Re: [PATCH 1/4] dma-buf: Remove kmap kerneldoc vestiges

["Christian König" <christian.koenig@amd.com>]

Am 14.12.20 um 17:01 schrieb Daniel Vetter:
> On Mon, Dec 14, 2020 at 11:33:10AM +0100, Christian König wrote:
>> Am 11.12.20 um 16:58 schrieb Daniel Vetter:
>>> Also try to clarify a bit when dma_buf_begin/end_cpu_access should
>>> be called.
>>>
>>> Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
>>> Cc: Thomas Zimmermann <tzimmermann@suse.de>
>>> Cc: Sumit Semwal <sumit.semwal@linaro.org>
>>> Cc: "Christian König" <christian.koenig@amd.com>
>>> Cc: linux-media@vger.kernel.org
>>> Cc: linaro-mm-sig@lists.linaro.org
>>> ---
>>>    drivers/dma-buf/dma-buf.c | 20 ++++++++++++++------
>>>    include/linux/dma-buf.h   | 25 +++++++++----------------
>>>    2 files changed, 23 insertions(+), 22 deletions(-)
>>>
>>> diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
>>> index e63684d4cd90..a12fdffa130f 100644
>>> --- a/drivers/dma-buf/dma-buf.c
>>> +++ b/drivers/dma-buf/dma-buf.c
>>> @@ -1001,15 +1001,15 @@ EXPORT_SYMBOL_GPL(dma_buf_move_notify);
>>>     *   vmalloc space might be limited and result in vmap calls failing.
>>>     *
>>>     *   Interfaces::
>>> + *
>>>     *      void \*dma_buf_vmap(struct dma_buf \*dmabuf)
>>>     *      void dma_buf_vunmap(struct dma_buf \*dmabuf, void \*vaddr)
>>>     *
>>>     *   The vmap call can fail if there is no vmap support in the exporter, or if
>>> - *   it runs out of vmalloc space. Fallback to kmap should be implemented. Note
>>> - *   that the dma-buf layer keeps a reference count for all vmap access and
>>> - *   calls down into the exporter's vmap function only when no vmapping exists,
>>> - *   and only unmaps it once. Protection against concurrent vmap/vunmap calls is
>>> - *   provided by taking the dma_buf->lock mutex.
>>> + *   it runs out of vmalloc space. Note that the dma-buf layer keeps a reference
>>> + *   count for all vmap access and calls down into the exporter's vmap function
>>> + *   only when no vmapping exists, and only unmaps it once. Protection against
>>> + *   concurrent vmap/vunmap calls is provided by taking the &dma_buf.lock mutex.
>> Who is talking the lock? The caller of the dma_buf_vmap/vunmap() functions,
>> the functions itself or the callback inside the exporter?
> That's the part I didn't change at all here, just re-laid out the line
> breaking. I only removed the outdated kmap section here.

I just wanted to point out that this still isn't described here very very.


> Should I do another patch and remove this one sentence here (it's kinda
> pointless and generally we don't muse about implementation details that
> callers don't care about)?

Na, works like this for me.

> I did try and do a cursory review of the dma-buf docs, but this is kinda
> not meant as an all-out revamp. Just a few things I've noticed while
> reviewing Thomas' vmap_local stuff.


Fell free to add an Acked-by: Christian König <christian.koenig@amd.com> 
to the series.

Christian.

