linux-kernel - Re: [PATCH 3/4] dmaengine: qcom_bam

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Date:	Tue, 01 Dec 2015 21:22:26 +0100
From:	Arnd Bergmann <arnd@...db.de>
To:	Andy Gross <agross@...eaurora.org>
Cc:	Stanimir Varbanov <stanimir.varbanov@...aro.org>,
	linux-arm-msm@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org, devicetree@...r.kernel.org,
	dmaengine@...r.kernel.org, Vinod Koul <vinod.koul@...el.com>,
	Rob Herring <robh+dt@...nel.org>,
	Rob Herring <robh@...nel.org>,
	Mark Rutland <mark.rutland@....com>,
	Pawel Moll <pawel.moll@....com>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Archit Taneja <architt@...eaurora.org>
Subject: Re: [PATCH 3/4] dmaengine: qcom_bam_dma: use correct pipe FIFO size

On Tuesday 01 December 2015 11:25:35 Andy Gross wrote:
> On Tue, Dec 01, 2015 at 11:28:32AM +0100, Arnd Bergmann wrote:
> > On Tuesday 01 December 2015 11:14:58 Stanimir Varbanov wrote:
> > > 
> > > diff --git a/drivers/dma/qcom_bam_dma.c b/drivers/dma/qcom_bam_dma.c
> > > index 0f06f3b7a72b..6d290de9ab2b 100644
> > > --- a/drivers/dma/qcom_bam_dma.c
> > > +++ b/drivers/dma/qcom_bam_dma.c
> > > @@ -458,7 +458,7 @@ static void bam_chan_init_hw(struct bam_chan *bchan,
> > >          */
> > >         writel_relaxed(ALIGN(bchan->fifo_phys, sizeof(struct bam_desc_hw)),
> > >                         bam_addr(bdev, bchan->id, BAM_P_DESC_FIFO_ADDR));
> > > -       writel_relaxed(BAM_DESC_FIFO_SIZE,
> > > +       writel_relaxed(BAM_MAX_DATA_SIZE,
> > >                         bam_addr(bdev, bchan->id, BAM_P_FIFO_SIZES));
> > >  
> > >         /* enable the per pipe interrupts, enable EOT, ERR, and INT irqs */
> > 
> > I'm looking at that now and fail to see why these have to use writel_relaxed().
> 
> At some point I believe I got a comment about using (readl/writel)_relaxed
> instead of readl/writel.  So I used these instead.  Has the wind direction
> changed?  =)

Yes.

> Using the readl/writel is nice w.r.t. having the implicit barriers, especially
> with the funky 1K boundary on reordering of operations that can occur on Kraits.
> This can hit you on accesses even within the same IP block.

We had a couple of bugs that we should not have had when drivers were mindlessly
converted, so generally speaking at least I try to get people to only use
the relaxed functions for the hot path when they can show an advantage as well
as the fact that it's safe to use.

	Arnd
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