linux-kernel - Re: [PATCH 2/2] dw_dmac: add cyclic API to DW DMA driver

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Message-ID: <20090401102830.6a749ad2@hskinnemoen-d830>
Date:	Wed, 1 Apr 2009 10:28:30 +0200
From:	Haavard Skinnemoen <haavard.skinnemoen@...el.com>
To:	"Sosnowski, Maciej" <maciej.sosnowski@...el.com>
Cc:	Hans-Christian Egtvedt <hans-christian.egtvedt@...el.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"Williams, Dan J" <dan.j.williams@...el.com>
Subject: Re: [PATCH 2/2] dw_dmac: add cyclic API to DW DMA driver

Sosnowski, Maciej wrote:
> > +void dw_dma_cyclic_stop(struct dma_chan *chan)
> > +{
> > +	struct dw_dma_chan	*dwc = to_dw_dma_chan(chan);
> > +	struct dw_dma		*dw = to_dw_dma(dwc->chan.device);
> > +
> > +	channel_clear_bit(dw, CH_EN, dwc->mask);
> > +	while (dma_readl(dw, CH_EN) & dwc->mask)
> > +		cpu_relax();
> > +}  
> 
> Don't you need locks in dw_dma_cyclic_stop?

Good question. On one hand, if cyclic_start() can race with
cyclic_stop(), the client probably has more serious issues to deal
with. On the other hand, if something ever manages to set the CH_EN bit
at the wrong moment, the loop may never finish.

So it's probably safest to wrap it in spin_lock_bh(). You should
probably add a note that this function can not be called from interrupt
context too.

Haavard
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