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

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Date:	Fri, 28 Feb 2014 12:02:52 +0000
From:	David Laight <David.Laight@...LAB.COM>
To:	'Marc Kleine-Budde' <mkl@...gutronix.de>,
	'Geert Uytterhoeven' <geert@...ux-m68k.org>
CC:	Sergei Shtylyov <sergei.shtylyov@...entembedded.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"wg@...ndegger.com" <wg@...ndegger.com>,
	"linux-can@...r.kernel.org" <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

From: Marc Kleine-Budde
> On 02/28/2014 12:47 PM, David Laight wrote:
> > From: Geert Uytterhoeven
> >> 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 ;-)
> >
> > Looks to me as though it should be defined as a 32bit field and then
> > the appropriate bit definitions and masks applied.
> 
> Ack, 32bit yes, but no field (as in http://en.wikipedia.org/wiki/Bit_field).

Yes, I meant 'field' as in 'structure member', not bit-field.
I learnt a long time ago that bit-fields are rarely, if ever, the construct
you are looking for.
1) A packed lookup table where the code for one access was larger than the table.
2) foo->two_bit_field = 3; compiled into code that first zeroed the two bits
   (caused an ISR to corrupt a linked list).
Never mind the issues about the actual order of the bits.

	David