netdev - Re: [PATCH 2/2] can: m_can: fix missed interrupts with m_can

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Message-ID: <20240919-tourmaline-jaguar-of-reverence-4875d2-mkl@pengutronix.de>
Date: Thu, 19 Sep 2024 10:47:06 +0200
From: Marc Kleine-Budde <mkl@...gutronix.de>
To: Matthias Schiffer <matthias.schiffer@...tq-group.com>
Cc: Chandrasekar Ramakrishnan <rcsekar@...sung.com>, 
	Vincent Mailhol <mailhol.vincent@...adoo.fr>, "David S. Miller" <davem@...emloft.net>, 
	Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, 
	Paolo Abeni <pabeni@...hat.com>, Martin Hundebøll <martin@...nix.com>, 
	Markus Schneider-Pargmann <msp@...libre.com>, "Felipe Balbi (Intel)" <balbi@...nel.org>, 
	Raymond Tan <raymond.tan@...el.com>, Jarkko Nikula <jarkko.nikula@...ux.intel.com>, 
	linux-can@...r.kernel.org, netdev@...r.kernel.org, linux-kernel@...r.kernel.org, 
	linux@...tq-group.com
Subject: Re: [PATCH 2/2] can: m_can: fix missed interrupts with m_can_pci

On 18.09.2024 16:21:54, 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
> 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@...tq-group.com>
> ---
>  drivers/net/can/m_can/m_can.c | 18 +++++++++++++-----
>  1 file changed, 13 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..363732517c3c5 100644
> --- a/drivers/net/can/m_can/m_can.c
> +++ b/drivers/net/can/m_can/m_can.c
> @@ -1207,20 +1207,28 @@ 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.
> +	 */
> +	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);
> +	}

This probably causes a measurable overhead on peripheral devices, think
about limiting this to !peripheral devices or introduce a new quirk that
is only set for the PCI devices.

Marc

> +
>  	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);
>  
> -- 
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