io-mapping: Provide iomap_local variant

Similar to kmap local provide a iomap local variant which only disables
migration, but neither disables pagefaults nor preemption.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20201118204007.561220818@linutronix.de

1 files changed, 45 insertions, 29 deletions

diff --git a/Documentation/driver-api/io-mapping.rst b/Documentation/driver-api/io-mapping.rst
index e33b88268554..a0cfb15988df 100644
--- a/Documentation/driver-api/io-mapping.rst
+++ b/Documentation/driver-api/io-mapping.rst
@@ -20,55 +20,71 @@ A mapping object is created during driver initialization using::
 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.
+This _wc variant provides a mapping which may only be used with
+io_mapping_map_atomic_wc(), io_mapping_map_local_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::
+With this mapping object, individual pages can be mapped either temporarily
+or long term, depending on the requirements. Of course, temporary maps are
+more efficient. They come in two flavours::
+
+	void *io_mapping_map_local_wc(struct io_mapping *mapping,
+				      unsigned long offset)
 
 	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.
+'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
+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.
+Temporary mappings are only valid in the context of the caller. The mapping
+is not guaranteed to be globaly visible.
 
-::
+io_mapping_map_local_wc() has a side effect on X86 32bit as it disables
+migration to make the mapping code work. No caller can rely on this side
+effect.
 
-	void io_mapping_unmap_atomic(void *vaddr)
+io_mapping_map_atomic_wc() has the side effect of disabling preemption and
+pagefaults. Don't use in new code. Use io_mapping_map_local_wc() instead.
 
-'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.
+Nested mappings need to be undone in reverse order because the mapping
+code uses a stack for keeping track of them::
 
-If you need to sleep while holding the lock, you can use the non-atomic
-variant, although they may be significantly slower.
+ addr1 = io_mapping_map_local_wc(map1, offset1);
+ addr2 = io_mapping_map_local_wc(map2, offset2);
+ ...
+ io_mapping_unmap_local(addr2);
+ io_mapping_unmap_local(addr1);
 
-::
+The mappings are released with::
+
+	void io_mapping_unmap_local(void *vaddr)
+	void io_mapping_unmap_atomic(void *vaddr)
+
+'vaddr' must be the value returned by the last io_mapping_map_local_wc() or
+io_mapping_map_atomic_wc() call. This unmaps the specified mapping and
+undoes the side effects of the mapping functions.
+
+If you need to sleep while holding a mapping, you can use the regular
+variant, although this 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.
-
+This works like io_mapping_map_atomic/local_wc() except it has no side
+effects and the pointer is globaly visible.
 
-::
+The mappings are released with::
 
 	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.
+Use for pages mapped with io_mapping_map_wc().
 
 At driver close time, the io_mapping object must be freed::