Re: [PATCH v3 04/14] dmaengine: Add metadata_ops for dma_async_tx_descriptor

From: Tero Kristo
Date: Wed Oct 09 2019 - 03:37:25 EST

On 01/10/2019 09:16, Peter Ujfalusi wrote:
The metadata is best described as side band data or parameters traveling
alongside the data DMAd by the DMA engine. It is data
which is understood by the peripheral and the peripheral driver only, the
DMA engine see it only as data block and it is not interpreting it in any

The metadata can be different per descriptor as it is a parameter for the
data being transferred.

If the DMA supports per descriptor metadata it can implement the attach,
get_ptr/set_len callbacks.

Client drivers must only use either attach or get_ptr/set_len to avoid

Client driver can check if a given metadata mode is supported by the
channel during probe time with
dmaengine_is_metadata_mode_supported(chan, DESC_METADATA_CLIENT);
dmaengine_is_metadata_mode_supported(chan, DESC_METADATA_ENGINE);

and based on this information can use either mode.

Wrappers are also added for the metadata_ops.

To be used in DESC_METADATA_CLIENT mode:

To be used in DESC_METADATA_ENGINE mode:

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@xxxxxx>

Again couple of typos below, but other than that:

Reviewed-by: Tero Kristo <t-kristo@xxxxxx>

drivers/dma/dmaengine.c | 73 ++++++++++++++++++++++++++
include/linux/dmaengine.h | 108 ++++++++++++++++++++++++++++++++++++++


+ * @DESC_METADATA_ENGINE - the metadata buffer is allocated/managed by the DMA
+ * driver. The client driver can ask for the pointer, maximum size and the
+ * currently used size of the metadata and can directly update or read it.
+ * dmaengine_desc_get_metadata_ptr() and dmaengine_desc_set_metadata_len() is
+ * provided as helper functions.
+ *
+ * Client drivers interested to use this mode can follow:
+ * 1. prepare the descriptor (dmaengine_prep_*)
+ * 2. use dmaengine_desc_get_metadata_ptr() to get the pointer to the engine's
+ * metadata area
+ * 3. update the metadata at the pointer
+ * 4. use dmaengine_desc_set_metadata_len() to tell the DMA engine the amount
+ * of data the client has placed into the metadata buffer
+ * 5. submit the transfer
+ * 1. prepare the descriptor (dmaengine_prep_*)
+ * 2. submit the transfer
+ * 3. on transfer completion, use dmaengine_desc_get_metadata_ptr() to get the
+ * pointer to the engine's metadata are

are = area?

+ * 4. Read out the metadate from the pointer

metadate = metadata?

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