From: "Luis R. Rodriguez" <mcgrof@kernel.org>
To: bp@alien8.de, mingo@kernel.org, tglx@linutronix.de, hpa@zytor.com
Cc: toshi.kani@hp.com, paulmck@linux.vnet.ibm.com,
airlied@redhat.com, benh@kernel.crashing.org, mst@redhat.com,
vinod.koul@intel.com, jgross@suse.com, daniel.vetter@ffwll.ch,
luto@amacapital.net, linux-fbdev@vger.kernel.org,
linux-arch@vger.kernel.org, linux-kernel@vger.kernel.org,
x86@kernel.org, linux-doc@vger.kernel.org, corbet@lwn.net,
"Luis R. Rodriguez" <mcgrof@kernel.org>
Subject: [PATCH] x86: PAT: Documentation: update overlapping ioremap hack recommendation
Date: Thu, 3 Mar 2016 13:21:48 -0800 [thread overview]
Message-ID: <1457040108-27358-1-git-send-email-mcgrof@kernel.org> (raw)
Message-ID: <20160303212148.bqpFbmA6tkO8z5ZH5o_lB2QD5R3O7aQBWS-C6rd_vw0@z> (raw)
The current documentation refers to using set_memor_wc() as a
possible hole strategy when you have overlapping ioremap() regions,
that's incorrect as set_memory_*() helpers can only be used on RAM,
not IO memory. This fixes that, and updates the documention to
*strongly* discourage overlapping ioremap() memory uses, but also
documents a possible solution should there really be no other
option.
Signed-off-by: Luis R. Rodriguez <mcgrof@kernel.org>
---
Documentation/x86/pat.txt | 25 ++++++++++++++-----------
1 file changed, 14 insertions(+), 11 deletions(-)
diff --git a/Documentation/x86/pat.txt b/Documentation/x86/pat.txt
index 54944c71b819..8a26b4cdccf6 100644
--- a/Documentation/x86/pat.txt
+++ b/Documentation/x86/pat.txt
@@ -113,18 +113,21 @@ MTRR effects on PAT / non-PAT systems
-------------------------------------
The following table provides the effects of using write-combining MTRRs when
-using ioremap*() calls on x86 for both non-PAT and PAT systems. Ideally
-mtrr_add() usage will be phased out in favor of arch_phys_wc_add() which will
-be a no-op on PAT enabled systems. The region over which a arch_phys_wc_add()
+using ioremap*() calls on x86 for both non-PAT and PAT systems. As of v4.3
+mtrr_add() has been phased out in favor of arch_phys_wc_add(), these calls are
+a no-op on PAT enabled systems. The region over which an arch_phys_wc_add()
is made, should already have been ioremapped with WC attributes or PAT entries,
-this can be done by using ioremap_wc() / set_memory_wc(). Devices which
-combine areas of IO memory desired to remain uncacheable with areas where
-write-combining is desirable should consider use of ioremap_uc() followed by
-set_memory_wc() to white-list effective write-combined areas. Such use is
-nevertheless discouraged as the effective memory type is considered
-implementation defined, yet this strategy can be used as last resort on devices
-with size-constrained regions where otherwise MTRR write-combining would
-otherwise not be effective.
+this can be done by using ioremap_wc(). Devices which combine areas of IO
+memory desired to remain uncacheable with areas where write-combining is
+desirable should consider use of ioremap_wc() followed by an overlapping
+ioremap_uc() "hole". For an example of this strategy refer to commit 3cc2dac5be
+("drivers/video/fbdev/atyfb: Replace MTRR UC hole with strong UC").
+Such use is nevertheless heavily discouraged as the effective memory type is
+considered implementation defined. This strategy should only be used used as a
+last resort measure.
+
+Note you cannot use set_memory_*() helpers on ioremap'd regions, even though
+its use currently gives no hint of an error.
----------------------------------------------------------------------
MTRR Non-PAT PAT Linux ioremap value Effective memory type
--
2.7.2
next reply other threads:[~2016-03-03 21:22 UTC|newest]
Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-03-03 21:21 Luis R. Rodriguez [this message]
2016-03-03 21:21 ` [PATCH] x86: PAT: Documentation: update overlapping ioremap hack recommendation Luis R. Rodriguez
2016-03-03 21:42 ` Paul E. McKenney
2016-03-03 21:42 ` Paul E. McKenney
2016-03-04 19:23 ` Luis R. Rodriguez
2016-03-04 19:23 ` Luis R. Rodriguez
2016-03-04 21:09 ` Paul E. McKenney
2016-03-04 22:45 ` [PATCH v2] x86: PAT: Documentation: rewrite "MTRR effects on PAT / non-PAT systems" Luis R. Rodriguez
2016-03-04 22:45 ` Luis R. Rodriguez
2016-03-05 0:03 ` Paul E. McKenney
2016-03-05 0:03 ` Paul E. McKenney
2016-03-05 1:03 ` Luis R. Rodriguez
2016-03-05 1:03 ` Luis R. Rodriguez
2016-03-05 4:39 ` Elliott, Robert (Persistent Memory)
2016-03-15 22:21 ` Luis R. Rodriguez
2016-03-15 22:21 ` Luis R. Rodriguez
2016-03-05 11:52 ` Ingo Molnar
2016-03-05 11:52 ` Ingo Molnar
2016-03-15 22:24 ` Luis R. Rodriguez
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