Hello Jakub,
On 08.01.21 17:32, Jakub Kicinski wrote:
> On Fri, 8 Jan 2021 11:07:26 +0100 Ahmad Fatoum wrote:
>>>>> +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).
>
> ____cacheline_aligned adds a big architecture dependent padding at the
> end of this struct, so the size of this structure is architecture
> dependent. Besides using packed forced the compiler to use byte by byte
> loads on architectures without unaligned access, so __packed is not
> free.
https://godbolt.org/z/j68x8n
seems to indicate that explicit alignment "overrules" packed's implicit
alignment of 1 as
there isn't any byte-by-byte access generated for a struct
that is both packed and cacheline aligned. packed only structs are accessed
byte-by-byte however.
Did I get something wrong in my testcase?
I compiled with ARM gcc 8.2 -mno-unaligned-access -fno-strict-aliasing -O2
Cheers,
Ahmad
--
Pengutronix e.K. | |
Steuerwalder Str. 21 | http://www.pengutronix.de/ |
31137 Hildesheim, Germany | Phone: +49-5121-206917-0 |
Amtsgericht Hildesheim, HRA 2686 | Fax: +49-5121-206917-5555 |
