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+
+PAT (Page Attribute Table)
+
+x86 Page Attribute Table (PAT) allows for setting the memory attribute at the
+page level granularity. PAT is complementary to the MTRR settings which allows
+for setting of memory types over physical address ranges. However, PAT is
+more flexible than MTRR due to its capability to set attributes at page level
+and also due to the fact that there are no hardware limitations on number of
+such attribute settings allowed. Added flexibility comes with guidelines for
+not having memory type aliasing for the same physical memory with multiple
+virtual addresses.
+
+PAT allows for different types of memory attributes. The most commonly used
+ones that will be supported at this time are Write-back, Uncached,
+Write-combined and Uncached Minus.
+
+
+PAT APIs
+--------
+
+There are many different APIs in the kernel that allows setting of memory
+attributes at the page level. In order to avoid aliasing, these interfaces
+should be used thoughtfully. Below is a table of interfaces available,
+their intended usage and their memory attribute relationships. Internally,
+these APIs use a reserve_memtype()/free_memtype() interface on the physical
+address range to avoid any aliasing.
+
+
+-------------------------------------------------------------------
+API                    |    RAM   |  ACPI,...  |  Reserved/Holes  |
+-----------------------|----------|------------|------------------|
+                       |          |            |                  |
+ioremap                |    --    |    UC-     |       UC-        |
+                       |          |            |                  |
+ioremap_cache          |    --    |    WB      |       WB         |
+                       |          |            |                  |
+ioremap_nocache        |    --    |    UC-     |       UC-        |
+                       |          |            |                  |
+ioremap_wc             |    --    |    --      |       WC         |
+                       |          |            |                  |
+set_memory_uc          |    UC-   |    --      |       --         |
+ set_memory_wb         |          |            |                  |
+                       |          |            |                  |
+set_memory_wc          |    WC    |    --      |       --         |
+ set_memory_wb         |          |            |                  |
+                       |          |            |                  |
+pci sysfs resource     |    --    |    --      |       UC-        |
+                       |          |            |                  |
+pci sysfs resource_wc  |    --    |    --      |       WC         |
+ is IORESOURCE_PREFETCH|          |            |                  |
+                       |          |            |                  |
+pci proc               |    --    |    --      |       UC-        |
+ !PCIIOC_WRITE_COMBINE |          |            |                  |
+                       |          |            |                  |
+pci proc               |    --    |    --      |       WC         |
+ PCIIOC_WRITE_COMBINE  |          |            |                  |
+                       |          |            |                  |
+/dev/mem               |    --    |  WB/WC/UC- |    WB/WC/UC-     |
+ read-write            |          |            |                  |
+                       |          |            |                  |
+/dev/mem               |    --    |    UC-     |       UC-        |
+ mmap SYNC flag        |          |            |                  |
+                       |          |            |                  |
+/dev/mem               |    --    |  WB/WC/UC- |    WB/WC/UC-     |
+ mmap !SYNC flag       |          |(from exist-|  (from exist-    |
+ and                   |          |  ing alias)|    ing alias)    |
+ any alias to this area|          |            |                  |
+                       |          |            |                  |
+/dev/mem               |    --    |    WB      |       WB         |
+ mmap !SYNC flag       |          |            |                  |
+ no alias to this area |          |            |                  |
+ and                   |          |            |                  |
+ MTRR says WB          |          |            |                  |
+                       |          |            |                  |
+/dev/mem               |    --    |    --      |       UC-        |
+ mmap !SYNC flag       |          |            |                  |
+ no alias to this area |          |            |                  |
+ and                   |          |            |                  |
+ MTRR says !WB         |          |            |                  |
+                       |          |            |                  |
+-------------------------------------------------------------------
+
+Advanced APIs for drivers
+-------------------------
+A. Exporting pages to users 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, drivers 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 works with IO (non RAM) regions. If driver
+wants to export a 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
+are strictly enforced by the kernel. Some others are not really enforced
+today, but may be enforced in future.
+
+For ioremap and pci access through /sys or /proc - The actual type returned
+can be more restrictive, in case of any existing aliasing for that address.
+For example: If there is an existing uncached mapping, a new ioremap_wc can
+return uncached mapping in place of write-combine requested.
+
+set_memory_[uc|wc] and set_memory_wb should be used in pairs, where driver will
+first make a region uc or wc and switch it back to wb after use.
+
+Over time writes to /proc/mtrr will be deprecated in favor of using PAT based
+interfaces. Users writing to /proc/mtrr are suggested to use above interfaces.
+
+Drivers should use ioremap_[uc|wc] to access PCI BARs with [uc|wc] access
+types.
+
+Drivers should use set_memory_[uc|wc] to set access type for RAM ranges.
+
+
+PAT debugging
+-------------
+
+With CONFIG_DEBUG_FS enabled, PAT memtype list can be examined by
+
+# mount -t debugfs debugfs /sys/kernel/debug
+# cat /sys/kernel/debug/x86/pat_memtype_list
+PAT memtype list:
+uncached-minus @ 0x7fadf000-0x7fae0000
+uncached-minus @ 0x7fb19000-0x7fb1a000
+uncached-minus @ 0x7fb1a000-0x7fb1b000
+uncached-minus @ 0x7fb1b000-0x7fb1c000
+uncached-minus @ 0x7fb1c000-0x7fb1d000
+uncached-minus @ 0x7fb1d000-0x7fb1e000
+uncached-minus @ 0x7fb1e000-0x7fb25000
+uncached-minus @ 0x7fb25000-0x7fb26000
+uncached-minus @ 0x7fb26000-0x7fb27000
+uncached-minus @ 0x7fb27000-0x7fb28000
+uncached-minus @ 0x7fb28000-0x7fb2e000
+uncached-minus @ 0x7fb2e000-0x7fb2f000
+uncached-minus @ 0x7fb2f000-0x7fb30000
+uncached-minus @ 0x7fb31000-0x7fb32000
+uncached-minus @ 0x80000000-0x90000000
+
+This list shows physical address ranges and various PAT settings used to
+access those physical address ranges.
+
+Another, more verbose way of getting PAT related debug messages is with
+"debugpat" boot parameter. With this parameter, various debug messages are
+printed to dmesg log.
+