linux-kernel - [PATCH v6 4/7] dmaengine: Add provider documentation on cookie assignment

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Date:   Fri, 28 Oct 2022 13:48:09 -0700
From:   Ben Walker <benjamin.walker@...el.com>
To:     vkoul@...nel.org
Cc:     dmaengine@...r.kernel.org, linux-kernel@...r.kernel.org,
        Dave Jiang <dave.jiang@...el.com>
Subject: [PATCH v6 4/7] dmaengine: Add provider documentation on cookie assignment

Clarify the rules on assigning cookies to DMA transactions.

Signed-off-by: Ben Walker <benjamin.walker@...el.com>
Reviewed-by: Dave Jiang <dave.jiang@...el.com>
---
 .../driver-api/dmaengine/provider.rst         | 45 +++++++++++++++----
 1 file changed, 37 insertions(+), 8 deletions(-)

diff --git a/Documentation/driver-api/dmaengine/provider.rst b/Documentation/driver-api/dmaengine/provider.rst
index 1d0da2777921d..a5539f816d125 100644
--- a/Documentation/driver-api/dmaengine/provider.rst
+++ b/Documentation/driver-api/dmaengine/provider.rst
@@ -417,7 +417,9 @@ supported.
 
     - tx_submit: A pointer to a function you have to implement,
       that is supposed to push the current transaction descriptor to a
-      pending queue, waiting for issue_pending to be called.
+      pending queue, waiting for issue_pending to be called. Each
+      descriptor is given a cookie to identify it. See the section
+      "Cookie Management" below.
 
   - In this structure the function pointer callback_result can be
     initialized in order for the submitter to be notified that a
@@ -522,6 +524,40 @@ supported.
 
   - May sleep.
 
+Cookie Management
+------------------
+
+When a transaction is queued for submission via tx_submit(), the provider
+must assign that transaction a cookie (dma_cookie_t) to uniquely identify it.
+The provider is allowed to perform this assignment however it wants, but for
+convenience the following utility functions are available to create
+monotonically increasing cookies
+
+  .. code-block:: c
+
+    void dma_cookie_init(struct dma_chan *chan);
+
+  Called once at channel creation
+
+  .. code-block:: c
+
+    dma_cookie_t dma_cookie_assign(struct dma_async_tx_descriptor *tx);
+
+  Assign a cookie to the given descriptor
+
+  .. code-block:: c
+
+    void dma_cookie_complete(struct dma_async_tx_descriptor *tx);
+
+  Mark the descriptor as complete and invalidate the cookie
+
+  .. code-block:: c
+
+    enum dma_status dma_cookie_status(struct dma_chan *chan,
+      dma_cookie_t cookie, struct dma_tx_state *state);
+
+  Report the status of the cookie and filling in state, if not NULL.
+
 
 Misc notes
 ==========
@@ -537,13 +573,6 @@ where to put them)
 - Makes sure that dependent operations are run before marking it
   as complete.
 
-dma_cookie_t
-
-- it's a DMA transaction ID.
-
-- The value can be chosen by the provider, or use the helper APIs
-  such as dma_cookie_assign() and dma_cookie_complete().
-
 DMA_CTRL_ACK
 
 - If clear, the descriptor cannot be reused by provider until the
-- 
2.37.3