Re: [PATCH] Documentation: dmaengine: Add a documentation for the dma controller API

[Vinod Koul]

On Sat, Aug 02, 2014 at 04:49:25PM +0200, Maxime Ripard wrote:
> > - residue callculation, though situation is much better now but still lots
> >   of driver do it worng and folks do get it wrong
> 
> What mistake in often made regarding the residue calculation?
- residue is for the descriptor asked
- not for the current active one
- we can store the size locally and return that when not processing
- arg txstate can be null

> Another extra questions arose during starting this.
> 
> In the case of the call to device_control, especially in the
> DMA_SLAVE_CONFIG case, but that also applies to pause/resume, are the
> changes supposed to be immediates or can they happen later?
> 
> I actually have in mind the case where we would use a vchan, that
> might or might not be actually mapped to a physical channel at the
> moment where the DMA_SLAVE_CONFIG call is made. In the case where it's
> not mapped and not transfering anything, it's pretty trivial, to
> handle, but in the case where it's actually mapped to a physical
> channel, should we push the new configuration to the physical channel
> right away, or can it wait until the transfer ends ?
Ah no. The model doesn't assume the config will be written runtime. So the
descriptor which are already prepared doesn't change with DMA_SLAVE_CONFIG
call. Only new descriptors will take those values

But yes the pause/resume would be syncronous

HTH
-- 
~Vinod

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