Re: [PATCH] Fix eisa_mmap evaluation, add memory existence check

[Christian Franke]

Robert Millan wrote:
> On Tue, Oct 23, 2007 at 09:06:16PM +0200, Christian Franke wrote:
>   
> > +/* Check memory address */
> > +static int
> > +addr_is_valid (grub_addr_t addr)
> > +{
> > +  volatile unsigned char * p = (volatile unsigned char *)addr;
> > +  unsigned char x, y;
> > +  x = *p;
> > +  *p = x ^ 0xcf;
> > +  y = *p;
> > +  *p = x;
> > +  return y == (x ^ 0xcf);
> > +}
> > +
> >     
>
> I have a feeling this might be dangerous.  Any comments on the warnings
> listed here:
>
>   
> http://www.osdev.org/wiki/How_Do_I_Determine_The_Amount_Of_RAM#Counting_RAM_by_direct_probing
>
>   ?
>
> Specially the bit about memory-mapped PCI.
>   

This code does not check any memory-mapped PCI. It does only check the 
boundary returned by the BIOS memory map evaluation code (which didn't 
work in the E801 case).

Definitely not mandatory, but a IMO recommended assert-type check for 
experimental code.



> Besides, if we really want it, perhaps it should be in a separate file so that
> other ports can use it if it's needed ?
>   

Good point.


> Ah, and why 0xcf instead of 0xff ?
>
>   

... or 0xaa or 0x55.

c.f. :-)