[parisc-linux-cvs] U bit and memory mapped IO

[Thomas Bogendoerfer]

On Sun, Mar 03, 2002 at 11:04:07PM -0700, John Marvin wrote:
> > Thomas Bogendoerfer wrote:
> > >       arch/parisc/kernel: entry.S
> > >
> > > Log message:
> > > set U bit in TLB for uncached memory. This is needed for mmaped IO space
> > > (framebuffers)
> >
> > I'm surprised this is needed for "F-space". (ie 0xf...).
> > My understanding is the CPU is hardcoded to not cache F-space.
> > I thought this was true of both PA1.1 and PA2.0 machines.
> > Can anyone confirm/deny this?
> 
> U bit is only necessary for F space access on PCXL2. It is not necessary
> on any other processor.

then the PA8000 in my C160 is sort of a strange CPU ? Without the U bit
set it HPMCs, when accessing the framebuffer from userspace. I read the
PA2.0 docs and there it is noted, that the behaviour of an unset U-bit for
IO memory space is undefined.

Thomas.