linux-kernel - Re: [Outreachy kernel] [PATCH] staging: fbtft: Replace udelay with preferred usleep

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Message-ID: <20200331001637.6bf108ed@elisabeth>
Date:   Tue, 31 Mar 2020 00:16:37 +0200
From:   Stefano Brivio <sbrivio@...hat.com>
To:     "John B. Wyatt IV" <jbwyatt4@...il.com>
Cc:     Julia Lawall <julia.lawall@...ia.fr>,
        Soumyajit Deb <debsoumyajit100@...il.com>,
        outreachy-kernel@...glegroups.com,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Payal Kshirsagar <payal.s.kshirsagar.98@...il.com>,
        dri-devel@...ts.freedesktop.org, linux-fbdev@...r.kernel.org,
        devel@...verdev.osuosl.org, linux-kernel@...r.kernel.org
Subject: Re: [Outreachy kernel] [PATCH] staging: fbtft: Replace udelay with
 preferred usleep_range

On Mon, 30 Mar 2020 15:03:55 -0700
"John B. Wyatt IV" <jbwyatt4@...il.com> wrote:

> On Mon, 2020-03-30 at 19:40 +0200, Stefano Brivio wrote:
> > On Sun, 29 Mar 2020 12:37:18 +0200 (CEST)
> > Julia Lawall <julia.lawall@...ia.fr> wrote:
> >   
> > > On Sun, 29 Mar 2020, Soumyajit Deb wrote:
> > >   
> > > > I had the same doubt the other day about the replacement of
> > > > udelay() with
> > > > usleep_range(). The corresponding range for the single argument
> > > > value of
> > > > udelay() is quite confusing as I couldn't decide the range. But
> > > > as much as I
> > > > noticed checkpatch.pl gives warning for replacing udelay() with
> > > > usleep_range() by checking the argument value of udelay(). In the
> > > > documentation, it is written udelay() should be used for a sleep
> > > > time of at
> > > > most 10 microseconds but between 10 microseconds and 20
> > > > milliseconds,
> > > > usleep_range() should be used. 
> > > > I think the range is code specific and will depend on what range
> > > > is
> > > > acceptable and doesn't break the code.
> > > >  Please correct me if I am wrong.    
> > > 
> > > The range depends on the associated hardware.  
> > 
> > John, by the way, here you could have checked the datasheet of this
> > LCD
> > controller. It's a pair of those:
> > 	https://www.sparkfun.com/datasheets/LCD/ks0108b.pdf
> 
> No I have not. This datasheet is a little over my head honestly.
> 
> What would you recommend to get familiar with datasheets like this?

Well, you don't necessarily have to, there are many subsystems in the
kernel which are almost completely abstracted away from hardware.

If you're interested, look around yourself for something simple chip,
or get something that you can easily plug on a "maker board", Raspberry
Pi, something like that. Perhaps via I²C or SPI.

Some types of sensors (temperature, pressure) have very simple
datasheets. If you are allergic to hardware, try:
	$ ls -Ssl drivers/iio/*

pick the smallest sensor driver in the category that is the most likely
to spark your interest, and go through it, checking it against the
datasheet, at some point it will make sense.

-- 
Stefano