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    SubjectRe: [PATCH 02/13] dmaengine: Introduce dma_request_slave_channel_compat_reason()
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    On Wednesday 18 November 2015 16:41:35 Peter Ujfalusi wrote:
    > On 11/18/2015 04:29 PM, Arnd Bergmann wrote:
    > > On Wednesday 18 November 2015 16:21:26 Peter Ujfalusi wrote:
    > >> 2. non slave channel requests, where only the functionality matters, like
    > >> memcpy, interleaved, memset, etc.
    > >> We could have a simple:
    > >> dma_request_channel(mask);
    > >>
    > >> But looking at the drivers using dmaengine legacy dma_request_channel() API:
    > >> Some sets DMA_INTERRUPT or DMA_PRIVATE or DMA_SG along with DMA_SLAVE:
    > >> drivers/misc/carma/carma-fpga.c DMA_INTERRUPT|DMA_SLAVE|DMA_SG
    > >> drivers/misc/carma/carma-fpga-program.c DMA_MEMCPY|DMA_SLAVE|DMA_SG
    > >> drivers/media/platform/soc_camera/mx3_camera.c DMA_SLAVE|DMA_PRIVATE
    > >> sound/soc/intel/common/sst-firmware.c DMA_SLAVE|DMA_MEMCPY
    > >>
    > >> as examples.
    > >> Not sure how valid are these...

    I just had a look myself. carma has been removed fortunately in linux-next,
    so we don't have to worry about that any more.

    I assume that the sst-firmware.c case is a mistake, it should just use a
    plain DMA_SLAVE and not DMA_MEMCPY.

    Aside from these, everyone else uses either DMA_CYCLIC in addition to
    DMA_SLAVE, which seems valid, or they use DMA_PRIVATE, which I think is
    redundant in slave drivers and can be removed.

    > > It's usually not much harder to separate out the legacy case from
    > > the normal dma_request_slave_channel_reason(), so those drivers don't
    > > really need to use the unified compat API.
    >
    > The current dma_request_slave_channel()/_reason() is not the 'legacy' API.
    > Currently there is no way to get the reason why the dma channel request fails
    > when using the _compat() version of the API, which is used by drivers which
    > can be used in DT or in legacy mode as well. Sure, they all could have local
    > if(){}else{} for handling this, but it is not a nice thing.
    >
    > As it was discussed instead of adding the _reason() version for the _compat
    > call, we should simplify the dmaengine API for getting the channel and at the
    > same time we will have ERR_PTR returned instead of NULL.

    What I meant was that we don't need to handle them with the unified
    simple interface. The users of DMA_CYCLIC can just keep using
    an internal helper that only deals with the legacy case, or use
    dma_request_slave() or whatever is the new API for the DT case.

    Arnd


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