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

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Message-ID: <20230416-failing-washbasin-e4fa5caea267-mkl@pengutronix.de>
Date:   Sun, 16 Apr 2023 17:35:33 +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 14:33:11, Oliver Hartkopp wrote:
> 
> 
> On 4/14/23 20:20, Marc Kleine-Budde wrote:
> > On 13.04.2023 17:30:51, Judith Mendez wrote:
> > > Add a hrtimer to MCAN struct. Each MCAN will have its own
> > > hrtimer instantiated if there is no hardware interrupt found.
> > > 
> > > The hrtimer will generate a software interrupt every 1 ms. In
> > 
> > Are you sure about the 1ms?

I had the 5ms that are actually used in the code in mind. But this is a
good 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?

> if there is some FIFO involved, right?

Yes, the mcan core has a FIFO. In the current driver the FIFO
configuration is done via device tree and fixed after that. And I don't
know the size of the available RAM in the mcan IP core on that TI SoC.

Marc

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