linux-kernel - Re: [PATCH] DMA: add help function to check whether dma controller registered

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Message-ID: <20130805055607.GC31469@rizhao-lap>
Date:	Mon, 5 Aug 2013 13:56:09 +0800
From:	Richard Zhao <rizhao@...dia.com>
To:	Stephen Warren <swarren@...dotorg.org>
CC:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"vinod.koul@...el.com" <vinod.koul@...el.com>,
	"djbw@...com" <djbw@...com>,
	"grant.likely@...aro.org" <grant.likely@...aro.org>,
	"rob.herring@...xeda.com" <rob.herring@...xeda.com>
Subject: Re: [PATCH] DMA: add help function to check whether dma controller
 registered

On Sat, Aug 03, 2013 at 03:59:59AM +0800, Stephen Warren wrote:
> On 08/02/2013 12:04 AM, Richard Zhao wrote:
> > DMA client device driver usually needs to know at probe time whether
> > dma controller has been registered to deffer probe. So add a help
> > function of_dma_check_controller.
> > 
> > DMA request channel functions can also used to check it, but they
> > are usually called at open() time.
> 
> > diff --git a/drivers/dma/of-dma.c b/drivers/dma/of-dma.c
> > index e1c4d3b..b6828c1 100644
> 
> > +int of_dma_check_controller(struct device *dev, const char *name)
> 
> 
> > +	for (i = 0; i < count; i++) {
> > +		if (of_dma_match_channel(np, name, i, &dma_spec))
> > +			continue;
> > +
> > +		mutex_lock(&of_dma_lock);
> > +		ofdma = of_dma_find_controller(&dma_spec);
> > +		mutex_unlock(&of_dma_lock);
> > +		of_node_put(dma_spec.np);
> 
> Do we need to add the following here:
> 
> if (ofdma)
> 	break
> 
> To ensure that as soon as a successful match is found, the loop exits?
> Otherwise, if there are multiple providers for that name, and the first
> N are registered but the last isn't, this function will still return
> failure.
Right. Thanks.
> 
> > +	if (ofdma)
> > +		return 0;
> > +	else
> > +		return -ENODEV;
> 
> That probably should be -EPROBE_DEFER?
> 
> Although, what about differentiating between "entry not found by
> of_dma_match_channel" and "controller not yet probed"?
I thought it might be called in non-probe functions.  If no people
against it, I'll change it to EPROBE_DEFER.

Thanks
Richard
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