On 03.08.2022 17:32:59, Sebastian Würl wrote:
> 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, an 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.
Nitpick: On mcp2515 the buffer is freed automatically.
With this change the interrupt flags are _always_ checked a 2nd time,
even if buffer 0 is not serviced. See below for a change proposal.
> 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
>
> Signed-off-by: Sebastian Würl <sebastian.wuerl@xxxxxxxxxxxxx>
> ---
> drivers/net/can/spi/mcp251x.c | 21 ++++++++++++++-------
> 1 file changed, 14 insertions(+), 7 deletions(-)
>
> diff --git a/drivers/net/can/spi/mcp251x.c b/drivers/net/can/spi/mcp251x.c
> index 666a4505a55a..687aafef4717 100644
> --- a/drivers/net/can/spi/mcp251x.c
> +++ b/drivers/net/can/spi/mcp251x.c
> @@ -1063,17 +1063,14 @@ static irqreturn_t mcp251x_can_ist(int irq, void *dev_id)
> mutex_lock(&priv->mcp_lock);
> while (!priv->force_quit) {
> enum can_state new_state;
> - u8 intf, eflag;
> + u8 intf, intf0, intf1, eflag, eflag0, eflag1;
> u8 clear_intf = 0;
> int can_id = 0, data1 = 0;
>
> - mcp251x_read_2regs(spi, CANINTF, &intf, &eflag);
> -
> - /* mask out flags we don't care about */
> - intf &= CANINTF_RX | CANINTF_TX | CANINTF_ERR;
> + mcp251x_read_2regs(spi, CANINTF, &intf0, &eflag0);
>
> /* receive buffer 0 */
> - if (intf & CANINTF_RX0IF) {
> + if (intf0 & CANINTF_RX0IF) {
> mcp251x_hw_rx(spi, 0);
> /* Free one buffer ASAP
> * (The MCP2515/25625 does this automatically.)
> @@ -1083,14 +1080,24 @@ static irqreturn_t mcp251x_can_ist(int irq, void *dev_id)
> CANINTF_RX0IF, 0x00);
> }
>
> + /* intf needs to be read again to avoid a race condition */
> + mcp251x_read_2regs(spi, CANINTF, &intf1, &eflag1);
I think we only have to re-read the interrupt flag if we actually read
data from buffer 0. So what about moving this into the if () { } above?
> +
> /* receive buffer 1 */
> - if (intf & CANINTF_RX1IF) {
> + if (intf1 & CANINTF_RX1IF) {
> mcp251x_hw_rx(spi, 1);
> /* The MCP2515/25625 does this automatically. */
> if (mcp251x_is_2510(spi))
> clear_intf |= CANINTF_RX1IF;
> }
>
> + /* combine flags from both operations for error handling */
> + intf = intf0 | intf1;
> + eflag = eflag0 | eflag1;
> +
> + /* mask out flags we don't care about */
> + intf &= CANINTF_RX | CANINTF_TX | CANINTF_ERR;
> +
> /* any error or tx interrupt we need to clear? */
> if (intf & (CANINTF_ERR | CANINTF_TX))
> clear_intf |= intf & (CANINTF_ERR | CANINTF_TX);
Marc
--
Pengutronix e.K. | Marc Kleine-Budde |
Embedded Linux | https://www.pengutronix.de |
Vertretung West/Dortmund | Phone: +49-231-2826-924 |
Amtsgericht Hildesheim, HRA 2686 | Fax: +49-5121-206917-5555 |
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