Re: coccinelle and bitmask arithmetic

Julia Lawall
2013-01-30T11:21:26Z

On Wed, 30 Jan 2013, Russell King - ARM Linux wrote:

> On Wed, Jan 30, 2013 at 09:21:28AM +0100, walter harms wrote:
> > Great hit Joe :)
> >
> > Sometimes i am really surprised what code can be found
> > in the kernal and it is still working.
> > Having no clue of the code i suspect somebody tries to
> > check is mask outside the range it should read
> > PHYS_OFFSET |( SZ_64M - 1)
> > maybe someone should tell them that
> > 1+1=10 while 1|1=1
> > It does not seem to matter here (or ... ?)
>
> This PCI host is only used on one platform (ARMCORE).
>
> For this, PHYS_OFFSET will be a value with only the top few bits of a
> 32-bit word set (such as 0xc0000000) - it's certainly not going to have
> any bits set below bit 26 on the platform this driver gets used on.
> "SZ_64M - 1" is the size of the window that RAM appears.
>
> So, _either_ logical OR or addition works.
>
> If we _did_ end up with a PHYS_OFFSET with bits less than bit 26 set
> here, we'd have bigger problems - because the base of RAM in PCI space
> will not correspond with PHYS_OFFSET and all the DMA mapping stuff breaks.

The "problem" is that the computation is done inconsistently within the
same file. Sometimes with + and sometimes with |.

julia