linux-kernel - Re: [RFC PATCH 5/5] can: m_can: Add hrtimer to generate software interrupt

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Message-ID: <20230417-taking-relieving-f2c8532864c0-mkl@pengutronix.de>
Date:   Mon, 17 Apr 2023 09:26:13 +0200
From:   Marc Kleine-Budde <mkl@...gutronix.de>
To:     Oliver Hartkopp <socketcan@...tkopp.net>
Cc:     Judith Mendez <jm@...com>,
        Chandrasekar Ramakrishnan <rcsekar@...sung.com>,
        Nishanth Menon <nm@...com>,
        Vignesh Raghavendra <vigneshr@...com>,
        Andrew Davis <afd@...com>,
        Wolfgang Grandegger <wg@...ndegger.com>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        linux-can@...r.kernel.org, linux-kernel@...r.kernel.org,
        devicetree@...r.kernel.org, netdev@...r.kernel.org,
        Schuyler Patton <spatton@...com>
Subject: Re: [RFC PATCH 5/5] can: m_can: Add hrtimer to generate software
 interrupt

On 16.04.2023 21:46:40, Oliver Hartkopp wrote:
> > I had the 5ms that are actually used in the code in mind. But this is a
> > good calculation.
> 
> @Judith: Can you acknowledge the value calculation?
> 
> > > The "shortest" 11 bit CAN ID CAN frame is a Classical CAN frame with DLC = 0
> > > and 1 Mbit/s (arbitration) bitrate. This should be 48 bits @1Mbit => ~50
> > > usecs
> > > 
> > > So it should be something about
> > > 
> > >      50 usecs * (FIFO queue len - 2)
> > 
> > Where does the "2" come from?
> 
> I thought about handling the FIFO earlier than it gets completely "full".
> 
> The fetching routine would need some time too and the hrtimer could also
> jitter to some extend.

I was assuming something like this.

I would argue that the polling time should be:

    50 µs * FIFO length - IRQ overhead.

The max IRQ overhead depends on your SoC and kernel configuration.

regards,
Marc

-- 
Pengutronix e.K.                 | Marc Kleine-Budde          |
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