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Date:	Thu, 23 Oct 2014 16:38:39 +0300
From:	Laurent Pinchart <laurent.pinchart@...asonboard.com>
To:	Maxime Ripard <maxime.ripard@...e-electrons.com>
Cc:	dmaengine@...r.kernel.org, Vinod Koul <vinod.koul@...el.com>,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	Antoine Ténart <antoine@...e-electrons.com>,
	lars@...afoo.de, Russell King <linux@....linux.org.uk>,
	Dan Williams <dan.j.williams@...el.com>
Subject: Re: [PATCH v3 58/59] dmaengine: Add a warning for drivers not using the generic slave caps retrieval

Hi Maxime,

On Thursday 23 October 2014 15:19:33 Maxime Ripard wrote:
> On Thu, Oct 23, 2014 at 12:21:31AM +0300, Laurent Pinchart wrote:
> > Hi Maxime,
> > 
> > Thank you for the patch.
> > 
> > On Wednesday 22 October 2014 17:44:12 Maxime Ripard wrote:
> > > For the slave caps retrieval to be really useful, most drivers need to
> > > implement it.
> > > 
> > > Hence, we need to be slightly more aggressive, and trigger a warning at
> > > registration time for drivers that don't fill their caps infos in order
> > > to
> > > encourage them to implement it.
> > > 
> > > Signed-off-by: Maxime Ripard <maxime.ripard@...e-electrons.com>
> > > ---
> > > 
> > >  drivers/dma/dmaengine.c | 3 +++
> > >  1 file changed, 3 insertions(+)
> > > 
> > > diff --git a/drivers/dma/dmaengine.c b/drivers/dma/dmaengine.c
> > > index 98e9431f85ec..4e18981b16bd 100644
> > > --- a/drivers/dma/dmaengine.c
> > > +++ b/drivers/dma/dmaengine.c
> > > @@ -827,6 +827,9 @@ int dma_async_device_register(struct dma_device
> > > *device) BUG_ON(!device->device_issue_pending);
> > > 
> > >  	BUG_ON(!device->dev);
> > > 
> > > +	WARN(dma_has_cap(DMA_SLAVE, device->cap_mask) &&
> > > !device->generic_slave_caps,
> > > +	     "this driver doesn't support generic slave capabilities
> > > reporting\n");
> > > +
> > 
> > This might be slightly too aggressive.
> 
> I disagree with that. If we want at some point to have the drivers
> implement it, we should be aggressive (and note that we don't break
> anything, the driver will still work as it used to).

I meant too aggressive given the possibility that a driver might need its own 
implementation if not all channels have the same capability.

> > I agree with your previous comment on all DMA engine drivers
> > returning the same capabilities for all channels, but it might not
> > be true anymore in the future, in which case drivers will need to
> > implement a custom slave caps function. We could delay support for
> > that to when it's needed though.
> 
> Actually, my next patch (59) is removing such possibility... And you
> acked it ;)
> 
> Is it still something we should be concerned about?

Maybe not now, we can always add that possibility back in the future if 
needed.

-- 
Regards,

Laurent Pinchart

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