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

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Message-ID: <52DD9F3C.7000408@cogentembedded.com>
Date:	Tue, 21 Jan 2014 01:12:12 +0300
From:	Sergei Shtylyov <sergei.shtylyov@...entembedded.com>
To:	David Miller <davem@...emloft.net>, mkl@...gutronix.de
CC:	geert@...ux-m68k.org, 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

Hello.

On 01/20/2014 10:16 PM, David Miller wrote:

>>>>>> - 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.

    They will. But how about the following 'struct rcar_can_regs'?

> Thanks.

WBR, Sergei

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