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Date:   Sat,  8 Jan 2022 22:43:45 +0100
From:   Marc Kleine-Budde <>
        Dario Binacchi <>,
        Marc Kleine-Budde <>
Subject: [PATCH net-next 22/22] docs: networking: device drivers: can: add flexcan

From: Dario Binacchi <>

Add initial documentation for Flexcan driver.

Signed-off-by: Dario Binacchi <>
Signed-off-by: Marc Kleine-Budde <>
 .../device_drivers/can/freescale/flexcan.rst  | 54 +++++++++++++++++++
 .../networking/device_drivers/can/index.rst   |  2 +
 2 files changed, 56 insertions(+)
 create mode 100644 Documentation/networking/device_drivers/can/freescale/flexcan.rst

diff --git a/Documentation/networking/device_drivers/can/freescale/flexcan.rst b/Documentation/networking/device_drivers/can/freescale/flexcan.rst
new file mode 100644
index 000000000000..4e3eec6cecd2
--- /dev/null
+++ b/Documentation/networking/device_drivers/can/freescale/flexcan.rst
@@ -0,0 +1,54 @@
+.. SPDX-License-Identifier: GPL-2.0+
+Flexcan CAN Controller driver
+Authors: Marc Kleine-Budde <>,
+Dario Binacchi <>
+On/off RTR frames reception
+For most flexcan IP cores the driver supports 2 RX modes:
+- mailbox
+The older flexcan cores (integrated into the i.MX25, i.MX28, i.MX35
+and i.MX53 SOCs) only receive RTR frames if the controller is
+configured for RX-FIFO mode.
+The RX FIFO mode uses a hardware FIFO with a depth of 6 CAN frames,
+while the mailbox mode uses a software FIFO with a depth of up to 62
+CAN frames. With the help of the bigger buffer, the mailbox mode
+performs better under high system load situations.
+As reception of RTR frames is part of the CAN standard, all flexcan
+cores come up in a mode where RTR reception is possible.
+With the "rx-rtr" private flag the ability to receive RTR frames can
+be waived at the expense of losing the ability to receive RTR
+messages. This trade off is beneficial in certain use cases.
+"rx-rtr" on
+  Receive RTR frames. (default)
+  The CAN controller can and will receive RTR frames.
+  On some IP cores the controller cannot receive RTR frames in the
+  more performant "RX mailbox" mode and will use "RX FIFO" mode
+  instead.
+"rx-rtr" off
+  Waive ability to receive RTR frames. (not supported on all IP cores)
+  This mode activates the "RX mailbox mode" for better performance, on
+  some IP cores RTR frames cannot be received anymore.
+The setting can only be changed if the interface is down::
+    ip link set dev can0 down
+    ethtool --set-priv-flags can0 rx-rtr {off|on}
+    ip link set dev can0 up
diff --git a/Documentation/networking/device_drivers/can/index.rst b/Documentation/networking/device_drivers/can/index.rst
index 218276818968..58b6e0ad3030 100644
--- a/Documentation/networking/device_drivers/can/index.rst
+++ b/Documentation/networking/device_drivers/can/index.rst
@@ -10,6 +10,8 @@ Contents:
 .. toctree::
    :maxdepth: 2
+   freescale/flexcan
 .. only::  subproject and html

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