linux-kernel - Re: [PATCH v9 09/10] serial: sc16is7xx: add post reset delay

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Date:   Mon, 31 Jul 2023 13:00:59 -0400
From:   Hugo Villeneuve <hugo@...ovil.com>
To:     Greg KH <gregkh@...uxfoundation.org>
Cc:     robh+dt@...nel.org, krzysztof.kozlowski+dt@...aro.org,
        conor+dt@...nel.org, jirislaby@...nel.org, jringle@...dpoint.com,
        isaac.true@...onical.com, jesse.sung@...onical.com,
        l.perczak@...lintechnologies.com, tomasz.mon@...lingroup.com,
        linux-serial@...r.kernel.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-gpio@...r.kernel.org,
        Hugo Villeneuve <hvilleneuve@...onoff.com>,
        Lech Perczak <lech.perczak@...lingroup.com>,
        andy.shevchenko@...il.com
Subject: Re: [PATCH v9 09/10] serial: sc16is7xx: add post reset delay

On Mon, 31 Jul 2023 17:57:50 +0200
Greg KH <gregkh@...uxfoundation.org> wrote:

> On Tue, Jul 25, 2023 at 10:23:41AM -0400, Hugo Villeneuve wrote:
> > From: Hugo Villeneuve <hvilleneuve@...onoff.com>
> > 
> > Make sure we wait at least 3us before initiating communication with
> > the device after reset.
> 
> That says what you do, but not _why_ you do it?

You are right, it is not clear. I will add a note that it is
recommended to do so by the manufacturer in the device datasheet.

> Please read the kernel documentation again for how to write a good
> changelog text.  It's usually the hardest part of submitting a patch.

Yes.


> > Signed-off-by: Hugo Villeneuve <hvilleneuve@...onoff.com>
> > Reviewed-by: Lech Perczak <lech.perczak@...lingroup.com>
> > Tested-by: Lech Perczak <lech.perczak@...lingroup.com>
> > ---
> >  drivers/tty/serial/sc16is7xx.c | 6 ++++++
> >  1 file changed, 6 insertions(+)
> > 
> > diff --git a/drivers/tty/serial/sc16is7xx.c b/drivers/tty/serial/sc16is7xx.c
> > index 49213be60baf..718e982e1efe 100644
> > --- a/drivers/tty/serial/sc16is7xx.c
> > +++ b/drivers/tty/serial/sc16is7xx.c
> > @@ -1526,6 +1526,12 @@ static int sc16is7xx_probe(struct device *dev,
> >  	regmap_write(s->regmap, SC16IS7XX_IOCONTROL_REG << SC16IS7XX_REG_SHIFT,
> >  			SC16IS7XX_IOCONTROL_SRESET_BIT);
> >  
> > +	/*
> > +	 * After reset, the host must wait at least 3us before initializing a
> > +	 * communication with the device.
> > +	 */
> > +	usleep_range(5, 10);
> 
> 5, 10 is NOT 3us.

In v3, Andy Shevchenko suggested "I would put (5, 10) instead to relax
a bit the scheduler."

Should I add a comment to that effect:

/*
 * After reset, the datasheet indicates that the host must wait at least
 * 3us before initializing a communication with the device.
 * Use (5, 10) range to relax the scheduler.
 */

?

Hugo.