linux-kernel - Re: [PATCH 2.6.19 2/4] ehca: hcp_phyp.c: correct page mapping in 64k page mode

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Message-ID: <17746.52343.815568.368590@cargo.ozlabs.ibm.com>
Date:	Thu, 9 Nov 2006 17:36:39 +1100
From:	Paul Mackerras <paulus@...ba.org>
To:	Christoph Raisch <RAISCH@...ibm.com>
Cc:	Roland Dreier <rdreier@...co.com>, linuxppc-dev@...abs.org,
	linux-kernel@...r.kernel.org, openib-general@...nib.org
Subject: Re: [PATCH 2.6.19 2/4] ehca: hcp_phyp.c: correct page mapping in 64k
	page mode

Christoph Raisch writes:

> ioremap maps 4k pages on 4k kernels and on 64k pages on 64k kernels. So far
> the theory.
> 
> This is true for memory.

And for I/O. :)  ioremap updates the (Linux) page tables that map the
vmalloc/ioremap area, and that is at page granularity.  So there is in
fact no difference in the end result in the page tables whether you
ask to map a small amount inside a page, or the whole page.

> On POWER the ebus memory is mapped by H_ENTER.
> The hypervisor checks for 4k page size on H_ENTER, reason see above.

The next part of the story is that the low-level MMU code on System-P
(pSeries) machines only does the H_ENTER when you access an I/O
mapping.  It does H_ENTER for 4k pages for non-cacheable mappings,
and it only does the H_ENTER for the 4k subpages of a 64k page that
the kernel actually accesses.

So Roland is correct in his comment about how ioremap is called.

Regards,
Paul.
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