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

[Marek Vasut]

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 ?