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SubjectRe: [PATCH 06/13] DMAENGINE: driver for the ARM PL080/PL081 PrimeCells
On Wed, Dec 22, 2010 at 03:45:39PM -0800, Dan Williams wrote:
> This is listed in the dmaengine documentation [1], but I obviously
> missed this before merging. This also would have been caught by
> lockdep as required by SubmitChecklist. As far as corrective action
> before 2.6.37-final. It looks like this driver needs a full scrub
> which seems unreasonable to complete and test over the holidays before
> .37 lands. Linus we either need to mark this "depends on BROKEN" or
> revert it.
>
> Support for the DMA_COMPL flags are necessary if the DMA_MEMCPY
> capability is advertised, yes this driver got this wrong. I'll update
> the documentation to make this requirement clear, and audit the other
> drivers. With slave-only drivers the only usage model is one where
> the client driver owns dma-mapping. In the non-slave (opportunistic
> memcpy offload) case the client is unaware of the engine so the driver
> owns unmapping. The minimal fix is to disable memcpy offload.
>
> --
> Dan
>
> [1]
> 3.6 Constraints:
> 1/ Calls to async_<operation> are not permitted in IRQ context. Other
> contexts are permitted provided constraint #2 is not violated.
> 2/ Completion callback routines cannot submit new operations. This
> results in recursion in the synchronous case and spin_locks being
> acquired twice in the asynchronous case.

(2) seems to be more than a little annoying - it seems that DMA engine
drivers use a tasklet for running their DMA cleanup, which calls drivers
callbacks, and we're going to have to have a whole pile of taskets
in drivers just to be triggered from the completion callback. I can
see this adding an additional layer of complexity and a nice fine set
of shiney new races to deal with.


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