Re: [PATCH v4 01/13] dma-buf: Add vmap_local and vnumap_local operations

[Thomas Zimmermann <tzimmermann@suse.de>]

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Hi

Am 08.01.21 um 17:12 schrieb Ruhl, Michael J:
>> -----Original Message-----
>> From: dri-devel <dri-devel-bounces@lists.freedesktop.org> On Behalf Of
>> Thomas Zimmermann
>> Sent: Friday, January 8, 2021 4:43 AM
>> To: sumit.semwal@linaro.org; christian.koenig@amd.com;
>> airlied@redhat.com; daniel@ffwll.ch; maarten.lankhorst@linux.intel.com;
>> mripard@kernel.org; kraxel@redhat.com; hdegoede@redhat.com;
>> sean@poorly.run; eric@anholt.net; sam@ravnborg.org
>> Cc: Daniel Vetter <daniel.vetter@ffwll.ch>; dri-devel@lists.freedesktop.org;
>> virtualization@lists.linux-foundation.org; linaro-mm-sig@lists.linaro.org;
>> Thomas Zimmermann <tzimmermann@suse.de>; linux-
>> media@vger.kernel.org
>> Subject: [PATCH v4 01/13] dma-buf: Add vmap_local and vnumap_local
>> operations
>>
>> The existing dma-buf calls dma_buf_vmap() and dma_buf_vunmap() are
>> allowed to pin the buffer or acquire the buffer's reservation object
>> lock.
>>
>> This is a problem for callers that only require a short-term mapping
>> of the buffer without the pinning, or callers that have special locking
>> requirements. These may suffer from unnecessary overhead or interfere
>> with regular pin operations.
>>
>> The new interfaces dma_buf_vmap_local(), dma_buf_vunmapo_local(), and
>> their rsp callbacks in struct dma_buf_ops provide an alternative without
>> pinning or reservation locking. Callers are responsible for these
>> operations.
>>
>> v4:
>> 	* update documentation (Daniel)
>>
>> Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
>> Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
>> Suggested-by: Daniel Vetter <daniel.vetter@ffwll.ch>
>> ---
>> drivers/dma-buf/dma-buf.c | 81
>> +++++++++++++++++++++++++++++++++++++++
>> include/linux/dma-buf.h   | 34 ++++++++++++++++
>> 2 files changed, 115 insertions(+)
>>
>> diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
>> index b8465243eca2..01f9c74d97fa 100644
>> --- a/drivers/dma-buf/dma-buf.c
>> +++ b/drivers/dma-buf/dma-buf.c
>> @@ -1295,6 +1295,87 @@ void dma_buf_vunmap(struct dma_buf *dmabuf,
>> struct dma_buf_map *map)
>> }
>> EXPORT_SYMBOL_GPL(dma_buf_vunmap);
>>
>> +/**
>> + * dma_buf_vmap_local - Create virtual mapping for the buffer object into
>> kernel
>> + * address space.
>> + * @dmabuf:	[in]	buffer to vmap
>> + * @map:	[out]	returns the vmap pointer
>> + *
>> + * Unlike dma_buf_vmap() this is a short term mapping and will not pin
>> + * the buffer. The struct dma_resv for the @dmabuf must be locked until
>> + * dma_buf_vunmap_local() is called.
>> + *
>> + * Returns:
>> + * 0 on success, or a negative errno code otherwise.
>> + */
>> +int dma_buf_vmap_local(struct dma_buf *dmabuf, struct dma_buf_map
>> *map)
>> +{
>> +	struct dma_buf_map ptr;
>> +	int ret = 0;
>> +
>> +	dma_buf_map_clear(map);
>> +
>> +	if (WARN_ON(!dmabuf))
>> +		return -EINVAL;
>> +
>> +	dma_resv_assert_held(dmabuf->resv);
>> +
>> +	if (!dmabuf->ops->vmap_local)
>> +		return -EINVAL;
> 
> You are clearing the map, and then doing the above checks.
> 
> Is it ok to change the map info and then exit on error?

In vmap_local map argument returns the mapping's address. Callers are 
expected to check the return code. But I would expect a careless caller 
to not check, or check for map being cleared. Clearing it here first is 
the save thing to do.

