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

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Message-ID: <20230419-trimmer-fasting-928868e8cb81-mkl@pengutronix.de>
Date:   Wed, 19 Apr 2023 08:13:52 +0200
From:   Marc Kleine-Budde <mkl@...gutronix.de>
To:     "Mendez, Judith" <jm@...com>
Cc:     Oliver Hartkopp <socketcan@...tkopp.net>,
        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 18.04.2023 15:59:57, Mendez, Judith wrote:
> > > > > > > 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.
> > 
> 
> I am not sure how to estimate IRQ overhead, but FIFO length should be 64
> elements.

Ok

> 50 us * 62 is about 3.1 ms and we are using 1 ms timer polling interval.

Sounds good.

> Running a few benchmarks showed that using 0.5 ms timer polling interval
> starts to take a toll on CPU load, that is why I chose 1 ms polling
> interval.

However in the code you use 5 ms.

Marc

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