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, 5 deletions

diff --git a/drivers/net/can/usb/ems_usb.c b/drivers/net/can/usb/ems_usb.c
index c9b6adf5c1ec..7bedceffdfa3 100644
--- a/drivers/net/can/usb/ems_usb.c
+++ b/drivers/net/can/usb/ems_usb.c
@@ -230,7 +230,6 @@ struct ems_tx_urb_context {
 	struct ems_usb *dev;
 
 	u32 echo_index;
-	u8 dlc;
 };
 
 struct ems_usb {
@@ -517,9 +516,8 @@ static void ems_usb_write_bulk_callback(struct urb *urb)
 
 	/* transmission complete interrupt */
 	netdev->stats.tx_packets++;
-	netdev->stats.tx_bytes += context->dlc;
-
-	can_get_echo_skb(netdev, context->echo_index, NULL);
+	netdev->stats.tx_bytes += can_get_echo_skb(netdev, context->echo_index,
+						   NULL);
 
 	/* Release context */
 	context->echo_index = MAX_TX_URBS;
@@ -805,7 +803,6 @@ static netdev_tx_t ems_usb_start_xmit(struct sk_buff *skb, struct net_device *ne
 
 	context->dev = dev;
 	context->echo_index = i;
-	context->dlc = cf->len;
 
 	usb_fill_bulk_urb(urb, dev->udev, usb_sndbulkpipe(dev->udev, 2), buf,
 			  size, ems_usb_write_bulk_callback, context);