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

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Date:	Mon, 20 Jan 2014 13:05:43 +0100
From:	Marc Kleine-Budde <mkl@...gutronix.de>
To:	Ben Dooks <ben.dooks@...ethink.co.uk>
CC:	Geert Uytterhoeven <geert@...ux-m68k.org>,
	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 01/20/2014 01:02 PM, Ben Dooks wrote:
> On 20/01/14 11:58, Marc Kleine-Budde wrote:
>> On 01/20/2014 12:52 PM, Geert Uytterhoeven wrote:
>>> 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.
>>
>> 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.
> 
> I thought we'd gotten past the stage of ever writing register
> definitions as structures?

I don't care whether to use offsets defined by a #define or an enum or a
struct. What's the "official" preference? Is it documented somewhere?

Marc

-- 
Pengutronix e.K.                  | Marc Kleine-Budde           |
Industrial Linux Solutions        | Phone: +49-231-2826-924     |
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