[PATCH v4 3/4] dma: mmp_pdma: add support for residue reporting

[Vinod Koul]

On Sun, Aug 25, 2013 at 06:01:51PM +0100, Russell King - ARM Linux wrote:
> On Sun, Aug 25, 2013 at 09:41:04PM +0530, Vinod Koul wrote:
> > Also descriptor is returned for a transaction when you invoke .device_prep_xxx.
> > And each descriptor is assigned a cookie value.
Sorry this wasnt meant to be in same timeline, just stating that descriptor is
always assigned a cookie value and yes that will be done _only_ when descriptor
is submitted by invoking .tx_submit(). 

In dma driver, one should use cookie value to check status

Also, the reason for not assigning cookie in prepare is simple. If we preared
three trasactions A, B and C. But sumbitted A, C and then B then whole cookie
logic will fail, so it makes sense to assign cookie in an ordered fashion when
trasactions are submitted not when prepared.

Though i suspect in slave dma case, we would see ordered manner typically.

> The descriptor is not assigned a cookie in the preparation function - it
> is only assigned a cookie when it is submitted, which should always happen
> shortly after preparation.  The submission call returns the assigned
> cookie, so there's no reason for any driver to dereference the descriptor.
> 
> Any driver which does dereference the descriptor after submission is
> potentially a bug; the DMA engine owns it, and can kfree that descriptor
> any moment after submission, or even re-use it for another descriptor.
I would say after callback is invoked in case when one is set. For the ones when
no callback is set, yes after invoking tx_submit() is right one.

Clients should remeber the cookie value returned and use it to track the status
of transactions.

~Vinod
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