netdev - Re: [PATCH v5] can: add Renesas R-Car CAN driver

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Date:	Mon, 20 Jan 2014 12:52:51 +0100
From:	Geert Uytterhoeven <geert@...ux-m68k.org>
To:	Marc Kleine-Budde <mkl@...gutronix.de>
Cc:	Sergei Shtylyov <sergei.shtylyov@...entembedded.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	wg@...ndegger.com, linux-can@...r.kernel.org,
	Linux-sh list <linux-sh@...r.kernel.org>, vksavl@...il.com
Subject: Re: [PATCH v5] can: add Renesas R-Car CAN driver

On Mon, Jan 20, 2014 at 12:47 PM, Marc Kleine-Budde <mkl@...gutronix.de> wrote:
> On 01/20/2014 12:43 PM, Geert Uytterhoeven wrote:
>> On Thu, Dec 26, 2013 at 10:37 PM, Sergei Shtylyov
>> <sergei.shtylyov@...entembedded.com> wrote:
>>> Changes in version 3:
>>
>>> - added '__packed' to 'struct rcar_can_mbox_regs' and 'struct rcar_can_regs';
>>> - removed unneeded type cast in the probe() method.
>>
>>> +/* Mailbox registers structure */
>>> +struct rcar_can_mbox_regs {
>>> +       u32 id;         /* IDE and RTR bits, SID and EID */
>>> +       u8 stub;        /* Not used */
>>> +       u8 dlc;         /* Data Length Code - bits [0..3] */
>>> +       u8 data[8];     /* Data Bytes */
>>> +       u8 tsh;         /* Time Stamp Higher Byte */
>>> +       u8 tsl;         /* Time Stamp Lower Byte */
>>> +} __packed;
>>
>> Sorry, I missed the earlier discussion, but why the _packed?
>> One u32 and 12 bytes makes a nice multiple of 4.
>
> Better safe than sorry, it's the layout of the registers in hardware,
> don't let the compiler optimize here.

Actually __packed makes it less safe, as the compiler now assumes
the u32 id member is unaligned, and thus may turn 32-bit accesses into 4
byte accesses.

Fortunately it won't happen in this case as the code uses writel/readl to
acces the id member.

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds
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