Re: [PATCH] X86: MM: Add PAT Type write-through in combination with mtrr

Andreas Werner
2013-10-28T10:34:36Z

On Mon, Oct 28, 2013 at 11:17:49AM +0100, Ingo Molnar wrote:
>
> * Andreas Werner <wernerandy@gmx.de> wrote:
>
> > > IOW, you probably could use a WC buffer here too, as it would
> > > combine the writes coming from the FPGA.
> > >
> > > Btw, there's also mtrr_add(..., MTRR_TYPE_WRTHROUGH, ) if you
> > > must use a WT thing. Have you tried that?
> >
> > For reading i need to map the mmio with attributes that allow
> > cache-line read. Therefore i use WT. For the Virtual address i use
> > ioremap_cache in combination with this patch to get an effective
> > memory type of "Write-Through". This allows me to read from the
> > mmio with "PCIe burst". The write behaviour to this region do not
> > matter.
>
> And regular write-back cacheable isn't sufficient because the CPU
> could do things like prefetch your range automatically?
>
> If the reads are for packet data and not for commands, WB could
> still be beneficial as it should allow even higher bandwidth. (For
> non-data with real semantics WB is probably not good.)
>
> Thanks,
>
> Ingo

Yes the reads are only for packet data, the commands or configuration
registers are mapped non cachable.

I´ve tried WB, but on PCIe Tracer i could not see any burst access.
Thats the reason why i have created this patch.

Is there a chance to get this patch into the kernel? Or
is this solution so special?

Regards
Andy