netdev - Re: [PATCH] mlx4_en: fix transmit of packages when blue frame is enabled

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Date:	Mon, 10 Oct 2011 11:29:26 +0200
From:	Eli Cohen <eli@....mellanox.co.il>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:	David Laight <David.Laight@...LAB.COM>, netdev@...r.kernel.org,
	Eli Cohen <eli@...lanox.co.il>, linuxppc-dev@...ts.ozlabs.org,
	Thadeu Lima de Souza Cascardo <cascardo@...ux.vnet.ibm.com>,
	Yevgeny Petrilin <yevgenyp@...lanox.co.il>
Subject: Re: [PATCH] mlx4_en: fix transmit of packages when blue frame is
 enabled

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:
> 
> > 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?

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