On 07.01.21 23:38, Jakub Kicinski wrote:
> On Thu, 7 Jan 2021 22:17:15 +0100 Marc Kleine-Budde wrote:
>>> +struct __packed tcan4x5x_buf_cmd {
>>> + u8 cmd;
>>> + __be16 addr;
>>> + u8 len;
>>> +};
>>
>> This has to be packed, as I assume the compiler would add some space after the
>> "u8 cmd" to align the __be16 naturally.
>
> Ack, packing this one makes sense.
>
>>> +struct __packed tcan4x5x_map_buf {
>>> + struct tcan4x5x_buf_cmd cmd;
>>> + u8 data[256 * sizeof(u32)];
>>> +} ____cacheline_aligned;
>>
>> Due to the packing of the struct tcan4x5x_buf_cmd it should have a length of 4
>> bytes. Without __packed, will the "u8 data" come directly after the cmd?
>
> Yup, u8 with no alignment attribute will follow the previous
> field with no holes.
__packed has a documentation benefit though. It documents that the author
considers the current layout to be the only correct one. (and thus extra
care should be taken when modifying it).
>
>
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