Best regards
Thomas

> 
> Mike
> 
>> +	mutex_lock(&dmabuf->lock);
>> +	if (dmabuf->vmapping_counter) {
>> +		dmabuf->vmapping_counter++;
>> +		BUG_ON(dma_buf_map_is_null(&dmabuf->vmap_ptr));
>> +		*map = dmabuf->vmap_ptr;
>> +		goto out_unlock;
>> +	}
>> +
>> +	BUG_ON(dma_buf_map_is_set(&dmabuf->vmap_ptr));
>> +
>> +	ret = dmabuf->ops->vmap_local(dmabuf, &ptr);
>> +	if (WARN_ON_ONCE(ret))
>> +		goto out_unlock;
>> +
>> +	dmabuf->vmap_ptr = ptr;
>> +	dmabuf->vmapping_counter = 1;
>> +
>> +	*map = dmabuf->vmap_ptr;
>> +
>> +out_unlock:
>> +	mutex_unlock(&dmabuf->lock);
>> +	return ret;
>> +}
>> +EXPORT_SYMBOL_GPL(dma_buf_vmap_local);
>> +
>> +/**
>> + * dma_buf_vunmap_local - Unmap a vmap obtained by
>> dma_buf_vmap_local.
>> + * @dmabuf:	[in]	buffer to vunmap
>> + * @map:	[in]	vmap pointer to vunmap
>> + *
>> + * Release a mapping established with dma_buf_vmap_local().
>> + */
>> +void dma_buf_vunmap_local(struct dma_buf *dmabuf, struct
>> dma_buf_map *map)
>> +{
>> +	if (WARN_ON(!dmabuf))
>> +		return;
>> +
>> +	dma_resv_assert_held(dmabuf->resv);
>> +
>> +	BUG_ON(dma_buf_map_is_null(&dmabuf->vmap_ptr));
>> +	BUG_ON(dmabuf->vmapping_counter == 0);
>> +	BUG_ON(!dma_buf_map_is_equal(&dmabuf->vmap_ptr, map));
>> +
>> +	mutex_lock(&dmabuf->lock);
>> +	if (--dmabuf->vmapping_counter == 0) {
>> +		if (dmabuf->ops->vunmap_local)
>> +			dmabuf->ops->vunmap_local(dmabuf, map);
>> +		dma_buf_map_clear(&dmabuf->vmap_ptr);
>> +	}
>> +	mutex_unlock(&dmabuf->lock);
>> +}
>> +EXPORT_SYMBOL_GPL(dma_buf_vunmap_local);
>> +
>> #ifdef CONFIG_DEBUG_FS
>> static int dma_buf_debug_show(struct seq_file *s, void *unused)
>> {
>> diff --git a/include/linux/dma-buf.h b/include/linux/dma-buf.h
>> index 628681bf6c99..aeed754b5467 100644
>> --- a/include/linux/dma-buf.h
>> +++ b/include/linux/dma-buf.h
>> @@ -264,6 +264,38 @@ struct dma_buf_ops {
>>
>> 	int (*vmap)(struct dma_buf *dmabuf, struct dma_buf_map *map);
>> 	void (*vunmap)(struct dma_buf *dmabuf, struct dma_buf_map
>> *map);
>> +
>> +	/**
>> +	 * @vmap_local:
>> +	 *
>> +	 * Creates a virtual mapping for the buffer into kernel address space.
>> +	 *
>> +	 * This callback establishes short-term mappings for situations where
>> +	 * callers only use the buffer for a bounded amount of time; such as
>> +	 * updates to the framebuffer or reading back contained information.
>> +	 * In contrast to the regular @vmap callback, vmap_local does never
>> pin
>> +	 * the buffer to a specific domain or acquire the buffer's reservation
>> +	 * lock.
>> +	 *
>> +	 * This is called with the &dma_buf.resv object locked. Callers must
>> hold
>> +	 * the lock until after removing the mapping with @vunmap_local.
>> +	 *
>> +	 * This callback is optional.
>> +	 *
>> +	 * Returns:
>> +	 *
>> +	 * 0 on success or a negative error code on failure.
>> +	 */
>> +	int (*vmap_local)(struct dma_buf *dmabuf, struct dma_buf_map
>> *map);
>> +
>> +	/**
>> +	 * @vunmap_local:
>> +	 *
>> +	 * Removes a virtual mapping that was established by @vmap_local.
>> +	 *
>> +	 * This callback is optional.
>> +	 */
>> +	void (*vunmap_local)(struct dma_buf *dmabuf, struct dma_buf_map
>> *map);
>> };
>>
>> /**
>> @@ -501,4 +533,6 @@ int dma_buf_mmap(struct dma_buf *, struct
>> vm_area_struct *,
>> 		 unsigned long);
>> int dma_buf_vmap(struct dma_buf *dmabuf, struct dma_buf_map *map);
>> void dma_buf_vunmap(struct dma_buf *dmabuf, struct dma_buf_map
>> *map);
>> +int dma_buf_vmap_local(struct dma_buf *dmabuf, struct dma_buf_map
>> *map);
>> +void dma_buf_vunmap_local(struct dma_buf *dmabuf, struct
>> dma_buf_map *map);
>> #endif /* __DMA_BUF_H__ */
>> --
>> 2.29.2
>>
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> 

-- 
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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