From mboxrd@z Thu Jan 1 00:00:00 1970 From: Geert Uytterhoeven Date: Mon, 20 Jan 2014 12:05:41 +0000 Subject: Re: [PATCH v5] can: add Renesas R-Car CAN driver Message-Id: List-Id: References: <201312270037.15822.sergei.shtylyov@cogentembedded.com> <52DD0CC1.70205@pengutronix.de> <52DD0F72.1000005@pengutronix.de> <52DD1058.4090205@codethink.co.uk> In-Reply-To: <52DD1058.4090205@codethink.co.uk> MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: Ben Dooks Cc: Marc Kleine-Budde , Sergei Shtylyov , "netdev@vger.kernel.org" , wg@grandegger.com, linux-can@vger.kernel.org, Linux-sh list , Pavel Kiryukhin On Mon, Jan 20, 2014 at 1:02 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 >>> wrote: >>>> >>>> On 01/20/2014 12:43 PM, Geert Uytterhoeven wrote: >>>>> >>>>> On Thu, Dec 26, 2013 at 10:37 PM, Sergei Shtylyov >>>>> 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? It allows to use e.g. "readl(&base->field)", which is used all over the place. And sparse will inform you (read: the person who runs it) if you access it directly. Note that this driver does not use direct accesses. Gr{oetje,eeting}s, Geert -- Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org In personal conversations with technical people, I call myself a hacker. But when I'm talking to journalists I just say "programmer" or something like that. -- Linus Torvalds From mboxrd@z Thu Jan 1 00:00:00 1970 From: Geert Uytterhoeven Subject: Re: [PATCH v5] can: add Renesas R-Car CAN driver Date: Mon, 20 Jan 2014 13:05:41 +0100 Message-ID: References: <201312270037.15822.sergei.shtylyov@cogentembedded.com> <52DD0CC1.70205@pengutronix.de> <52DD0F72.1000005@pengutronix.de> <52DD1058.4090205@codethink.co.uk> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Return-path: Received: from mail-pb0-f49.google.com ([209.85.160.49]:60703 "EHLO mail-pb0-f49.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750721AbaATMFm (ORCPT ); Mon, 20 Jan 2014 07:05:42 -0500 In-Reply-To: <52DD1058.4090205@codethink.co.uk> Sender: linux-can-owner@vger.kernel.org List-ID: To: Ben Dooks Cc: Marc Kleine-Budde , Sergei Shtylyov , "netdev@vger.kernel.org" , wg@grandegger.com, linux-can@vger.kernel.org, Linux-sh list , Pavel Kiryukhin On Mon, Jan 20, 2014 at 1:02 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 >>> wrote: >>>> >>>> On 01/20/2014 12:43 PM, Geert Uytterhoeven wrote: >>>>> >>>>> On Thu, Dec 26, 2013 at 10:37 PM, Sergei Shtylyov >>>>> 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? It allows to use e.g. "readl(&base->field)", which is used all over the place. And sparse will inform you (read: the person who runs it) if you access it directly. Note that this driver does not use direct accesses. Gr{oetje,eeting}s, Geert -- Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org In personal conversations with technical people, I call myself a hacker. But when I'm talking to journalists I just say "programmer" or something like that. -- Linus Torvalds