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

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Message-ID: <25806ec7-64c5-3421-aea1-c0d431e3f27f@hartkopp.net>
Date:   Mon, 17 Apr 2023 19:34:03 +0200
From:   Oliver Hartkopp <socketcan@...tkopp.net>
To:     Marc Kleine-Budde <mkl@...gutronix.de>
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 17.04.23 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?

Best regards,
Oliver