PCI Memory mapping

Benjamin Herrenschmidt benh at kernel.crashing.org
Mon Apr 5 08:53:55 EST 2004

On Thu, 2004-04-01 at 22:33, Marc Leeman wrote:

> No real success, until I came accross the PCMBCR register which defines
> the number of PCI to Local Memory Read and Write Buffers. When I set
> both to 1 ( & 0xF0), the data copy back is no longer needed to assure
> data consistency between the PPC and DSP (default is 4 32 byte buffers).
> include/mpc824x.h:
> #define PCMBCR          0x800000e1  /* PCI/Memory Buffer Configuration
> Register */
> cpu/mpc824x/cpu_init.c:
> Double checking this by setting both on 4 again (during cpu_init of
> ppcboot) resulted in severe MPEG data corruption. These results at least
> seems to point in the direction you suggested.
> This is the only way I seem to be able to assure data consistency (the
> kernel copy back is no longer required), but the documentation also
> suggests that (obviously) this degrades performance and is mainly used
> for debugging.0
> Still, when reading the explanations that could case this behaviour
> (copy back buffer and filling the PCMRBs) , they point to cache
> operations, which, I thought, were disabled in the linux kernel for
> these particular PCI mapped buffers.
> By adding __asm__ __volatile__("eieio"); in user and kernel space,
> which lets the CCU buffers to be flushed, no change is observed (sect.
>; CCU Responses to the Processor Transactions).
> It looks to me as if I should disable the bus snooping, but as mentioned
> before, initialising this in ppc boot inhibits the NFS boot process.
> Can this be disabled at runtime (possibly with re-enabling it) and if
> so, is this a good practice to do so?

Looks like your north bridge is horribly buggy...


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