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

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Message-ID: <5ece3561-4690-a721-aa83-adf80d0be9f5@ti.com>
Date:   Tue, 18 Apr 2023 15:59:57 -0500
From:   "Mendez, Judith" <jm@...com>
To:     Marc Kleine-Budde <mkl@...gutronix.de>,
        Oliver Hartkopp <socketcan@...tkopp.net>
CC:     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

Hello Marc,

On 4/17/2023 2:26 PM, Marc Kleine-Budde wrote:
> On 17.04.2023 19:34:03, Oliver Hartkopp wrote:
>> 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?
> 
> 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.

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

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.

regards,
Judith