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

From: Marc Kleine-Budde <[email-masked]>
Date: Mon, 20 Jan 2014 12:58:42 +0100

On 01/20/2014 12:52 PM, Geert Uytterhoeven wrote:
> On Mon, Jan 20, 2014 at 12:47 PM, Marc Kleine-Budde <[email-masked]> wrote:
>> On 01/20/2014 12:43 PM, Geert Uytterhoeven wrote:
>>> On Thu, Dec 26, 2013 at 10:37 PM, Sergei Shtylyov
>>> <[email-masked]> 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.

Yes, as this are registers they must not be accessed directly. However
we can use "__attribute__ ((packed, aligned(4)))" to tell the compiler
that the base address of this struct is always aligned to 4 bytes.

Marc

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