On Wed, Dec 10, 2014 at 04:19:36PM +0200, Laurent Pinchart wrote:
> (CC'ing the dmaengine mailing list)
Thanks!
> 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@xxxxxxxxxxxxx> 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.
Note that the I2C drives uses subsys_initcall() for historic reasons,
while the DMA driver uses module_init(). This is hard to revert without
introducing potential regressions on older boards. So, the I2C DMA
support needs to handle deferred probe definately. I am with Laurent, I
don't see any other way, but I'd be glad to be enlightened...
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