linux-kernel - Re: [RFC PATCH 0/5] Enable multiple MCAN on AM62x

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Message-ID: <20230414-tubular-service-3404c64c6c62-mkl@pengutronix.de>
Date:   Fri, 14 Apr 2023 19:49:13 +0200
From:   Marc Kleine-Budde <mkl@...gutronix.de>
To:     Judith Mendez <jm@...com>
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 0/5] Enable multiple MCAN on AM62x

On 13.04.2023 17:30:46, Judith Mendez wrote:
> On AM62x there is one MCAN in MAIN domain and two in MCU domain.
> The MCANs in MCU domain were not enabled since there is no
> hardware interrupt routed to A53 GIC interrupt controller.
> Therefore A53 Linux cannot be interrupted by MCU MCANs.

Is this a general hardware limitation, that effects all MCU domain
peripherals? Is there a mailbox mechanism between the MCU and the MAIN
domain, would it be possible to pass the IRQ with a small firmware on
the MCU? Anyways, that's future optimization.

> This solution instantiates a hrtimer with 1 ms polling interval
> for a MCAN when there is no hardware interrupt. This hrtimer
> generates a recurring software interrupt which allows to call the
> isr. The isr will check if there is pending transaction by reading
> a register and proceed normally if there is.
> 
> On AM62x this series enables two MCU MCAN which will use the hrtimer
> implementation. MCANs with hardware interrupt routed to A53 Linux
> will continue to use the hardware interrupt as expected.
> 
> Timer polling method was tested on both classic CAN and CAN-FD
> at 125 KBPS, 250 KBPS, 1 MBPS and 2.5 MBPS with 4 MBPS bitrate
> switching.
> 
> Letency and CPU load benchmarks were tested on 3x MCAN on AM62x.
> 1 MBPS timer polling interval is the better timer polling interval
> since it has comparable latency to hardware interrupt with the worse
> case being 1ms + CAN frame propagation time and CPU load is not
> substantial. Latency can be improved further with less than 1 ms
> polling intervals, howerver it is at the cost of CPU usage since CPU
> load increases at 0.5 ms and lower polling periods than 1ms.

Some Linux input drivers have the property poll-interval, would it make
sense to ass this here too?

> Note that in terms of power, enabling MCU MCANs with timer-polling
> implementation might have negative impact since we will have to wake
> up every 1 ms whether there are CAN packets pending in the RX FIFO or
> not. This might prevent the CPU from entering into deeper idle states
> for extended periods of time.
> 
> This patch series depends on 'Enable CAN PHY transceiver driver':
> https://lore.kernel.org/lkml/775ec9ce-7668-429c-a977-6c8995968d6e@app.fastmail.com/T/

Marc

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