[patch 7/7] x86 PAT: update documentation to cover pgprot and remap_pfn related changes - v3

[venkatesh . pallipadi]

Add documentation related to pgprot_* change.

Signed-off-by: Venkatesh Pallipadi <venkatesh.pallipadi@xxxxxxxxx>
Signed-off-by: Suresh Siddha <suresh.b.siddha@xxxxxxxxx>
---
 Documentation/x86/pat.txt |   24 ++++++++++++++++++++++++
 1 file changed, 24 insertions(+)

Index: linux-2.6/Documentation/x86/pat.txt
===================================================================
--- linux-2.6.orig/Documentation/x86/pat.txt	2008-12-17 15:01:50.000000000 -0800
+++ linux-2.6/Documentation/x86/pat.txt	2008-12-17 17:23:16.000000000 -0800
@@ -80,6 +80,30 @@ pci proc               |    --    |    -
                        |          |            |                  |
 -------------------------------------------------------------------
 
+Advanced APIs for drivers
+-------------------------
+A. Exporting pages to user with remap_pfn_range, io_remap_pfn_range,
+vm_insert_pfn
+
+Drivers wanting to export some pages to userspace, do it by using mmap
+interface and a combination of
+1) pgprot_noncached()
+2) io_remap_pfn_range() or remap_pfn_range() or vm_insert_pfn()
+
+With pat support, a new API pgprot_writecombine is being added. So, driver can
+continue to use the above sequence, with either pgprot_noncached() or
+pgprot_writecombine() in step 1, followed by step 2.
+
+In addition, step 2 internally tracks the region as UC or WC in memtype
+list in order to ensure no conflicting mapping.
+
+Note that this set of APIs only work with IO (non RAM) regions. If driver
+wants to export RAM region, it has to do set_memory_uc() or set_memory_wc()
+as step 0 above and also track the usage of those pages and use set_memory_wb()
+before the page is freed to free pool.
+
+
+
 Notes:
 
 -- in the above table mean "Not suggested usage for the API". Some of the --'s

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