Re: [PATCH v2 2/2] can: m_can: fix missed interrupts with m_can_pci

[Markus Schneider-Pargmann]

Hi Matthias,

On Thu, Sep 19, 2024 at 01:27:28PM GMT, Matthias Schiffer wrote:
> The interrupt line of PCI devices is interpreted as edge-triggered,
> however the interrupt signal of the m_can controller integrated in Intel

I have a similar patch though for a different setup (I didn't send it
yet). I have a tcan chip wired to a pin that is only capable of edge
interrupts.

> Elkhart Lake CPUs appears to be generated level-triggered.
> 
> Consider the following sequence of events:
> 
> - IR register is read, interrupt X is set
> - A new interrupt Y is triggered in the m_can controller
> - IR register is written to acknowledge interrupt X. Y remains set in IR
> 
> As at no point in this sequence no interrupt flag is set in IR, the
> m_can interrupt line will never become deasserted, and no edge will ever
> be observed to trigger another run of the ISR. This was observed to
> result in the TX queue of the EHL m_can to get stuck under high load,
> because frames were queued to the hardware in m_can_start_xmit(), but
> m_can_finish_tx() was never run to account for their successful
> transmission.
> 
> To fix the issue, repeatedly read and acknowledge interrupts at the
> start of the ISR until no interrupt flags are set, so the next incoming
> interrupt will also result in an edge on the interrupt line.
> 
> Fixes: cab7ffc0324f ("can: m_can: add PCI glue driver for Intel Elkhart Lake")
> Signed-off-by: Matthias Schiffer <matthias.schiffer@xxxxxxxxxxxxxxx>
> ---
> 
> v2: introduce flag is_edge_triggered, so we can avoid the loop on !m_can_pci
> 
>  drivers/net/can/m_can/m_can.c     | 21 ++++++++++++++++-----
>  drivers/net/can/m_can/m_can.h     |  1 +
>  drivers/net/can/m_can/m_can_pci.c |  1 +
>  3 files changed, 18 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/net/can/m_can/m_can.c b/drivers/net/can/m_can/m_can.c
> index 47481afb9add3..2e182c3c98fed 100644
> --- a/drivers/net/can/m_can/m_can.c
> +++ b/drivers/net/can/m_can/m_can.c
> @@ -1207,20 +1207,31 @@ static void m_can_coalescing_update(struct m_can_classdev *cdev, u32 ir)
>  static int m_can_interrupt_handler(struct m_can_classdev *cdev)
>  {
>  	struct net_device *dev = cdev->net;
> -	u32 ir;
> +	u32 ir = 0, ir_read;
>  	int ret;
>  
>  	if (pm_runtime_suspended(cdev->dev))
>  		return IRQ_NONE;
>  
> -	ir = m_can_read(cdev, M_CAN_IR);
> +	/* For m_can_pci, the interrupt line is interpreted as edge-triggered,
> +	 * but the m_can controller generates them as level-triggered. We must
> +	 * observe that IR is 0 at least once to be sure that the next
> +	 * interrupt will generate an edge.
> +	 */

Could you please remove this hardware specific comment? As mentioned
above this will be independent of any specific hardware.

> +	while ((ir_read = m_can_read(cdev, M_CAN_IR)) != 0) {
> +		ir |= ir_read;
> +
> +		/* ACK all irqs */
> +		m_can_write(cdev, M_CAN_IR, ir);
> +
> +		if (!cdev->is_edge_triggered)
> +			break;
> +	}
> +
>  	m_can_coalescing_update(cdev, ir);
>  	if (!ir)
>  		return IRQ_NONE;
>  
> -	/* ACK all irqs */
> -	m_can_write(cdev, M_CAN_IR, ir);
> -
>  	if (cdev->ops->clear_interrupts)
>  		cdev->ops->clear_interrupts(cdev);
>  
> diff --git a/drivers/net/can/m_can/m_can.h b/drivers/net/can/m_can/m_can.h
> index 92b2bd8628e6b..8c17eb94d2f98 100644
> --- a/drivers/net/can/m_can/m_can.h
> +++ b/drivers/net/can/m_can/m_can.h
> @@ -99,6 +99,7 @@ struct m_can_classdev {
>  	int pm_clock_support;
>  	int pm_wake_source;
>  	int is_peripheral;
> +	bool is_edge_triggered;

To avoid confusion could you rename it to irq_edge_triggered or
something similar, to make clear that it is not about the chip but the
way the interrupt line is connected?

Also I am not sure it is possible, but could you use
irq_get_trigger_type() to see if it is a level or edge based interrupt?
Then we wouldn't need this additional parameter at all and could just
detect it in m_can.c.

Best
Markus

>  
>  	// Cached M_CAN_IE register content
>  	u32 active_interrupts;
> diff --git a/drivers/net/can/m_can/m_can_pci.c b/drivers/net/can/m_can/m_can_pci.c
> index d72fe771dfc7a..f98527981402a 100644
> --- a/drivers/net/can/m_can/m_can_pci.c
> +++ b/drivers/net/can/m_can/m_can_pci.c
> @@ -127,6 +127,7 @@ static int m_can_pci_probe(struct pci_dev *pci, const struct pci_device_id *id)
>  	mcan_class->pm_clock_support = 1;
>  	mcan_class->pm_wake_source = 0;
>  	mcan_class->can.clock.freq = id->driver_data;
> +	mcan_class->is_edge_triggered = true;
>  	mcan_class->ops = &m_can_pci_ops;
>  
>  	pci_set_drvdata(pci, mcan_class);
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