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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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