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

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Message-ID: <20230417-unsafe-porridge-0b712d137530-mkl@pengutronix.de>
Date:   Mon, 17 Apr 2023 21:26:04 +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 17.04.2023 19:34:03, Oliver Hartkopp wrote:
> On 17.04.23 09:26, Marc Kleine-Budde wrote:
> > 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.
> 
> I just tried an educated guess to prevent the FIFO to be filled up
> completely. How can you estimate the "IRQ overhead"? And how do you catch
> the CAN frames that are received while the IRQ is handled?

We're talking about polling, better call it "overhead" or "latency from
timer expiration until FIFO has at least one frame room". This value
depends on your system.

It depends on many, many factors, SoC, Kernel configuration (preempt RT,
powersaving, frequency scaling, system load. In your example it's 100
µs. I wanted to say there's an overhead (or latency) and we need enough
space in the FIFO, to cover it.

Marc

-- 
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