Re: [PATCH 1/6] dma: rcar-dma: add wait after stopping dma engine

[Laurent Pinchart <laurent.pinchart@ideasonboard.com>]

Hi Morimoto-san,

On Wednesday 08 Jun 2016 10:05:09 Kuninori Morimoto wrote:
> Hi Laurent
> # add Niklas
> 
> Sorry for my suport late response.
> Thanks Niklas for reminder
> 
> > (With a question for Morimoto-san below)
> 
> (snip)
> 
> > > >> +     } while (++i < NR_READS_TO_WAIT);
> > > > 
> > > > How long does the DMA engine typically need to stop ? Is there a safe
> > > > upper bound ?
> > > 
> > > I have tested it many times. The highest number of retries was 3 which
> > > occurred not very frequently. Normally it is 1-2
> > 
> > Morimoto-san, could you check with the hardware team if there's a worst
> > case guarantee regarding how long the channel could take to stop ?
> 
> You need to wait maximum 0.7ms for it.
> both for Gen2/Gen3.
> In worst case, it needs 8 request times.
> If 1 request is 20,000 cycle
> 
> 	20,000cycle x 8request x [4ns/cylce] = 640,000ns > 700us

And we're calling the wait function both from interrupt context and from user 
context with a spinlock held. I'm afraid that's not acceptable, we can't busy-
loop for so long with interrupts disabled.

-- 
Regards,

Laurent Pinchart