Re: [PATCH 06/13] DMAENGINE: driver for the ARM PL080/PL081 PrimeCells

[Russell King - ARM Linux <linux@arm.linux.org.uk>]

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.