Re: [PATCH 2/5] i2c: sh_mobile: add DMA support

[Laurent Pinchart <laurent.pinchart-ryLnwIuWjnjg/C1BVhZhaw@public.gmane.org>]

(CC'ing the dmaengine mailing list)

On Wednesday 10 December 2014 09:01:55 Wolfram Sang wrote:
> On Wed, Dec 10, 2014 at 02:44:01PM +0900, Magnus Damm wrote:
> > On Tue, Dec 9, 2014 at 11:09 PM, Wolfram Sang <wsa-z923LK4zBo2bacvFa/9K2g@public.gmane.org> wrote:
> >> On Tue, Dec 09, 2014 at 11:53:45AM +0100, Geert Uytterhoeven wrote:
> >>> On Fri, Nov 7, 2014 at 11:11 AM, Wolfram Sang wrote:
> >>>> --- a/drivers/i2c/busses/i2c-sh_mobile.c
> >>>> +++ b/drivers/i2c/busses/i2c-sh_mobile.c
> >>>> @@ -727,6 +886,21 @@ static int sh_mobile_i2c_probe(struct
> >>>> platform_device *dev)
> >>>>         if (ret)
> >>>>                 return ret;
> >>>> 
> >>>> +       /* Init DMA */
> >>>> +       sg_init_table(&pd->sg, 1);
> >>>> +       pd->dma_direction = DMA_NONE;
> >>>> +       ret = sh_mobile_i2c_request_dma_chan(pd->dev, DMA_DEV_TO_MEM,
> >>>> +                                            res->start + ICDR,
> >>>> &pd->dma_rx);
> >>>> +       if (ret == -EPROBE_DEFER)
> >>>> +               return ret;
> >>>> +
> >>>> +       ret = sh_mobile_i2c_request_dma_chan(pd->dev, DMA_MEM_TO_DEV,
> >>>> +                                            res->start + ICDR,
> >>>> &pd->dma_tx);
> >>>> +       if (ret == -EPROBE_DEFER) {
> >>>> +               sh_mobile_i2c_release_dma(pd);
> >>>> +               return ret;
> >>>> +       }
> >>>> +
> >>> 
> >>> If the DTS contains "dma" and "dma-names" properties, but
> >>> CONFIG_RCAR_DMAC is disabled, sh_mobile_i2c_request_dma_chan() returns
> >>> -EPROBE_DEFER, and the driver fails to initialize.
> >>> 
> >>> If I remove the "dma" and "dma-names" properties, the driver does fall
> >>> back to PIO mode.
> >>> 
> >>> I think this is a regression.
> >> 
> >> The only solution I can think of is to not bail out here and retry again
> >> before every transfer? Doesn't sound elegant, though...
> > 
> > I think we have to request for each and every transfer. And fall back
> > to PIO as default in a transparent way. This because the number of DMA
> > channels are limited compared to number of potential consumers, so
> > request failure may happen at any time.
> 
> AFAIR this scenario happens when submitting the transfer. The check
> for this is already in place. Requesting the channel is a different
> matter. Still, I'll cook up a patch and we will see what it looks
> like...

We could fix part of the issue by using virtual dma channels. In that case 
channel requests wouldn't fail anymore due to resource starvation with a large 
number of consumers. However, the request could still fail with -EPROBE_DEFER. 
For a driver that wants to fall back to PIO when DMA is unavailable I 
currently don't see another way than moving the channel request at the time of 
the transfer.

-- 
Regards,

Laurent Pinchart