x86: PAT: update documentation to cover pgprot and remap_pfn related changes - v3
Impact: Documentation only. Add documentation related to pgprot_* change. Signed-off-by: Venkatesh Pallipadi <venkatesh.pallipadi@intel.com> Signed-off-by: Suresh Siddha <suresh.b.siddha@intel.com> Signed-off-by: H. Peter Anvin <hpa@zytor.com>
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@ -80,6 +80,30 @@ pci proc | -- | -- | WC |
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-------------------------------------------------------------------
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Advanced APIs for drivers
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-------------------------
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A. Exporting pages to user with remap_pfn_range, io_remap_pfn_range,
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vm_insert_pfn
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Drivers wanting to export some pages to userspace, do it by using mmap
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interface and a combination of
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1) pgprot_noncached()
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2) io_remap_pfn_range() or remap_pfn_range() or vm_insert_pfn()
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With pat support, a new API pgprot_writecombine is being added. So, driver can
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continue to use the above sequence, with either pgprot_noncached() or
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pgprot_writecombine() in step 1, followed by step 2.
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In addition, step 2 internally tracks the region as UC or WC in memtype
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list in order to ensure no conflicting mapping.
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Note that this set of APIs only work with IO (non RAM) regions. If driver
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wants to export RAM region, it has to do set_memory_uc() or set_memory_wc()
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as step 0 above and also track the usage of those pages and use set_memory_wb()
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before the page is freed to free pool.
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Notes:
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-- in the above table mean "Not suggested usage for the API". Some of the --'s
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