On 3/30/26 10:27 PM, Akhil P Oommen wrote:
> On 3/30/2026 4:45 PM, Konrad Dybcio wrote:
>> On 3/27/26 1:13 AM, Akhil P Oommen wrote:
>>> Update the HFI definitions to support additional GMU based power
>>> features.
>>>
>>> Signed-off-by: Akhil P Oommen <[email protected]>
>>> ---
>>
>> Whether you want to proceed with bitfields or not:
>>
>> Reviewed-by: Konrad Dybcio <[email protected]>
>
> Thanks. I still feel it is bitfield layout is a 'bit' complicated.
>
> I did an experiment:
>
> #include <stdio.h>
> #include <stdint.h>
> #include <stddef.h>
>
> struct packed_u8 {
> uint16_t mitigation_time;
> uint8_t current_rating : 6;
> uint8_t num_phases : 6;
> uint8_t path_notification : 1;
> uint8_t extra_features : 3;
> } __packed;
>
>
> void main() {
>
> struct packed_u8 data = {
> .mitigation_time = 0xffff,
> .current_rating = 0x3f, /* all 6 bits set */
> .num_phases = 0x3f,
> .path_notification = 1,
> .extra_features = 0x7,
> };
>
> printf("Akhil 0x%x\n", *((uint32_t *) &data));
> }
>
> The output I got in Kaanapali is: Akhil 0x7f3fffff
>
> This means that the compiler inserted a padding between current_rating
> and num_phases.
That's because __packed doesn't work outside the kernel - you're just
creating a variable named __packed, so this is effectively the same as:
struct foo {
int a;
int b;
unsigned char c : 2;
} __hotdog;
int main () {
printf("Akhil 0x%x\n", *((uint32_t *) &__hotdog));
}
Outside the kernel tree, you need to use the full annoying __attribute__((foo))
syntax:
include/linux/compiler_attributes.h:#define __packed
__attribute__((__packed__))
with that changed, we get:
Akhil 0xffffffff
which is the expected behavior
Konrad
