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

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Date:	Fri, 28 Feb 2014 15:16:23 +0400
From:	Sergei Shtylyov <sergei.shtylyov@...entembedded.com>
To:	Marc Kleine-Budde <mkl@...gutronix.de>, netdev@...r.kernel.org,
	wg@...ndegger.com, linux-can@...r.kernel.org
CC:	linux-sh@...r.kernel.org, vksavl@...il.com
Subject: Re: [PATCH v5] can: add Renesas R-Car CAN driver

Hello.

On 28-02-2014 13:08, Marc Kleine-Budde wrote:

>>>> 1. According to documentation BCR is the 24-bit register.
>>>> Actually we can consider some 32-bit register that combines BCR and
>>>> CLKR but according to documentation there are two separate registers.
>>>> 2. BCR has 8- ,16-, and 32-bit access (according to documentation).
>>>> 3. This is the algorithm that the documentation suggests.
>>>> 4. We had a driver version with byte access but 32-bit access seems
>>>> shorter.

>>> Please use a normal read-modify-write 32 bit access.

>>     IMO, reading 32-bits is futile, as we're going to completely
>> overwrite those 24 bits that constitute BCR. So I kept the 8-bit CLKR
>> read but removed the CLKR write in the end. I've also added a comment
>> clarifying why CLKR is positioned in the LSBs of 32-bit word (while it's
>> address would assume MSBs).
>> The host bus is big-endian but byte-swaps at least 16- and 32-bit
>> accesses, so that read[wl]()/write[wl]() work. 8-bit accesses are not
>> byte swapped, despite what the figure in the manual shows.

> 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()?

> Marc

WBR, Sergei

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