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

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Date:	Mon, 20 Jan 2014 11:16:14 -0800 (PST)
From:	David Miller <davem@...emloft.net>
To:	mkl@...gutronix.de
Cc:	geert@...ux-m68k.org, sergei.shtylyov@...entembedded.com,
	netdev@...r.kernel.org, wg@...ndegger.com,
	linux-can@...r.kernel.org, linux-sh@...r.kernel.org,
	vksavl@...il.com
Subject: Re: [PATCH v5] can: add Renesas R-Car CAN driver

From: Marc Kleine-Budde <mkl@...gutronix.de>
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 <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 truly think using packed here is rediculous, please remove it unless
you can prove that things won't work without it.

Thanks.
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