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

From: Tomasz Figa
Date: Wed May 15 2013 - 18:02:52 EST


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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