netdev - Re: [PATCH V2] CAN: Add Flexcan CAN controller driver

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Message-ID: <20090728141855.GV2714@pengutronix.de>
Date:	Tue, 28 Jul 2009 16:18:55 +0200
From:	Sascha Hauer <s.hauer@...gutronix.de>
To:	Oliver Hartkopp <socketcan@...tkopp.net>
Cc:	Socketcan-core@...ts.berlios.de,
	Linux Netdev List <netdev@...r.kernel.org>
Subject: Re: [PATCH V2] CAN: Add Flexcan CAN controller driver

On Tue, Jul 28, 2009 at 03:50:48PM +0200, Oliver Hartkopp wrote:
> Sascha Hauer wrote:
> > Hi Oliver,
> > 
> > On Tue, Jul 28, 2009 at 03:21:40PM +0200, Oliver Hartkopp wrote:
> >> Sascha Hauer wrote:
> >>> Hi,
> >>>
> >>> Here is the second version of the flexcan driver.
> >> Hi Sascha,
> >>
> >> some more points i forgot to mention, sorry ...
> >>
> >>
> >>> +/* Structure of the message buffer */
> >>> +struct flexcan_mb {
> >>> +	u32	can_dlc;
> >>> +	u32	can_id;
> >>> +	u32	data[2];
> >>> +};
> >> This looks really hackish and does not reflect the structure of a flexcan
> >> message buffer! The data is 'u8' and the name of 'dlc' for the
> >> description/flag register is bad.
> >>
> > 
> > see below..
> 
> Especially can_dlc, can_id and data[] are known from struct can_frame which
> really can confuse here ...
> 
> > 
> >>> +
> >>> +static int flexcan_start_xmit(struct sk_buff *skb, struct net_device *dev)
> >>> +{
> >>> +	struct can_frame *frame = (struct can_frame *)skb->data;
> >>> +	struct flexcan_priv *priv = netdev_priv(dev);
> >>> +	struct flexcan_regs __iomem *regs = priv->base;
> >>> +	u32 can_id;
> >>> +	u32 dlc = MB_CNT_CODE(0xc) | (frame->can_dlc << 16);
> >> Naming this variable 'dlc' does not hit the point. See below.
> >>
> >>> +	u32 *can_data;
> >> Really this needs to be fixed up by defining a proper mailbox struct.
> >>
> >>
> >>> +
> >>> +	netif_stop_queue(dev);
> >>> +
> >>> +	if (frame->can_id & CAN_EFF_FLAG) {
> >>> +		can_id = frame->can_id & MB_ID_EXT;
> >> Please use CAN_EFF_MASK here.
> > 
> > I used MX_ID_EXT intentionally because it it flexcan specific and just
> > happens to be the same as CAN_EFF_MASK. I can change it if you like.
> 
> Yes, i've seen that. I would tend to use CAN_EFF_MASK here as you apply it on
> frame->can_id.
> 
> When you get it from the controller MB_ID_EXT_MASK would be the better one.
> 
> > 
> >>
> >>> +		dlc |= MB_CNT_IDE | MB_CNT_SRR;
> >>> +	} else {
> >>> +		can_id = (frame->can_id & CAN_SFF_MASK) << 18;
> >>> +	}
> >> Just nitpicking for Kernel coding style:
> >> remove the last '{' and '}' pair.
> > 
> > No, Documentation/CondingStyle suggests that if one branch needs braces
> > the other branch should use them, too.
> 
> Sorry. Didn't know that.
> 
> > 
> >>> +
> >>> +	if (frame->can_id & CAN_RTR_FLAG)
> >>> +		dlc |= MB_CNT_RTR;
> >>> +
> >>> +	writel(dlc, &regs->cantxfg[TX_BUF_ID].can_dlc);
> >>> +	writel(can_id, &regs->cantxfg[TX_BUF_ID].can_id);
> >>> +
> >>> +	can_data = (u32 *)frame->data;
> >>> +	writel(cpu_to_be32(*can_data), &regs->cantxfg[TX_BUF_ID].data[0]);
> >>> +	writel(cpu_to_be32(*(can_data + 1)), &regs->cantxfg[TX_BUF_ID].data[1]);
> >> IMHO it is not really transparent, that this is a correct handling to copy the
> >>   can_frame.data[] on all architectures. I bet creating a for-statement
> >> regarding the dlc is not slower and makes really clear, what's going on here.
> > 
> > This is indeed a problem here. The original Coldfire code I used as a
> > template used a loop around unsigned char * which did the wrong thing
> > for me.
> 
> This could be a good starting point for an investigation ;-)
> 
> > So yes, this is not generic here, but I have no idea how the
> > generic code looks like. As Coldfire is big endian this doesn't seem
> > that wrong.
> 
> I would try to define a proper flexcan_mb struct like
> 
> struct flexcan_mb {
> 	u8 code;
> 	u8 ctrl;
> 	u16 timestamp;
> 	u32 id;
> 	u8 data[8];
> }

I can't properly define it like this because I have a little endian
system and u8 code is on offset 3 on my hardware. I just checked it
because I always get confused with endian problems ;)

I hope you're not suggesting me to do something like

#ifdef __LITTLE_ENDIAN
struct flexcan_mb {
	u16 timestamp;
	u8 ctrl;
	u8 code;
	u32 id;
	u8 data[8];
}
#else
struct flexcan_mb {
	u8 code;
	u8 ctrl;
	u16 timestamp;
	u32 id;
	u8 data[8];
}
#endif

(which would still require endian specific handling for the actual CAN
data)

> 
> And then see how it looks like ;-)

Well, I would have to do a le/be conversion manually whereas cpu_to_be32
*should* do the right thing. I don't have any ColdFire hardware to test
though.

The more I think about it the more I think that my original code does
the right thing(tm)

Sascha

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