[RFC PATCH v2] ACPICA / Hardware: Fix old register check in acpi_hw_get_access_bit_width()

From: Lv Zheng
Date: Tue May 31 2016 - 03:06:12 EST

The address check in acpi_hw_get_access_bit_width() should be byte width
based, not bit width based. This patch fixes this mistake.

For those who want to review acpi_hw_access_bit_width(), here is the
concerns and the design details of the function:

It is supposed that the GAS Address field should be aligned to the byte
width indicated by the GAS AccessSize field. Similarly, for the old non
GAS register, it is supposed that its Address should be aligned to its
For the "AccessSize = 0 (meaning ANY)" case, we try to return the maximum
instruction width (64 for MMIO or 32 for PIO) or the user expected access
bit width (64 for acpi_read()/acpi_write() or 32 for acpi_hw_read()/
acpi_hw_write()) for futher operation and it is supposed that the GAS
Address field should always be aligned to the maximum expected access bit
width (otherwise it can't be ANY).

The problem is in acpi_tb_init_generic_address(), where the non GAS
register's Length is converted into the GAS BitWidth field, its Address is
converted into the GAS Address field, and the GAS AccessSize field is left
0 but most of the register actually cannot be accessed using "ANY"

As a conclusion, when AccessSize = 0 (ANY), the Address should either be
aligned to the BitWidth (wrong conversion) or aligned to 32 (PIO) or 64
(MMIO). Since BitWidth for the wrong conversion is 8,16,32, the Address
of the real GAS should always be aligned to 8,16,32, the address alignment
check is not necessary. But we in fact could enhance the check for a future
case where max_bit_width could be 64 for a PIO access issued from

Fixes: b314a172ee96 ("ACPICA: Hardware: Add optimized access bit width support")
Cc: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
Cc: Mike Marshall <hubcap@xxxxxxxxxxxx>
Suggested-by: Jan Beulich <jbeulich@xxxxxxxx>
Signed-off-by: Lv Zheng <lv.zheng@xxxxxxxxx>
drivers/acpi/acpica/hwregs.c | 16 +++++++---------
1 file changed, 7 insertions(+), 9 deletions(-)

diff --git a/drivers/acpi/acpica/hwregs.c b/drivers/acpi/acpica/hwregs.c
index 0f18dbc..0553c0b 100644
--- a/drivers/acpi/acpica/hwregs.c
+++ b/drivers/acpi/acpica/hwregs.c
@@ -86,24 +86,22 @@ acpi_hw_get_access_bit_width(struct acpi_generic_address *reg, u8 max_bit_width)
u64 address;

if (!reg->access_width) {
+ if (reg->space_id == ACPI_ADR_SPACE_SYSTEM_IO) {
+ max_bit_width = 32;
+ }
* Detect old register descriptors where only the bit_width field
* makes senses. The target address is copied to handle possible
* alignment issues.
ACPI_MOVE_64_TO_64(&address, &reg->address);
- if (!reg->bit_offset && reg->bit_width &&
+ if (reg->bit_width < max_bit_width &&
+ !reg->bit_offset && reg->bit_width &&
ACPI_IS_POWER_OF_TWO(reg->bit_width) &&
- ACPI_IS_ALIGNED(reg->bit_width, 8) &&
- ACPI_IS_ALIGNED(address, reg->bit_width)) {
+ ACPI_IS_ALIGNED(reg->bit_width, 8)) {
return (reg->bit_width);
- } else {
- if (reg->space_id == ACPI_ADR_SPACE_SYSTEM_IO) {
- return (32);
- } else {
- return (max_bit_width);
- }
+ return (max_bit_width);
} else {
return (1 << (reg->access_width + 2));