Messages in this thread Patch in this message | | | From | Geert Uytterhoeven <> | Subject | [PATCH/RFC] of: Shrink struct of_device_id | Date | Wed, 10 Nov 2021 17:23:17 +0100 |
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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@glider.be> --- 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
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