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Date:   Mon, 17 Apr 2023 09:26:13 +0200
From:   Marc Kleine-Budde <mkl@pengutronix.de>
To:     Oliver Hartkopp <socketcan@hartkopp.net>
Cc:     Judith Mendez <jm@ti.com>,
        Chandrasekar Ramakrishnan <rcsekar@samsung.com>,
        Nishanth Menon <nm@ti.com>,
        Vignesh Raghavendra <vigneshr@ti.com>,
        Andrew Davis <afd@ti.com>,
        Wolfgang Grandegger <wg@grandegger.com>,
        Rob Herring <robh+dt@kernel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@linaro.org>,
        linux-can@vger.kernel.org, linux-kernel@vger.kernel.org,
        devicetree@vger.kernel.org, netdev@vger.kernel.org,
        Schuyler Patton <spatton@ti.com>
Subject: Re: [RFC PATCH 5/5] can: m_can: Add hrtimer to generate software
 interrupt
Message-ID: <20230417-taking-relieving-f2c8532864c0-mkl@pengutronix.de>
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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.
>=20
> @Judith: Can you acknowledge the value calculation?
>=20
> > > The "shortest" 11 bit CAN ID CAN frame is a Classical CAN frame with =
DLC =3D 0
> > > and 1 Mbit/s (arbitration) bitrate. This should be 48 bits @1Mbit =3D=
> ~50
> > > usecs
> > >=20
> > > So it should be something about
> > >=20
> > >      50 usecs * (FIFO queue len - 2)
> >=20
> > Where does the "2" come from?
>=20
> I thought about handling the FIFO earlier than it gets completely "full".
>=20
> 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 =C2=B5s * FIFO length - IRQ overhead.

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

regards,
Marc

--=20
Pengutronix e.K.                 | Marc Kleine-Budde          |
Embedded Linux                   | https://www.pengutronix.de |
Vertretung N=C3=BCrnberg              | Phone: +49-5121-206917-129 |
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