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

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Date:	Mon, 20 Jan 2014 13:35:26 +0100
From:	Marc Kleine-Budde <mkl@...gutronix.de>
To:	David Laight <David.Laight@...LAB.COM>,
	'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" <wg@...ndegger.com>,
	"linux-can@...r.kernel.org" <linux-can@...r.kernel.org>,
	Linux-sh list <linux-sh@...r.kernel.org>,
	"vksavl@...il.com" <vksavl@...il.com>
Subject: Re: [PATCH v5] can: add Renesas R-Car CAN driver

On 01/20/2014 01:13 PM, David Laight wrote:
> From: 
>> 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.
> 
> Why not just add a compile time check against the size of the
> structure - that will ensure that no padding is accidentally added.

And what do when the check fails? Add an the __packed (again)? :D

>>>> 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.
> 
> Which means that it will be aligned (and must be aligned).
> So the packed is completely useless and pointless.

Then we'll remove the packed.

Marc
-- 
Pengutronix e.K.                  | Marc Kleine-Budde           |
Industrial Linux Solutions        | Phone: +49-231-2826-924     |
Vertretung West/Dortmund          | Fax:   +49-5121-206917-5555 |
Amtsgericht Hildesheim, HRA 2686  | http://www.pengutronix.de   |


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