This patch tries to works around erratum DS80000789E 6 of the
mcp2518fd, the other variants of the chip family (mcp2517fd and
mcp251863) are probably also affected.

In the bad case, the driver reads a too large head index. In the
original code, the driver always trusted the read value, which caused
old, already processed CAN frames or new, incompletely written CAN
frames to be (re-)processed.

To work around this issue, keep a per FIFO timestamp [1] of the last
valid received CAN frame and compare against the timestamp of every
received CAN frame. If an old CAN frame is detected, abort the
iteration and mark the number of valid CAN frames as processed in the
chip by incrementing the FIFO's tail index.

Further tests showed that this workaround can recognize old CAN
frames, but a small time window remains in which partially written CAN
frames [2] are not recognized but then processed. These CAN frames
have the correct data and time stamps, but the DLC has not yet been
updated.

[1] As the raw timestamp overflows every 107 seconds (at the usual
    clock rate of 40 MHz) convert it to nanoseconds with the
    timecounter framework and use this to detect stale CAN frames.

Link: https://lore.kernel.org/all/BL3PR11MB64844C1C95CA3BDADAE4D8CCFBC99@BL3PR11MB6484.namprd11.prod.outlook.com [2]
Reported-by: Stefan Althöfer <Stefan.Althoefer@janztec.com>
Closes: https://lore.kernel.org/all/FR0P281MB1966273C216630B120ABB6E197E89@FR0P281MB1966.DEUP281.PROD.OUTLOOK.COMTested-by: Stefan Althöfer <Stefan.Althoefer@janztec.com>
Tested-by: Thomas Kopp <thomas.kopp@microchip.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>