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

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Message-ID: <20210107143851.51675f8d@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com>
Date:   Thu, 7 Jan 2021 14:38:51 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     Marc Kleine-Budde <mkl@...gutronix.de>
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 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.