Message-Id: <20200302183105.27628-2-pragat.pandya@gmail.com>
Date:   Tue,  3 Mar 2020 00:01:04 +0530
From:   Pragat Pandya <pragat.pandya@...il.com>
To:     corbet@....net
Cc:     linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-kernel-mentees@...ts.linuxfoundation.org,
        skhan@...uxfoundation.org, Pragat Pandya <pragat.pandya@...il.com>
Subject: [PATCH v2 1/2] Documentation: Add io-mapping.rst to driver-api manual

Add io-mapping.rst under Documentation/driver-api and reference it from
Sphinx TOC Tree present in Documentation/driver-api/index.rst

Signed-off-by: Pragat Pandya <pragat.pandya@...il.com>
---
 Documentation/driver-api/index.rst      |  1 +
 Documentation/driver-api/io-mapping.rst | 97 +++++++++++++++++++++++++
 2 files changed, 98 insertions(+)
 create mode 100644 Documentation/driver-api/io-mapping.rst

diff --git a/Documentation/driver-api/index.rst b/Documentation/driver-api/index.rst
index 0ebe205efd0c..e9da95004632 100644
--- a/Documentation/driver-api/index.rst
+++ b/Documentation/driver-api/index.rst
@@ -79,6 +79,7 @@ available subsections can be seen below.
    ipmb
    isa
    isapnp
+   io-mapping
    generic-counter
    lightnvm-pblk
    memory-devices/index
diff --git a/Documentation/driver-api/io-mapping.rst b/Documentation/driver-api/io-mapping.rst
new file mode 100644
index 000000000000..a966239f04e4
--- /dev/null
+++ b/Documentation/driver-api/io-mapping.rst
@@ -0,0 +1,97 @@
+========================
+The io_mapping functions
+========================
+
+API
+===
+
+The io_mapping functions in linux/io-mapping.h provide an abstraction for
+efficiently mapping small regions of an I/O device to the CPU. The initial
+usage is to support the large graphics aperture on 32-bit processors where
+ioremap_wc cannot be used to statically map the entire aperture to the CPU
+as it would consume too much of the kernel address space.
+
+A mapping object is created during driver initialization using::
+
+	struct io_mapping *io_mapping_create_wc(unsigned long base,
+						unsigned long size)
+
+'base' is the bus address of the region to be made
+mappable, while 'size' indicates how large a mapping region to
+enable. Both are in bytes.
+
+This _wc variant provides a mapping which may only be used
+with the io_mapping_map_atomic_wc or io_mapping_map_wc.
+
+With this mapping object, individual pages can be mapped either atomically
+or not, depending on the necessary scheduling environment. Of course, atomic
+maps are more efficient::
+
+	void *io_mapping_map_atomic_wc(struct io_mapping *mapping,
+				       unsigned long offset)
+
+'offset' is the offset within the defined mapping region.
+Accessing addresses beyond the region specified in the
+creation function yields undefined results. Using an offset
+which is not page aligned yields an undefined result. The
+return value points to a single page in CPU address space.
+
+This _wc variant returns a write-combining map to the
+page and may only be used with mappings created by
+io_mapping_create_wc
+
+Note that the task may not sleep while holding this page
+mapped.
+
+::
+
+	void io_mapping_unmap_atomic(void *vaddr)
+
+'vaddr' must be the value returned by the last
+io_mapping_map_atomic_wc call. This unmaps the specified
+page and allows the task to sleep once again.
+
+If you need to sleep while holding the lock, you can use the non-atomic
+variant, although they may be significantly slower.
+
+::
+
+	void *io_mapping_map_wc(struct io_mapping *mapping,
+				unsigned long offset)
+
+This works like io_mapping_map_atomic_wc except it allows
+the task to sleep while holding the page mapped.
+
+
+::
+
+	void io_mapping_unmap(void *vaddr)
+
+This works like io_mapping_unmap_atomic, except it is used
+for pages mapped with io_mapping_map_wc.
+
+At driver close time, the io_mapping object must be freed::
+
+	void io_mapping_free(struct io_mapping *mapping)
+
+Current Implementation
+======================
+
+The initial implementation of these functions uses existing mapping
+mechanisms and so provides only an abstraction layer and no new
+functionality.
+
+On 64-bit processors, io_mapping_create_wc calls ioremap_wc for the whole
+range, creating a permanent kernel-visible mapping to the resource. The
+map_atomic and map functions add the requested offset to the base of the
+virtual address returned by ioremap_wc.
+
+On 32-bit processors with HIGHMEM defined, io_mapping_map_atomic_wc uses
+kmap_atomic_pfn to map the specified page in an atomic fashion;
+kmap_atomic_pfn isn't really supposed to be used with device pages, but it
+provides an efficient mapping for this usage.
+
+On 32-bit processors without HIGHMEM defined, io_mapping_map_atomic_wc and
+io_mapping_map_wc both use ioremap_wc, a terribly inefficient function which
+performs an IPI to inform all processors about the new mapping. This results
+in a significant performance penalty.
-- 
2.17.1

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