Re: [RFC v2 5/5] Atmel MCI: Driver for Atmel on-chip MMC controllers

["Dan Williams" <dan.j.williams@intel.com>]

On Feb 13, 2008 2:06 PM, Haavard Skinnemoen <hskinnemoen@atmel.com> wrote:
> On Wed, 13 Feb 2008 12:11:58 -0700
> "Dan Williams" <dan.j.williams@intel.com> wrote:
>
> > > +               desc = chan->device->device_prep_slave(chan,
> > > +                               sg_dma_address(sg), direction,
> > > +                               DMA_SLAVE_WIDTH_32BIT,
> > > +                               sg_dma_len(sg), dma_flags);
> > > +               desc->txd.callback = NULL;
> > > +               list_add_tail(&desc->client_node,
> > > +                               &host->dma.data_descs);
> > > +       }
> >
> > Need to handle device_prep_slave returning NULL?
>
> You're right, we definitely need to handle that. Which probably means
> we need to prepare an interrupt descriptor first that we can throw in
> when we're unable to obtain more descriptors, and submit the rest from
> the callback.
>
> Except we're not allowed to submit anything from the callback. Ouch.
>
> How can we solve that? Set up a work queue and submit it from there?
> Trigger a different tasklet?
>
> In any case, I guess I need to implement support for interrupt
> descriptors in the dw_dmac driver.
>

Well, the other two possibilities are:

1/ Spin/sleep until a descriptor shows up
2/ Fall back to PIO for a few transfers

Descriptor availability is improved if the code interleaves allocation
and submission.  Currently it looks like we wait until all descriptors
for the scatterlist are allocated before we start submitting.