> -Daniel
>
>> Christian.
>>
>>>     *
>>>     * - For full compatibility on the importer side with existing userspace
>>>     *   interfaces, which might already support mmap'ing buffers. This is needed in
>>> @@ -1098,6 +1098,11 @@ static int __dma_buf_begin_cpu_access(struct dma_buf *dmabuf,
>>>     * dma_buf_end_cpu_access(). Only when cpu access is braketed by both calls is
>>>     * it guaranteed to be coherent with other DMA access.
>>>     *
>>> + * This function will also wait for any DMA transactions tracked through
>>> + * implicit synchronization in &dma_buf.resv. For DMA transactions with explicit
>>> + * synchronization this function will only ensure cache coherency, callers must
>>> + * ensure synchronization with such DMA transactions on their own.
>>> + *
>>>     * Can return negative error values, returns 0 on success.
>>>     */
>>>    int dma_buf_begin_cpu_access(struct dma_buf *dmabuf,
>>> @@ -1199,7 +1204,10 @@ EXPORT_SYMBOL_GPL(dma_buf_mmap);
>>>     * This call may fail due to lack of virtual mapping address space.
>>>     * These calls are optional in drivers. The intended use for them
>>>     * is for mapping objects linear in kernel space for high use objects.
>>> - * Please attempt to use kmap/kunmap before thinking about these interfaces.
>>> + *
>>> + * To ensure coherency users must call dma_buf_begin_cpu_access() and
>>> + * dma_buf_end_cpu_access() around any cpu access performed through this
>>> + * mapping.
>>>     *
>>>     * Returns 0 on success, or a negative errno code otherwise.
>>>     */
>>> diff --git a/include/linux/dma-buf.h b/include/linux/dma-buf.h
>>> index cf72699cb2bc..7eca37c8b10c 100644
>>> --- a/include/linux/dma-buf.h
>>> +++ b/include/linux/dma-buf.h
>>> @@ -183,24 +183,19 @@ struct dma_buf_ops {
>>>    	 * @begin_cpu_access:
>>>    	 *
>>>    	 * This is called from dma_buf_begin_cpu_access() and allows the
>>> -	 * exporter to ensure that the memory is actually available for cpu
>>> -	 * access - the exporter might need to allocate or swap-in and pin the
>>> -	 * backing storage. The exporter also needs to ensure that cpu access is
>>> -	 * coherent for the access direction. The direction can be used by the
>>> -	 * exporter to optimize the cache flushing, i.e. access with a different
>>> +	 * exporter to ensure that the memory is actually coherent for cpu
>>> +	 * access. The exporter also needs to ensure that cpu access is coherent
>>> +	 * for the access direction. The direction can be used by the exporter
>>> +	 * to optimize the cache flushing, i.e. access with a different
>>>    	 * direction (read instead of write) might return stale or even bogus
>>>    	 * data (e.g. when the exporter needs to copy the data to temporary
>>>    	 * storage).
>>>    	 *
>>> -	 * This callback is optional.
>>> +	 * Note that this is both called through the DMA_BUF_IOCTL_SYNC IOCTL
>>> +	 * command for userspace mappings established through @mmap, and also
>>> +	 * for kernel mappings established with @vmap.
>>>    	 *
>>> -	 * FIXME: This is both called through the DMA_BUF_IOCTL_SYNC command
>>> -	 * from userspace (where storage shouldn't be pinned to avoid handing
>>> -	 * de-factor mlock rights to userspace) and for the kernel-internal
>>> -	 * users of the various kmap interfaces, where the backing storage must
>>> -	 * be pinned to guarantee that the atomic kmap calls can succeed. Since
>>> -	 * there's no in-kernel users of the kmap interfaces yet this isn't a
>>> -	 * real problem.
>>> +	 * This callback is optional.
>>>    	 *
>>>    	 * Returns:
>>>    	 *
>>> @@ -216,9 +211,7 @@ struct dma_buf_ops {
>>>    	 *
>>>    	 * This is called from dma_buf_end_cpu_access() when the importer is
>>>    	 * done accessing the CPU. The exporter can use this to flush caches and
>>> -	 * unpin any resources pinned in @begin_cpu_access.
>>> -	 * The result of any dma_buf kmap calls after end_cpu_access is
>>> -	 * undefined.
>>> +	 * undo anything else done in @begin_cpu_access.
>>>    	 *
>>>    	 * This callback is optional.
>>>    	 *