[PATCH] drm/panel: ilitek-ili9881c: Use u8 for lane count

[Neil Armstrong]

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.

Neil