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Date: Wed, 28 Oct 2020 17:20:24 +0100 From: Jan Kundrát <jan.kundrat@...net.cz> To: Thomas Petazzoni <thomas.petazzoni@...tlin.com> 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 On čtvrtek 1. října 2020 9:44:15 CEST, Thomas Petazzoni wrote: > Currently, the RX interrupt logic uses the RXEMPTY interrupt, with the > RXEMPTYINV bit set, which means we get an RX interrupt as soon as the > RX FIFO is non-empty. > > However, with the MAX310X having a FIFO of 128 bytes, this makes very > poor use of the FIFO: we trigger an interrupt as soon as the RX FIFO > has one byte, which means a lot of interrupts, each only collecting a > few bytes from the FIFO, causing a significant CPU load. 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) > + /* 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); With kind regards, Jan
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