***SPAM*** Re: [ofa-general] [PATCH] IB/mlx4: Use pgprot_writecombine() for BlueFlame pages

[Jack Morgenstein]

On Sunday 29 March 2009 20:06, Or Gerlitz wrote:
> On Sun, Mar 29, 2009 at 7:35 PM, Roland Dreier <rdreier at cisco.com> wrote:
> > This should bring mainline kernel small message latency to the same
> > level that OFED gets with the PAT support it hacks in.
> 
> Interesting... so the ofed support for blue flame (we are talking on
> PIO, correct?) is a hack that magically works even on 2.6.18 et al
> older kernels? does it have any side effects?
> 
> Or.
The problem was that it was not standardized for all drivers (as a set
of kernel procedures available for all).
The hack worked by replacing one of the entries (modes) in the PAT with
the WC (write combining) mode, and then allocating memory with
pgprot_wc() to set the PAT reference to the WC PAT.

The mlx4 driver does the PAT replacement at driver start, and undoes the
replacement at driver unload.  However, if other device drivers use the
same hack, then things get messed up -- the driver unload creates problems
for these other drivers (or their unload creates problems for us).

You can see the write combining driver code in file:
  kernel_patches/fixes/mlx4_0010_add_wc.patch

- Jack