[PATCH v3 1/3] mtd: maps: only point() maps read through the simple accessors

[Orgad Shaneh <orgads@gmail.com>]

map_is_linear() only checks that a map has a physical address. Map
drivers that install their own accessors keep one, and a pointer
bypasses whatever those accessors do:

- physmap's IXP4xx support: the expansion bus only takes 16-bit
  accesses (the reason it cannot use memcpy_fromio()), and on
  little-endian kernels every access also flips address bit 1 and
  swaps bytes;
- physmap's Gemini support enables the flash pins only inside its
  accessors;
- lantiq-flash serializes the external bus against PCI and copies byte
  by byte because the EBU cannot take a prefetching memcpy;
- cobalt-flash reads at an offset inside a PCI BAR while leaving phys 0.

cfi_cmdset_0001 hands such pointers to jffs2 today, and the IXP4xx
boards carry Intel StrataFlash. Add map_is_simple(), true when the map
reads through the simple accessors (always true without
CONFIG_MTD_COMPLEX_MAPPINGS, where the accessors are inline), and
require it before enabling point(). Maps with custom accessors fall
back to copying through them; on big-endian IXP4xx, where pointed reads
happened to work, jffs2 mounts get slower.

Fixes: 9d3b5086f6d4 ("mtd: physmap_of_gemini: Handle pin control")
Fixes: 2aba2f2a704d ("mtd: physmap_of: add a hook for Intel IXP4xx flash probing")
Assisted-by: LLM
Signed-off-by: Orgad Shaneh <orgads@gmail.com>
Cc: Linus Walleij <linusw@kernel.org>
Cc: Imre Kaloz <kaloz@openwrt.org>
Cc: Hans Ulli Kroll <ulli.kroll@googlemail.com>
Cc: John Crispin <john@phrozen.org>
---
v3: Assisted-by in the documented form.
v2: no change.

 drivers/mtd/chips/cfi_cmdset_0001.c |  2 +-
 drivers/mtd/maps/map_funcs.c        | 12 ++++++++++++
 include/linux/mtd/map.h             |  2 ++
 3 files changed, 15 insertions(+), 1 deletion(-)

diff --git a/drivers/mtd/chips/cfi_cmdset_0001.c b/drivers/mtd/chips/cfi_cmdset_0001.c
index b73596a..3f3364c 100644
--- a/drivers/mtd/chips/cfi_cmdset_0001.c
+++ b/drivers/mtd/chips/cfi_cmdset_0001.c
@@ -299,7 +299,7 @@ static void fixup_LH28F640BF(struct mtd_info *mtd)
 static void fixup_use_point(struct mtd_info *mtd)
 {
 	struct map_info *map = mtd->priv;
-	if (!mtd->_point && map_is_linear(map)) {
+	if (!mtd->_point && map_is_linear(map) && map_is_simple(map)) {
 		mtd->_point   = cfi_intelext_point;
 		mtd->_unpoint = cfi_intelext_unpoint;
 	}
diff --git a/drivers/mtd/maps/map_funcs.c b/drivers/mtd/maps/map_funcs.c
index 1a4add9..ccf1dbf 100644
--- a/drivers/mtd/maps/map_funcs.c
+++ b/drivers/mtd/maps/map_funcs.c
@@ -41,5 +41,17 @@ void simple_map_init(struct map_info *map)
 }
 
 EXPORT_SYMBOL(simple_map_init);
+
+/*
+ * True when every access goes through the simple accessors, so a reader
+ * may use map->virt directly; a map with its own accessors (byte swaps,
+ * bus locking, address fixups) must not be pointed at.
+ */
+bool map_is_simple(struct map_info *map)
+{
+	return map->read == simple_map_read &&
+	       map->copy_from == simple_map_copy_from;
+}
+EXPORT_SYMBOL(map_is_simple);
 MODULE_DESCRIPTION("Out-of-line map I/O");
 MODULE_LICENSE("GPL");
diff --git a/include/linux/mtd/map.h b/include/linux/mtd/map.h
index 75b0b2a..ebee3fe 100644
--- a/include/linux/mtd/map.h
+++ b/include/linux/mtd/map.h
@@ -449,6 +449,7 @@ static inline void inline_map_copy_to(struct map_info *map, unsigned long to, co
 #define map_copy_to(map, to, from, len) (map)->copy_to(map, to, from, len)
 
 extern void simple_map_init(struct map_info *);
+bool map_is_simple(struct map_info *map);
 #define map_is_linear(map) (map->phys != NO_XIP)
 
 #else
@@ -460,6 +461,7 @@ extern void simple_map_init(struct map_info *);
 
 #define simple_map_init(map) BUG_ON(!map_bankwidth_supported((map)->bankwidth))
 #define map_is_linear(map) ({ (void)(map); 1; })
+#define map_is_simple(map) ({ (void)(map); 1; })
 
 #endif /* !CONFIG_MTD_COMPLEX_MAPPINGS */
 
-- 
2.53.0