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

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Message-ID: <c98003bf-e62a-ab6a-a526-1f3ed0bb1ab7@pengutronix.de>
Date:   Thu, 7 Jan 2021 22:17:15 +0100
From:   Marc Kleine-Budde <mkl@...gutronix.de>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     netdev@...r.kernel.org, davem@...emloft.net,
        linux-can@...r.kernel.org, kernel@...gutronix.de,
        Dan Murphy <dmurphy@...com>, Sean Nyekjaer <sean@...nix.com>
Subject: Re: [net-next 15/19] can: tcan4x5x: rework SPI access

On 1/7/21 8:06 PM, Jakub Kicinski wrote:
> On Thu, 7 Jan 2021 11:00:35 -0800 Jakub Kicinski wrote:
>> On Thu,  7 Jan 2021 10:48:56 +0100 Marc Kleine-Budde wrote:
>>> +struct __packed tcan4x5x_map_buf {
>>> +	struct tcan4x5x_buf_cmd cmd;
>>> +	u8 data[256 * sizeof(u32)];
>>> +} ____cacheline_aligned;  
>>
>> Interesting attribute combo, I must say.
> 
> Looking at the rest of the patch I don't really see a reason for
> __packed.  Perhaps it can be dropped?

It's the stream of bytes send via SPI to the chip. Here are both structs for
reference:

> +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.

> +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?

Marc

-- 
Pengutronix e.K.                 | Marc Kleine-Budde           |
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