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

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Message-Id: <201305160045.03934.heiko@sntech.de>
Date:	Thu, 16 May 2013 00:45:03 +0200
From:	Heiko Stübner <heiko@...ech.de>
To:	Tomasz Figa <tomasz.figa@...il.com>
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

Am Donnerstag, 16. Mai 2013, 00:02:40 schrieb Tomasz Figa:
> 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?)

this is for calculating the remaining bytes of the transaction. which is used 
in s3c24xx_dma_tx_status. 

And when looking at it again, I can't really fathom why I did it this way with 
decoding the DSZ from the dcon value of the s3c24xx_txd again instead of 
simply using the width value of the same struct .... 

So it can be much simpler as
	(...)
     u32 tc = readl(phy->base + DSTAT) & DSTAT_CURRTC_MASK;
	return tc * txd->width;

getting rid of this stuff alltogether


still puzzled how I came up with this strangeness in the first place
Heiko
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