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

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Message-ID: <53107B7B.8040800@cogentembedded.com>
Date:	Fri, 28 Feb 2014 16:05:15 +0400
From:	Sergei Shtylyov <sergei.shtylyov@...entembedded.com>
To:	Marc Kleine-Budde <mkl@...gutronix.de>,
	Geert Uytterhoeven <geert@...ux-m68k.org>
CC:	"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

Hello.

On 28-02-2014 15:49, Marc Kleine-Budde 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).

    That doesn't matter but yes.

>>> 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.

    It seems you can't figure that out yourself. :-)

>>> 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 ;-)

> Doh! Yes, correct.
> The point is, just read/modify the correct bits/write, should just work.

    That's what I do. But I completely fail to see the point of reading BCR 
which is completely overwritten. I would like to consider the pointless 
discussion about 32-bit read complete now.

> The driver has to be tested on BE ARM anyways, as this isn't the only 32
> bit reg.

    This is not a 32-bit register but 24- and 8-bit one. I'm afraid we won't 
be able to test on BE soon as the machine we're debugging on is LE only. 
Anyway, for the big-endian configured Superhyway bus the big-endian HPB bus 
the CAN controller resides on shouldn't swap bytes.

> Marc

WBR, Sergei

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