On Mon, Oct 10, 2011 at 11:24:05AM +0200, Benjamin Herrenschmidt wrote:

On Mon, 2011-10-10 at 11:16 +0200, Eli Cohen wrote:

quoted

Until then I think we need to have the logic working right on ppc and
measure if blue flame buys us any improvement in ppc. If that's not
the case (e.g because write combining is not working), then maybe we
should avoid using blueflame in ppc.
Could any of the guys from IBM check this and give us feedback?

I don't have the necessary hardware myself to test that but maybe Thadeu
can.

Note that for WC to work, things must be mapped non-guarded. You can do
that by using ioremap_prot() with pgprot_noncached_wc(PAGE_KERNEL) or
ioremap_wc() (dunno how "generic" the later is).

I use the io mapping API:

at driver statrt:
        priv->bf_mapping = io_mapping_create_wc(bf_start, bf_len);
        if (!priv->bf_mapping)
                err = -ENOMEM;

and then:
        uar->bf_map = io_mapping_map_wc(priv->bf_mapping, uar->index << PAGE_SHIFT);

        
Will this work on ppc?

quoted

From there, you should get write combining provided that you don't have

barriers between every access (ie those copy operations in their current
form should do the trick).

Cheers,
Ben.

quoted

quoted

Maybe it's time for us to revive those discussions about providing a
good set of relaxed MMIO accessors with explicit barriers :-)

Cheers,
Ben.