From ef399305a5bf64231059c63a53f3384eb59c2458 Mon Sep 17 00:00:00 2001 From: Bogdan Mahomet Date: Sat, 22 Aug 2026 03:15:30 +0300 Subject: [PATCH] docs: dmaengine: document ownership of the descriptor flags field The flags field of struct dma_async_tx_descriptor has carried a TODO since the provider documentation was added in 2014, asking whether a driver may modify the field itself or whether it must always hold the flags passed in the arguments. The field belongs to the client. dma_async_tx_descriptor_init() does not touch it, so the provider has to set it, and providers store the flags they were given verbatim; vchan_tx_prep() does this for every driver built on top of virt-dma. Afterwards providers only read the field: the DMA_PREP_* bits are consumed while the descriptor is being prepared, and DMA_CTRL_ACK and DMA_CTRL_REUSE are tested with async_tx_test_ack() and dmaengine_desc_test_reuse() before a descriptor is recycled or freed. Once a descriptor has been prepared, nothing in the tree changes the field except the client helpers in include/linux/dmaengine.h: async_tx_ack(), async_tx_clear_ack(), dmaengine_desc_set_reuse() and dmaengine_desc_clear_reuse(). Document that rule in place of the TODO. Signed-off-by: Bogdan Mahomet Reviewed-by: Frank Li Link: https://patch.msgid.link/20260822001536.29269-1-bogdan.mahomet.dev@gmail.com Signed-off-by: Vinod Koul --- Documentation/driver-api/dmaengine/provider.rst | 8 +++++--- 1 file changed, 5 insertions(+), 3 deletions(-) diff --git a/Documentation/driver-api/dmaengine/provider.rst b/Documentation/driver-api/dmaengine/provider.rst index 22d235fb478f..2897ade67b39 100644 --- a/Documentation/driver-api/dmaengine/provider.rst +++ b/Documentation/driver-api/dmaengine/provider.rst @@ -413,9 +413,11 @@ supported. - You'll also need to set following fields in this structure: - - flags: - TODO: Can it be modified by the driver itself, or - should it be always the flags passed in the arguments + - flags: this field belongs to the client. The driver stores the + flags passed in the arguments and must not modify them. It reads + the DMA_PREP_* bits while preparing the descriptor, and must only + test DMA_CTRL_ACK and DMA_CTRL_REUSE, which the client may change + after the descriptor has been prepared. - tx_submit: A pointer to a function you have to implement, that is supposed to push the current transaction descriptor to a