The mcp251x driver uses both receiving mailboxes of the CAN controller
chips. For retrieving the CAN frames from the controller via SPI, it checks
once per interrupt which mailboxes have been filled and will retrieve the
messages accordingly.

This introduces a race condition, as another CAN frame can enter mailbox 1
while mailbox 0 is emptied. If now another CAN frame enters mailbox 0 until
the interrupt handler is called next, mailbox 0 is emptied before
mailbox 1, leading to out-of-order CAN frames in the network device.

This is fixed by checking the interrupt flags once again after freeing
mailbox 0, to correctly also empty mailbox 1 before leaving the handler.

For reproducing the bug I created the following setup:
 - Two CAN devices, one Raspberry Pi with MCP2515, the other can be any.
 - Setup CAN to 1 MHz
 - Spam bursts of 5 CAN-messages with increasing CAN-ids
 - Continue sending the bursts while sleeping a second between the bursts
 - Check on the RPi whether the received messages have increasing CAN-ids
 - Without this patch, every burst of messages will contain a flipped pair

v3: https://lore.kernel.org/all/20220804075914.67569-1-sebastian.wuerl@ororatech.com
v2: https://lore.kernel.org/all/20220804064803.63157-1-sebastian.wuerl@ororatech.com
v1: https://lore.kernel.org/all/20220803153300.58732-1-sebastian.wuerl@ororatech.com

Fixes: bf66f373 ("can: mcp251x: Move to threaded interrupts instead of workqueues.")
Signed-off-by: Sebastian Würl <sebastian.wuerl@ororatech.com>
Link: https://lore.kernel.org/all/20220804081411.68567-1-sebastian.wuerl@ororatech.com
[mkl: reduce scope of intf1, eflag1]
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>