linux-kernel - howto use ioremap

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Message-ID: <4841143E.5080003@myri.com>
Date:	Sat, 31 May 2008 11:02:54 +0200
From:	Brice Goglin <brice@...i.com>
To:	LKML <linux-kernel@...r.kernel.org>, netdev@...r.kernel.org,
	"venkatesh.pallipadi@...el.com" <venkatesh.pallipadi@...el.com>
Subject: howto use ioremap_wc?

Hello,

We're looking at using ioremap_wc() in myri10ge. No drivers seem to be
using it yet, so I'd like to get some clarification regarding ioremap_wc
failures, MTRR and so on.

What we currently do is mtrr_add() and then ioremap. Depending on the
mtrr_add() success, we use the "wc_fifo" or regular PIO with fences to
submit requests to the NIC. How are we supposed to switch this to
ioremap_wc?

Are we sure that if the arch supports _wc, it does not return success
when the underlying plain ioremap worked but setting up _wc() failed? If
so, why does it revert to ioremap_nocache when PAT isn't enabled?

Can we keep the mtrr_add() in the regular path? Or are we supposed to
drop it when the arch provides ioremap_wc and it did not fail?

thanks,
Brice

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