[v3 1/2] dma: mmp_pdma: add support for residue reporting

[Chao Xie]

On Fri, Aug 16, 2013 at 2:12 AM, Daniel Mack <zonque at gmail.com> wrote:
> On 15.08.2013 10:08, Chao Xie wrote:
>> In the function mmp_pdma_tx_submit
>> It will assign cookie for every descriptors by
>> list_for_each_entry(child, &desc->tx_list, node) {
>>                 cookie = dma_cookie_assign(&child->async_tx);
>>         }
>> So not all the descriptors in same transaction will share same cookie.
>
> Hmm, yes. You're right. However, I see quite some more issues with the
> driver as it stands, especially due to assumptions that are made for
> multiple transactions on one channel.
>
> For example, let's think about two transactions with multiple
> descriptors each, and both transactions are present in the chain_running
> list. Each transaction has the ENDIRQEN bit set on the last descriptor only.
>
> Currently, when an interrupt is caught, the code will only look at the
> very last entry in the running list and complete its cookie, and then
> dispose the entire running chain. Hence, the other transaction's cookie
> will never complete. Even worse: as an interrupt will always be for the
> first descriptor in the chain with the ENDIRQEN bit set, we complete the
> second transaction that is in fact still running (and call the
> completion handler for it), while we don't do exactly that for the first
> one.
>
> Am I right with the above or do I make faulty assumptions? I'll think
> about how the driver can be changed in order to fix this, but it will be
> more invasive than I thought.
>
I think you are right. I include the orignal author for the driver, and maybe
he can comment at it.

>
> Thanks for sharing your ideas,
> Daniel
>