Re: c_can driver sometimes sends first two bytes filled with zeros

[Andy Haydon <ahaydon@moog.com>]

Hi Wolfgang,

Wolfgang Grandegger <wg <at> grandegger.com> writes:

> 
> Making "priv->write_regs() atomic is something I would try as well. My 
> suspicion is that something goes wrong writing(/reading) the controller 
> registers.

We have also been looking at a similar issue over the last few days. In our 
case, bytes 0 and 1 and bytes 4 and 5 of an 8 byte message can be corrupted 
randomly - not necessarily zeroed. Bytes 2 and 3 and bytes 6 and 7 are never
corrupted. The Tx task is running as a high priority real time thread in 
PREEMPT_RT.

I've found that so far the only thing that fixes this issue is to perform 
the writes to the data registers as 32 bit operations something like this 
(for 3.18.20):

--- a/drivers/net/can/c_can/c_can.c	2015-08-07 20:08:04.000000000 +0100
+++ b/drivers/net/can/c_can/c_can.c	2016-06-09 14:46:51.497895357 +0100
@@ -331,9 +332,19 @@
 
 	priv->write_reg(priv, C_CAN_IFACE(MSGCTRL_REG, iface), ctrl);
 
-	for (i = 0; i < frame->can_dlc; i += 2) {
-		priv->write_reg(priv, C_CAN_IFACE(DATA1_REG, iface) + i / 2,
-				frame->data[i] | (frame->data[i + 1] << 8));
+// this doesn't work for Cyclone V
+//	for (i = 0; i < frame->can_dlc; i += 2) {
+//		priv->write_reg(priv, C_CAN_IFACE(DATA1_REG, iface) + i / 2,
+//				frame->data[i] | (frame->data[i + 1] << 8));
+
+// replaced with this as a workaround
+	u32 data = 0;
+	for (i = 0; i < frame->can_dlc; i += 4) {
+		data = (u32)frame->data[i];
+		data |= (u32)frame->data[i + 1] << 8;
+		data |= (u32)frame->data[i + 2] << 16;
+		data |= (u32)frame->data[i + 3] << 24;
+		priv->write_reg32(priv, C_CAN_IFACE(DATA1_REG, iface) + i / 
2, data);
 	}
 }
 
At the moment I can't explain this behaviour, or if this is really the only 
way to fix the issue for the Cyclone V. 
The tests that I've done so far seem to indicate that the data is correct 
in the driver right up to the point at which it is actually written to the 
CAN peripheral registers, so I think that your suspicion is correct.

Regards,
Andy