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@xxxxxxxxx>
Cc: Thomas Zimmermann <tzimmermann@xxxxxxx>
Cc: Sumit Semwal <sumit.semwal@xxxxxxxxxx>
Cc: "Christian König" <christian.koenig@xxxxxxx>
Cc: linux-media@xxxxxxxxxxxxxxx
Cc: linaro-mm-sig@xxxxxxxxxxxxxxxx
---
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?
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.
*
