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

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Message-ID: <f58e8dce-898c-8797-5293-1001c9a75381@hartkopp.net>
Date:   Sun, 16 Apr 2023 21:46:40 +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 16.04.23 17:35, Marc Kleine-Budde wrote:
> 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.

@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.

>> 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
> 

Best regards,
Oliver