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

[Neil Armstrong]

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.

Neil