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1 | |
| 2 | PAT (Page Attribute Table) |
| 3 | |
| 4 | x86 Page Attribute Table (PAT) allows for setting the memory attribute at the |
| 5 | page level granularity. PAT is complementary to the MTRR settings which allows |
| 6 | for setting of memory types over physical address ranges. However, PAT is |
| 7 | more flexible than MTRR due to its capability to set attributes at page level |
| 8 | and also due to the fact that there are no hardware limitations on number of |
| 9 | such attribute settings allowed. Added flexibility comes with guidelines for |
| 10 | not having memory type aliasing for the same physical memory with multiple |
| 11 | virtual addresses. |
| 12 | |
| 13 | PAT allows for different types of memory attributes. The most commonly used |
| 14 | ones that will be supported at this time are Write-back, Uncached, |
| 15 | Write-combined and Uncached Minus. |
| 16 | |
| 17 | There are many different APIs in the kernel that allows setting of memory |
| 18 | attributes at the page level. In order to avoid aliasing, these interfaces |
| 19 | should be used thoughtfully. Below is a table of interfaces available, |
| 20 | their intended usage and their memory attribute relationships. Internally, |
| 21 | these APIs use a reserve_memtype()/free_memtype() interface on the physical |
| 22 | address range to avoid any aliasing. |
| 23 | |
| 24 | |
| 25 | ------------------------------------------------------------------- |
| 26 | API | RAM | ACPI,... | Reserved/Holes | |
| 27 | -----------------------|----------|------------|------------------| |
| 28 | | | | | |
| 29 | ioremap | -- | UC | UC | |
| 30 | | | | | |
| 31 | ioremap_cache | -- | WB | WB | |
| 32 | | | | | |
| 33 | ioremap_nocache | -- | UC | UC | |
| 34 | | | | | |
| 35 | ioremap_wc | -- | -- | WC | |
| 36 | | | | | |
| 37 | set_memory_uc | UC | -- | -- | |
| 38 | set_memory_wb | | | | |
| 39 | | | | | |
| 40 | set_memory_wc | WC | -- | -- | |
| 41 | set_memory_wb | | | | |
| 42 | | | | | |
| 43 | pci sysfs resource | -- | -- | UC | |
| 44 | | | | | |
| 45 | pci sysfs resource_wc | -- | -- | WC | |
| 46 | is IORESOURCE_PREFETCH| | | | |
| 47 | | | | | |
| 48 | pci proc | -- | -- | UC | |
| 49 | !PCIIOC_WRITE_COMBINE | | | | |
| 50 | | | | | |
| 51 | pci proc | -- | -- | WC | |
| 52 | PCIIOC_WRITE_COMBINE | | | | |
| 53 | | | | | |
| 54 | /dev/mem | -- | UC | UC | |
| 55 | read-write | | | | |
| 56 | | | | | |
| 57 | /dev/mem | -- | UC | UC | |
| 58 | mmap SYNC flag | | | | |
| 59 | | | | | |
| 60 | /dev/mem | -- | WB/WC/UC | WB/WC/UC | |
| 61 | mmap !SYNC flag | |(from exist-| (from exist- | |
| 62 | and | | ing alias)| ing alias) | |
| 63 | any alias to this area| | | | |
| 64 | | | | | |
| 65 | /dev/mem | -- | WB | WB | |
| 66 | mmap !SYNC flag | | | | |
| 67 | no alias to this area | | | | |
| 68 | and | | | | |
| 69 | MTRR says WB | | | | |
| 70 | | | | | |
| 71 | /dev/mem | -- | -- | UC_MINUS | |
| 72 | mmap !SYNC flag | | | | |
| 73 | no alias to this area | | | | |
| 74 | and | | | | |
| 75 | MTRR says !WB | | | | |
| 76 | | | | | |
| 77 | ------------------------------------------------------------------- |
| 78 | |
| 79 | Notes: |
| 80 | |
| 81 | -- in the above table mean "Not suggested usage for the API". Some of the --'s |
| 82 | are strictly enforced by the kernel. Some others are not really enforced |
| 83 | today, but may be enforced in future. |
| 84 | |
| 85 | For ioremap and pci access through /sys or /proc - The actual type returned |
| 86 | can be more restrictive, in case of any existing aliasing for that address. |
| 87 | For example: If there is an existing uncached mapping, a new ioremap_wc can |
| 88 | return uncached mapping in place of write-combine requested. |
| 89 | |
| 90 | set_memory_[uc|wc] and set_memory_wb should be used in pairs, where driver will |
| 91 | first make a region uc or wc and switch it back to wb after use. |
| 92 | |
| 93 | Over time writes to /proc/mtrr will be deprecated in favor of using PAT based |
| 94 | interfaces. Users writing to /proc/mtrr are suggested to use above interfaces. |
| 95 | |
| 96 | Drivers should use ioremap_[uc|wc] to access PCI BARs with [uc|wc] access |
| 97 | types. |
| 98 | |
| 99 | Drivers should use set_memory_[uc|wc] to set access type for RAM ranges. |
| 100 | |
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