Re: [coreboot] [PATCH][v3] Check that CAR and ROM areas don'tcollide

[Carl-Daniel Hailfinger]

On 10.12.2008 20:39, Corey Osgood wrote:
> On Wed, Dec 10, 2008 at 1:29 PM, Myles Watson  wrote:
> > > -----Original Message-----
> > > From: [e-mail] [mailto:
> >
> > [e-mail]]
> > > On Behalf Of Carl-Daniel Hailfinger
> > > Sent: Wednesday, December 10, 2008 11:18 AM
> > > To: Corey Osgood
> > > Cc: Segher Boessenkool; coreboot
> > > Subject: Re: [coreboot] [PATCH][v3] Check that CAR and ROM areas
> > > don'tcollide
> > >
> > > Hi Segher,
> > >
> > > is the last test below with 0x100000000 (2^32) in the formula guaranteed
> > > to work or may cpp truncate the results to 32 bit? I'd rather avoid
> > > introducing a test that can never trigger.
> >
> > ...snip...
> > > What you actually want is this test:
> > > #if CONFIG_CARBASE + CONFIG_CARSIZE + (CONFIG_COREBOOT_ROMSIZE_KB * 1024)
> > > > 0x100000000
> >
> > To avoid that problem, maybe we should /1024 instead of *.
> > #if CONFIG_CARBASE/1024 + CONFIG_CARSIZE/1024 + CONFIG_COREBOOT_ROMSIZE_KB
> >
> > 1<<22
> >
> > I realize that 1<<22 isn't pretty, but the rest doesn't seem too bad.
> >
> > Thanks,
> > Myles
>
> I'm probably missing something, but I'm not seeing the off-by-one error in
> the original function. CONFIG_CARBASE and 0xffffffff are both addresses, you
> could say they're both off by one, but by comparing them the problem is
> negated. You could rearrange it to
>
> 0xffffffff - (CONFIG_CARBASE + CONFIG_CARSIZE) < CONFIG_COREBOOT_ROMSIZE_KB
> 1024
>
> which should, AFAIK, compare the remaining space with the ROM size. Right?

Let me demonstrate the off-by-one error for you with your original code:

#define CONFIG_CARBASE 0xffef0000
#define CONFIG_CARSIZE 0x00010000
#define CONFIG_COREBOOT_ROMSIZE_KB 1024
#if ((0xffffffff - (CONFIG_COREBOOT_ROMSIZE_KB * 1024)) < (CONFIG_CARBASE + CONFIG_CARSIZE))
#error Your current Cache-As-Ram base does not allow room to map the selected\
 chip size to memory. Please select a different chip size or move the CAR\
 base to another sane location.
#endif

Try to compile that snippet. It will error out although the
configuration is valid.
CAR is from 0xffef0000-0xffefffff (size 0x10000)
ROM is from 0xfff00000-0xffffffff (size 0x100000)

Regards,
Carl-Daniel