[PATCH/RFC] of: Shrink struct of_device_id

[Geert Uytterhoeven]

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 at 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