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

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Message-ID: <8185f3e1-d0b1-0ea4-ac45-f2ea0b63ced9@pengutronix.de>
Date:   Fri, 8 Jan 2021 11:07:26 +0100
From:   Ahmad Fatoum <a.fatoum@...gutronix.de>
To:     Jakub Kicinski <kuba@...nel.org>,
        Marc Kleine-Budde <mkl@...gutronix.de>
Cc:     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

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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