Re: [PATCH 08/32] dmaengine: ste_dma40: Optimise local MAX() macro

[Russell King - ARM Linux]

On Thu, Apr 18, 2013 at 12:46:03PM +0200, Arnd Bergmann wrote:
> On Thursday 18 April 2013, Lee Jones wrote:
> > The current implementation of the DMA40's local MAX() macro evaluates
> > its arguments more times than is necessary. This patch strips it
> > optimises it to only evaluate what's appropriate.
> 
> No, it does not.
> 
> > index b21a8a3..7b451b2 100644
> > --- a/drivers/dma/ste_dma40.c
> > +++ b/drivers/dma/ste_dma40.c
> > @@ -53,7 +53,7 @@
> >  #define D40_ALLOC_PHY          BIT(30)
> >  #define D40_ALLOC_LOG_FREE     BIT(0)
> >  
> > -#define MAX(a, b) (((a) < (b)) ? (b) : (a))
> > +#define MAX(a, b) ((a > b) ? a : b)
> 
> This just makes the macro worse in case you pass a complex expression
> in, which can now have additional unwanted effects. Just drop this patch.

Never got the original patch...

A much better idea is to get rid of that buggy MAX() macro altogether
and use the macros already provided by the kernel, which are safe from
side effects - but more importantly are _type_ _safe_.  The above goes
wrong when you consider 'a' and 'b' may have different signed-ness.

Consider:

	int val_in = -5;
	unsigned val = MAX(val_in, 5U);

The resulting value is (unsigned)-5, not (unsigned)5.

Best use the kernel's max() or max_t() _everywhere_.
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