On Wed, Feb 24, 2021 at 02:27:28PM +0000, Mariusz Madej wrote:

Hi,

I have a problem with m_can controller in my sama5d2 processor.
Under heavy can traffic it happens that my device starts to report (dmesg):

[   77.610000] m_can_platform f8054000.can can0: Rx FIFO 0 Message Lost
[   77.620000] m_can_platform f8054000.can can0: Rx FIFO 0 Message Lost
[   77.630000] m_can_platform f8054000.can can0: Rx FIFO 0 Message Lost
[   77.630000] m_can_platform f8054000.can can0: Rx FIFO 0 Message Lost
[   77.640000] m_can_platform f8054000.can can0: Rx FIFO 0 Message Lost
[   77.640000] m_can_platform f8054000.can can0: Rx FIFO 0 Message Lost
[   77.650000] m_can_platform f8054000.can can0: Rx FIFO 0 Message Lost
[   77.660000] m_can_platform f8054000.can can0: Rx FIFO 0 Message Lost
[   77.660000] m_can_platform f8054000.can can0: Rx FIFO 0 Message Lost

what causes large load problem in my system.

How heavy is this traffic? Is the bus operating at a very high bitrate?
Are there any other useful lines in dmesg?

The only place in m_can.c file, where interrupt register is cleared is function
called when interrupt arrives

static irqreturn_t m_can_isr(int irq, void *dev_id)
{
.
.
        /* ACK all irqs */
        if (ir & IR_ALL_INT)
                m_can_write(cdev, M_CAN_IR, ir);
.
.
}

But when we enter 'NAPI mode' in heavy load we are never get to this function
until load gets lower and interrupts are enabled again. In this situation,
this code:

The m_can driver handles the IRQ by offloading the RX to a NAPI queue,
so the RX procedure is deferred, and is scheduled to happen at a
(slightly) later time. As far as I understand it, interrupts are not
disabled at any point.

That is why we got so many messages in a row for so long time. So clearing
RXFS_RFL bit after warning is issued could be a solution.

RXFS_RFL is a flag in a status register, not an interrupt flag. There is
a corresponding interrupt flag, but that is cleared along with the rest,
at the top of m_can_isr.

I think you are losing messages because the traffic is too heavy for
your system to read out the messages fast enough. That is the usual
reason for seeing "Rx FIFO 0 Message Lost".

--
Regards,

Torin Cooper-Bennun
Software Engineer  | maxiluxsystems.com