On 21/06/2025 18:03, Marek Vasut wrote:
On 6/16/25 6:26 PM, Neil Armstrong wrote:
On 16/06/2025 18:05, Marek Vasut wrote:
On 6/16/25 1:45 PM, Neil Armstrong wrote:
On 13/06/2025 12:54, Marek Vasut wrote:
On 6/13/25 11:29 AM, Neil Armstrong wrote:
On 12/06/2025 01:49, Marek Vasut wrote:
Use u8 to hold lane count in struct ili9881c_desc {} to avoid
alignment gap between default_address_mode and lanes members.
The ili9881c controller can only operate up to 4 DSI lanes, so
there is no chance this value can ever be larger than 4. No
functional change.
The u8 will still take at least 4 bytes and cpu will still
do at least a 32bit memory access, so there's no point to change
it to u8.
Assuming this layout:
40 struct ili9881c_desc {
41 const struct ili9881c_instr *init;
42 const size_t init_length;
43 const struct drm_display_mode *mode;
44 const unsigned long mode_flags;
45 u8 default_address_mode;
46 u8 lanes;
47 };
I wrote a quick test:
$ cat test.c
#include <stdio.h>
#include <stdint.h>
struct foo {
void *a;
size_t b;
void *c;
unsigned long d;
uint8_t x;
unsigned long y; // ~= lanes
};
struct bar {
void *a;
size_t b;
void *c;
unsigned long d;
uint8_t x;
uint8_t y; // ~= lanes
};
int main(void)
{
printf("%d %d\n", sizeof(struct foo), sizeof(struct bar));
return 0;
}
With which I get these results on x86-64:
$ gcc -o test test.c && ./test
48 40
And on x86 32bit:
$ i686-linux-gnu-gcc -o test test.c && ./test
24 20
Maybe there is some improvement ?
Try again with code size included, and other archs since 99% of the users would
be an arm/riscv based boards.
Doesn't that mean, that one some systems it wins us a bit of memory utilization
improvement, and on other systems it has no impact ?
4 or 8 bytes less in a dynamically allocated struct which is by default aligned
on 64 bytes by default on x86, 128 on aarch64, 32/64/128 on arm32, 64 on riscv,
sorry this is negligible.
It is still not zero, so why tolerate the inefficiency when it can be improved ?
Is this change rejected ?
I won't nack it since it's technically correct, but won't ack it since it's an
useless change.
Neil
