linux-kernel - Re: [RFC 2/4] dma: add dmaengine driver for Samsung s3c24xx SoCs

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Message-ID: <45625193.0y3McEvmiD@flatron>
Date:	Thu, 16 May 2013 00:02:40 +0200
From:	Tomasz Figa <tomasz.figa@...il.com>
To:	Heiko Stübner <heiko@...ech.de>
Cc:	Sylwester Nawrocki <sylvester.nawrocki@...il.com>,
	Linus Walleij <linus.walleij@...aro.org>,
	Dan Williams <djbw@...com>, Vinod Koul <vinod.koul@...el.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	linux-samsung-soc <linux-samsung-soc@...r.kernel.org>,
	Kukjin Kim <kgene.kim@...sung.com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Russell King - ARM Linux <linux@....linux.org.uk>
Subject: Re: [RFC 2/4] dma: add dmaengine driver for Samsung s3c24xx SoCs

On Wednesday 15 of May 2013 23:48:31 Heiko Stübner wrote:
> Am Mittwoch, 15. Mai 2013, 23:20:08 schrieb Sylwester Nawrocki:
> > On 05/15/2013 10:31 PM, Heiko Stübner wrote:
> > >>> +       BUG();
> > >>> 
> > >> >  Isn't that a bit nasty. This macro should be used with care and
> > >> >  we
> > >> >  should recover if possible. dev_err()?
> > > 
> > > runtime_config already denies any settings not in the 1,2 or 4bytes
> > > range - the default-part should therefore never be reached. So if
> > > any other value magically appears in the register and triggers the
> > > default-part, something is seriously wrong. So my guess is, the BUG
> > > might be appropriate.
> > > 
> > > On the other hand the whole default+BUG part could also simply go
> > > away,
> > > for the same reasons.
> > 
> > IMHO BUG() is not needed at all. As Linus suggested dev_err() is such
> > case or WARN_ON() would be more appropriate. This has been discussed
> > in the past extensively, not sure if you are aware of the other
> > Linus' opinion on BUG()/BUG_ON() proliferation:
> > https://lkml.org/lkml/2012/9/27/461
> Very interesting read and I'll keep this in mind in the future. What
> about the other option ... i.e. simply getting rid of the whole "error
> handling", as the other code paths should already make sure that only
> valid values get written into the register.
> 
> Can the value change in the register somehow on its own without kernel
> intervention, or does this not happen?

Hmm, it depends on hardware, I guess. Not sure how it works on this 
particular IP.

Still, the mentioned BUG() was about a value in a driver-filled struct, 
wasn't it?

/* Quoting the the code for reference */

> +static u32 s3c24xx_dma_getbytes_chan(struct s3c24xx_dma_chan *s3cchan)
> +{
> +       struct s3c24xx_dma_phy *phy = s3cchan->phy;
> +       struct s3c24xx_txd *txd = s3cchan->at;
> +       u32 tc = readl(phy->base + DSTAT) & DSTAT_CURRTC_MASK;
> +
> +       switch (txd->dcon & DCON_DSZ_MASK) {
> +       case DCON_DSZ_BYTE:
> +               return tc;
> +       case DCON_DSZ_HALFWORD:
> +               return tc * 2;
> +       case DCON_DSZ_WORD:
> +               return tc * 4;
> +       default:
> +               break;
> +       }
> +
> +       BUG();

(Btw. I don't see anything setting the DCON_DSZ bits in this field. Am I 
missing something?)

Best regards,
Tomasz

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