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

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Date:	Fri, 28 Feb 2014 12:41:37 +0100
From:	Geert Uytterhoeven <geert@...ux-m68k.org>
To:	Marc Kleine-Budde <mkl@...gutronix.de>
Cc:	Sergei Shtylyov <sergei.shtylyov@...entembedded.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	wg@...ndegger.com, linux-can@...r.kernel.org,
	Linux-sh list <linux-sh@...r.kernel.org>,
	Pavel Kiryukhin <vksavl@...il.com>
Subject: Re: [PATCH v5] can: add Renesas R-Car CAN driver

On Fri, Feb 28, 2014 at 12:37 PM, Marc Kleine-Budde <mkl@...gutronix.de> wrote:
>>> A 32 bit read/modify/write is a standard operation, nothing special, no
>>> need to worry about byte swapping or anything like this.
>>
>>    Oh, really? 8-)
>>    Don't you know that read[bwlq]() assume little-endian memory layout
>> and to read from big-endian 32-bit register one normally needs readl_be()?
>
> I assume you are on little endian ARM only (for now).
>
> If you use a standard 32 bit read, then modify the correct bits in that
> 32 bit word and write it back, with the corresponding 32 bit write
> everything should be fine. For this usecase you just have yo figure out
> which 24 of the 32 bit are the one you have to change and which are the
> 8 that must not be modified.
>
> Looking at the register layout:
>
>> +     u8 bcr[3];      /* Bit Configuration Register */
>> +     u8 clkr;        /* Clock Select Register */
>
> I think clkr would be the lowest 8 bit and bcr[] are the upper 24.

That would be the outcome on big endian ;-)

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-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
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