Re: [PATCH] serial: imx: reduce RX interrupt frequency - Uwe Kleine-König

From: "Uwe Kleine-König" <u.kleine-koenig@pengutronix.de>
To: Sergey Organov <sorganov@gmail.com>
Cc: "Pengutronix Kernel Team" <kernel@pengutronix.de>,
	"Jiri Slaby" <jirislaby@kernel.org>,
	"Greg Kroah-Hartman" <gregkh@linuxfoundation.org>,
	"Tomasz Moń" <tomasz.mon@camlingroup.com>,
	k.drobinski@camlintechnologies.com,
	"NXP Linux Team" <linux-imx@nxp.com>,
	linux-serial@vger.kernel.org, "Shawn Guo" <shawnguo@kernel.org>,
	"Fabio Estevam" <festevam@gmail.com>,
	"Sascha Hauer" <s.hauer@pengutronix.de>
Subject: Re: [PATCH] serial: imx: reduce RX interrupt frequency
Date: Thu, 6 Jan 2022 16:05:52 +0100	[thread overview]
Message-ID: <20220106150552.vdt5qabe2xj4kasg@pengutronix.de> (raw)
In-Reply-To: <87tuei4882.fsf@osv.gnss.ru>

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On Wed, Jan 05, 2022 at 04:34:21PM +0300, Sergey Organov wrote:
> Uwe Kleine-König <u.kleine-koenig@pengutronix.de> writes:
> 
> > On Tue, Jan 04, 2022 at 12:38:01PM +0100, Greg Kroah-Hartman wrote:
> >> On Tue, Jan 04, 2022 at 12:13:06PM +0100, Tomasz Moń wrote:
> >> > On 04.01.2022 11:54, Greg Kroah-Hartman wrote:
> >> > > Why can't you do this dynamically based on the baud rate so as to always
> >> > > work properly for all speeds without increased delays for slower ones?
> >> > 
> >> > Could you please advise on which baud rates to consider as slow? Does it
> >> > sound good to have the old trigger level for rates up to and including
> >> > 115200 and the new one for faster ones?
> >> 
> >> You tell me, you are the one seeing this issue and are seeing delays on
> >> slower values with your change.  Do some testing to see where the curve
> >> is.
> >
> > Maybe it's more sensible to make this even more dynamic: e.g. keep it at
> > 1 as default and increase the water level when a certain irq frequency
> > is reached?
> 
> Too complex, and too many questions, I'm afraid. What is "irq
> frequency", exactly? For this particular driver, or overall system?
> Measured on what time interval? What is the threshold? Do we drop the
> water level back to 1 when "irq frequency" is down again? Will we just
> create re-configure storm at some conditions? Etc.....

It could be as easy as increasing the waterlevel by one if an RX irq
happens with USR1.AGTIM = 0 and reset to 1 if USR1.AGTIM = 1.

This makes sure that receiving at a low frequency makes the hardware
interrupt the CPU early, and a burst doesn't starve the CPU.

Best regards
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | https://www.pengutronix.de/ |

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