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 channels. [1]: http://lists.infradead.org/pipermail/linux-arm-kernel/2011-January/036753.html [2]: https://lkml.org/lkml/2012/12/6/71 Cc: Anatolij Gustschin <[email protected]> Cc: Anup Patel <[email protected]> Cc: Rameshwar Prasad Sahu <[email protected]> Cc: Saeed Bishara <[email protected]> Cc: Thomas Petazzoni <[email protected]> Reported-by: Russell King <[email protected]> Signed-off-by: Dan Williams <[email protected]> --- include/linux/async_tx.h | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/include/linux/async_tx.h b/include/linux/async_tx.h index 388574ea38ed..28e3cf1465ab 100644 --- a/include/linux/async_tx.h +++ b/include/linux/async_tx.h @@ -87,7 +87,7 @@ struct async_submit_ctl { void *scribble; }; -#ifdef CONFIG_DMA_ENGINE +#if defined(CONFIG_DMA_ENGINE) && !defined(CONFIG_ASYNC_TX_CHANNEL_SWITCH) #define async_tx_issue_pending_all dma_issue_pending_all /**
