Currently struct of_device_id is 196 (32-bit) or 200 (64-bit) bytes
large. It contains fixed-size strings for a name, a type, and a
compatible value, but the first two are barely used.
OF device ID tables contain multiple entries, plus an empty sentinel
entry.
Statistics for my current kernel source tree:
- 4487 tables with 16836 entries (3367200 bytes)
- 176 names (average 6.7 max 23 chars)
- 66 types (average 5.1 max 21 chars)
- 12192 compatible values (average 18.0 max 45 chars)
Taking into account the minimum needed size to store the strings, only
6.9% of the allocated space is used...
Reduce kernel size by reducing the sizes of the fixed strings by one
half.
This reduces the size of an ARM multi_v7_defconfig kernel by ca. 400
KiB. For a typical kernel supporting a single board, you can expect to
save 50-100 KiB.
Signed-off-by: Geert Uytterhoeven <geert+renesas@xxxxxxxxx>
---
Notes:
- While gcc complains if the non-NUL characters in a string do not fit
in the available space, it does not complain if there is no space to
store the string's NUL-terminator. However, that should be caught
during testing, as the affected entry won't ever match. The kernel
won't crash, as such strings will still be terminated by the
sentinel in the table.
- We could save even more by converting the strings from fixed-size
arrays to pointers, at the expense of making it harder to mark
entries __init. Given most drivers support binding and unbinding
and thus cannot use __init for of_device_id tables, perhaps that's
the way to go?
Thanks for your comments!
---
include/linux/mod_devicetable.h | 6 +++---
1 file changed, 3 insertions(+), 3 deletions(-)
diff --git a/include/linux/mod_devicetable.h b/include/linux/mod_devicetable.h
index ae2e75d15b219920..2bb2558d52d30d2b 100644
--- a/include/linux/mod_devicetable.h
+++ b/include/linux/mod_devicetable.h
@@ -266,9 +266,9 @@ struct sdw_device_id {
* Struct used for matching a device
*/
struct of_device_id {
- char name[32];
- char type[32];
- char compatible[128];
+ char name[24];
+ char type[24];
+ char compatible[48];
const void *data;
};
--
2.25.1
