can: do not increase tx_bytes statistics for RTR frames

The actual payload length of the CAN Remote Transmission Request (RTR)
frames is always 0, i.e. no payload is transmitted on the wire.
However, those RTR frames still use the DLC to indicate the length of
the requested frame.

As such, net_device_stats::tx_bytes should not be increased when
sending RTR frames.

The function can_get_echo_skb() already returns the correct length,
even for RTR frames (c.f. [1]). However, for historical reasons, the
drivers do not use can_get_echo_skb()'s return value and instead, most
of them store a temporary length (or dlc) in some local structure or
array. Using the return value of can_get_echo_skb() solves the
issue. After doing this, such length/dlc fields become unused and so
this patch does the adequate cleaning when needed.

This patch fixes all the CAN drivers.

Finally, can_get_echo_skb() is decorated with the __must_check
attribute in order to force future drivers to correctly use its return
value (else the compiler would emit a warning).

[1] commit ed3320cec279 ("can: dev: __can_get_echo_skb():
fix real payload length return value for RTR frames")

Link: https://lore.kernel.org/all/20211207121531.42941-6-mailhol.vincent@wanadoo.fr
Cc: Nicolas Ferre <nicolas.ferre@microchip.com>
Cc: Alexandre Belloni <alexandre.belloni@bootlin.com>
Cc: Ludovic Desroches <ludovic.desroches@microchip.com>
Cc: Maxime Ripard <mripard@kernel.org>
Cc: Chen-Yu Tsai <wens@csie.org>
Cc: Jernej Skrabec <jernej.skrabec@gmail.com>
Cc: Yasushi SHOJI <yashi@spacecubics.com>
Cc: Oliver Hartkopp <socketcan@hartkopp.net>
Cc: Stephane Grosjean <s.grosjean@peak-system.com>
Cc: Andreas Larsson <andreas@gaisler.com>
Tested-by: Jimmy Assarsson <extja@kvaser.com> # kvaser
Signed-off-by: Vincent Mailhol <mailhol.vincent@wanadoo.fr>
Acked-by: Stefan Mätje <stefan.maetje@esd.eu> # esd_usb2
Tested-by: Stefan Mätje <stefan.maetje@esd.eu> # esd_usb2
[mkl: add conversion for grcan]
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>

1 files changed, 2 insertions, 1 deletions

diff --git a/drivers/net/can/slcan.c b/drivers/net/can/slcan.c
index 5cf03458e948..d4c7ce998a34 100644
--- a/drivers/net/can/slcan.c
+++ b/drivers/net/can/slcan.c
@@ -290,6 +290,8 @@ static void slc_encaps(struct slcan *sl, struct can_frame *cf)
 	if (!(cf->can_id & CAN_RTR_FLAG)) {
 		for (i = 0; i < cf->len; i++)
 			pos = hex_byte_pack_upper(pos, cf->data[i]);
+
+		sl->dev->stats.tx_bytes += cf->len;
 	}
 
 	*pos++ = '\r';
@@ -306,7 +308,6 @@ static void slc_encaps(struct slcan *sl, struct can_frame *cf)
 	actual = sl->tty->ops->write(sl->tty, sl->xbuff, pos - sl->xbuff);
 	sl->xleft = (pos - sl->xbuff) - actual;
 	sl->xhead = sl->xbuff + actual;
-	sl->dev->stats.tx_bytes += cf->len;
 }
 
 /* Write out any remaining transmit buffer. Scheduled when tty is writable */