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    SubjectRe: [PATCH 16/18] dmaengine/amba-pl08x: Add support for sg len greater than one for slave transfers
    On Thu, Aug 18, 2011 at 02:08:19PM +0530, Viresh Kumar wrote:
    > On 8/14/2011 2:06 PM, Russell King - ARM Linux wrote:
    > > On Fri, Jul 29, 2011 at 04:19:26PM +0530, Viresh Kumar wrote:
    > >> Untill now, sg_len greater than one is not supported. This patch adds support to
    > >> do that.
    > >
    > > I'm not sure that this is the correct approach. memcpy()s can only be
    > > used with one single buffer, so the sg stuff for that (and the unmapping
    > > support) doesn't make sense at all.
    > >
    >
    > I am not sure if i get this completely. In memcpy, we still don't support
    > more than one sg. We have created a new member in txd, which keeps track of
    > data (addresses, len).

    Yes, but we shouldn't need to translate it into any kind of scatterlist.

    > While unmapping, we are unmapping only single dsg. I think i don't need to
    > use list_for_each_entry() there, as there is only one element present in list.
    > But can still keep that.

    Correct.

    > > The only place where this makes sense is the slave sg stuff. I wonder
    > > whether we can better deal with that by having the LLI setup code deal
    > > with one SG list entry at a time, and chain each together.
    > >
    > > Something I've also been pondering which is related to this is linking
    > > together DMA operations using the LLI chaining when interrupts and
    > > callbacks aren't required. It's a very similar problem. Other DMA
    > > engine drivers do this.
    >
    > Are you talking about linking all sg's together or linking multiple calls to
    > prep_dma_memcpy? I am handling the first.

    I'm talking about letting the hardware process as many txds as possible
    without calling back into the driver to have it setup the next txd.

    > > Finally, note that some of the PL08x code assumes that for any TXD,
    > > the LLI list is a contiguous array. See the first part of
    > > pl08x_getbytes_chan(). That needs fixing as soon as we start going to
    > > more than one SG list entry.
    >
    > We still have one contiguous array for LLI list. In pl08x_fill_llis_for_desc()
    > i am using same txd->llis_va for all sg's.

    That's ok then.


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