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

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Date:	Wed, 22 Jan 2014 11:52:33 +0000
From:	Ben Dooks <ben.dooks@...ethink.co.uk>
To:	Marc Kleine-Budde <mkl@...gutronix.de>
CC:	Sergei Shtylyov <sergei.shtylyov@...entembedded.com>,
	David Miller <davem@...emloft.net>, 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

On 20/01/14 21:17, Marc Kleine-Budde wrote:
> On 01/20/2014 11:12 PM, Sergei Shtylyov wrote:
> [...]
>
>>> 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'?
>
> The strcuts in question are:
>
>> +/* 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;
>> +
>> +struct rcar_can_regs {
>> +	struct rcar_can_mbox_regs mb[RCAR_CAN_N_MBX]; /* Mailbox registers */
>> +	u32 mkr_2_9[8];	/* Mask Registers 2-9 */
>> +	u32 fidcr[2];	/* FIFO Received ID Compare Register */
>> +	u32 mkivlr1;	/* Mask Invalid Register 1 */
>> +	u32 mier1;	/* Mailbox Interrupt Enable Register 1 */
>> +	u32 mkr_0_1[2];	/* Mask Registers 0-1 */
>> +	u32 mkivlr0;    /* Mask Invalid Register 0*/
>> +	u32 mier0;      /* Mailbox Interrupt Enable Register 0 */
>> +	u8 pad_440[0x3c0];
>> +	u8 mctl[64];	/* Message Control Registers */
>> +	u16 ctlr;	/* Control Register */
>> +	u16 str;	/* Status register */
>> +	u8 bcr[3];	/* Bit Configuration Register */
>> +	u8 clkr;	/* Clock Select Register */
>> +	u8 rfcr;	/* Receive FIFO Control Register */
>> +	u8 rfpcr;	/* Receive FIFO Pointer Control Register */
>> +	u8 tfcr;	/* Transmit FIFO Control Register */
>> +	u8 tfpcr;       /* Transmit FIFO Pointer Control Register */
>> +	u8 eier;	/* Error Interrupt Enable Register */
>> +	u8 eifr;	/* Error Interrupt Factor Judge Register */
>> +	u8 recr;	/* Receive Error Count Register */
>> +	u8 tecr;        /* Transmit Error Count Register */
>> +	u8 ecsr;	/* Error Code Store Register */
>> +	u8 cssr;	/* Channel Search Support Register */
>> +	u8 mssr;	/* Mailbox Search Status Register */
>> +	u8 msmr;	/* Mailbox Search Mode Register */
>> +	u16 tsr;	/* Time Stamp Register */
>> +	u8 afsr;	/* Acceptance Filter Support Register */
>> +	u8 pad_857;
>> +	u8 tcr;		/* Test Control Register */
>> +	u8 pad_859[7];
>> +	u8 ier;		/* Interrupt Enable Register */
>> +	u8 isr;		/* Interrupt Status Register */
>> +	u8 pad_862;
>> +	u8 mbsmr;	/* Mailbox Search Mask Register */
>> +} __packed;
>
> I think they should work without packed, too.

I think this discussion proves why this is not a good idea.

IIRC, a compiler has the right to pad, or re-order structure
elements as it sees fit depending on the architecture and options.


-- 
Ben Dooks				http://www.codethink.co.uk/
Senior Engineer				Codethink - Providing Genius
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