netdev - Re: [net-next 15/19] can: tcan4x5x: rework SPI access

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Date:   Mon, 11 Jan 2021 16:35:30 +0100
From:   Ahmad Fatoum <a.fatoum@...gutronix.de>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     Marc Kleine-Budde <mkl@...gutronix.de>, netdev@...r.kernel.org,
        linux-can@...r.kernel.org, Dan Murphy <dmurphy@...com>,
        kernel@...gutronix.de, Sean Nyekjaer <sean@...nix.com>,
        davem@...emloft.net
Subject: Re: [net-next 15/19] can: tcan4x5x: rework SPI access

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
 

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