Commit b802c841 authored by Dan Williams's avatar Dan Williams Committed by Vinod Koul

async_tx: deprecate broken support for channel switching

Back in 2011, Russell pointed out that the "async_tx channel switch"
capability was violating expectations of the dma mapping api [1]. At the
time the existing uses were reviewed as still usable, but that longer
term we needed a rework of the raid offload implementation. While some
of the framework for a fixed implementation was introduced in 2012 [2],
the wider rewrite never materialized.

There continues to be interest in raid offload with new dma/raid engine
drivers being submitted. Those drivers must not build on top of the
broken channel switching capability.

Prevent async_tx from using an offload engine if the channel switching
capability is enabled. This still allows the engine to be used for other
purposes, but the broken way async_tx uses these engines for raid will
be disabled. For configurations where this causes a performance
regression the only solution is to start the work of eliminating the
async_tx api and moving channel management into the raid code directly
where it can manage marshalling an operation stream between multiple dma


Cc: Anatolij Gustschin <>
Cc: Anup Patel <>
Cc: Rameshwar Prasad Sahu <>
Cc: Saeed Bishara <>
Cc: Thomas Petazzoni <>
Reported-by: default avatarRussell King <>
Signed-off-by: default avatarDan Williams <>
Signed-off-by: default avatarVinod Koul <>
parent dc312349
......@@ -87,7 +87,7 @@ struct async_submit_ctl {
void *scribble;
#define async_tx_issue_pending_all dma_issue_pending_all
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