
To: Daniel Vetter <daniel@ffwll.ch>
Cc: hdegoede@redhat.com, christian.koenig@amd.com,
dri-devel@lists.freedesktop.org, airlied@redhat.com
Subject: Re: [PATCH 1/8] drm/gem: Write down some rules for vmap usage
Date: Tue, 1 Dec 2020 09:15:10 +0100 [thread overview]
Message-ID: <b940ba0c-8c8e-7f8f-8545-9ff6f10959e2@suse.de> (raw)
In-Reply-To: <20201130153048.GS401619@phenom.ffwll.local>
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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@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.
>
The documentation for GEM vmap callbacks point here, so it was a good
point to start.
I know about the dependencies on dmabuf. But fixing everything now is
unrealistic. My hope for this patch is that we can find the necessary
rules and document them.
> Enforce it = assert_lock_held
That's probably the final step of many.
Best regards
Thomas
>
> 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.
>
> 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
>>
>
--
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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next prev parent reply other threads:[~2020-12-01 8:15 UTC|newest]
Thread overview: 31+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-11-30 12:04 [PATCH 0/8] drm/vram-helper: Lock GEM BOs while they are mapped Thomas Zimmermann
2020-11-30 12:04 ` [PATCH 1/8] drm/gem: Write down some rules for vmap usage Thomas Zimmermann
2020-11-30 15:30 ` Daniel Vetter
2020-11-30 15:33 ` Christian König
2020-12-01 8:32 ` Thomas Zimmermann
2020-12-01 9:10 ` Daniel Vetter
2020-12-01 9:40 ` Thomas Zimmermann
2020-12-01 10:00 ` Daniel Vetter
2020-12-01 10:27 ` Thomas Zimmermann
2020-12-01 10:34 ` Christian König
2020-12-01 11:30 ` Thomas Zimmermann
2020-12-01 12:14 ` Christian König
2020-12-01 12:33 ` Thomas Zimmermann
2020-12-01 12:38 ` Christian König
2020-12-01 12:51 ` Thomas Zimmermann
2020-12-01 12:53 ` Thomas Zimmermann
2020-12-01 13:05 ` Christian König
2020-12-01 16:54 ` Daniel Vetter
2020-12-01 12:05 ` Thomas Zimmermann
2020-12-01 9:13 ` Christian König
2020-12-01 9:30 ` Thomas Zimmermann
2020-12-01 8:15 ` Thomas Zimmermann [this message]
2020-11-30 12:04 ` [PATCH 2/8] drm/ast: Only map cursor BOs during updates Thomas Zimmermann
2020-11-30 12:04 ` [PATCH 3/8] drm/vram-helper: Provide drm_gem_vram_vmap_unlocked() Thomas Zimmermann
2020-11-30 12:04 ` [PATCH 4/8] drm/ast: Use drm_gem_vram_vmap_unlocked() in ast_cursor_show() Thomas Zimmermann
2020-11-30 12:04 ` [PATCH 5/8] drm/vboxvideo: Use drm_gem_vram_vmap_unlocked() in cursor update Thomas Zimmermann
2020-11-30 12:04 ` [PATCH 6/8] drm/vram-helper: Remove pinning and locking from drm_gem_vram_vmap() Thomas Zimmermann
2020-11-30 12:04 ` [PATCH 7/8] drm/vram-helper: Remove vmap reference counting Thomas Zimmermann
2020-11-30 12:04 ` [PATCH 8/8] drm/vram-helper: Simplify vmap implementation Thomas Zimmermann
2020-11-30 12:27 ` [PATCH 0/8] drm/vram-helper: Lock GEM BOs while they are mapped Christian König
2020-11-30 12:45 ` Thomas Zimmermann
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