linux-kernel - Re: [PATCH v2] serial: max310x: rework RX interrupt handling

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Date:   Wed, 28 Oct 2020 18:06:51 +0100
From:   Thomas Petazzoni <thomas.petazzoni@...tlin.com>
To:     Jan Kundrát <jan.kundrat@...net.cz>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jirislaby@...nel.org>,
        <linux-serial@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        Mark Brown <broonie@...nel.org>,
        Andy Shevchenko <andy.shevchenko@...il.com>,
        <linux-spi@...r.kernel.org>
Subject: Re: [PATCH v2] serial: max310x: rework RX interrupt handling

Hello,

On Wed, 28 Oct 2020 17:20:24 +0100
Jan Kundrát <jan.kundrat@...net.cz> wrote:

> Thanks for taking the time to write this patch. We're using MAX14830 on a 
> Clearfog Base board via a 26 MHz SPI bus. Our code polls a custom 
> peripheral over UART at 115200 baud ten times a second; the messages are 
> typically shorter than 50 chars. Before this patch, `perf top --sort 
> comm,dso` showed about 28% CPU load for the corresponding SPI kthread, 
> after applying this patch it's between 3 and 5%. That's cool :).
> 
> Tested-by: Jan Kundrát <jan.kundrat@...net.cz>
> Reviewed-by: Jan Kundrát <jan.kundrat@...net.cz>
> 
> (but see below, please)

Thanks for your review and testing, and glad to hear that it also
improves the CPU load on your use-case.

> > +	/* Enable LSR, RX FIFO trigger, CTS change interrupts */
> > +	val = MAX310X_IRQ_LSR_BIT  | MAX310X_IRQ_RXFIFO_BIT | 
> > MAX310X_IRQ_TXEMPTY_BIT;
> >  	max310x_port_write(port, MAX310X_IRQEN_REG, val | MAX310X_IRQ_CTS_BIT);  
> 
> This comment doesn't fully match that code, and also the effective value 
> that is written to the register is split into two statements. What about 
> just:
> 
> +	/* Enable LSR, RX FIFO trigger, TX FIFO empty, CTS change interrupts */
> + 	max310x_port_write(port, MAX310X_IRQEN_REG, MAX310X_IRQ_LSR_BIT | 
> MAX310X_IRQ_RXFIFO_BIT | MAX310X_IRQ_TXEMPTY_BIT | MAX310X_IRQ_CTS_BIT);

Indeed, the comment should be updated, I'll fix that. Regarding the
effective value computed in two steps, it was already the case in the
current code:

	/* Enable RX, TX, CTS change interrupts */
	val = MAX310X_IRQ_RXEMPTY_BIT | MAX310X_IRQ_TXEMPTY_BIT;
        max310x_port_write(port, MAX310X_IRQEN_REG, val | MAX310X_IRQ_CTS_BIT);

but granted, that's not an excuse not to fix it.

On my way to send a v3 :-)

Thanks again,

Thomas
-- 
Thomas Petazzoni, CTO, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com