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

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Date:   Sun, 29 Mar 2020 15:22:41 +0300
From:   Andy Shevchenko <andy.shevchenko@...il.com>
To:     Sam Muhammed <jane.pnx9@...il.com>
Cc:     Julia Lawall <julia.lawall@...ia.fr>,
        Soumyajit Deb <debsoumyajit100@...il.com>,
        John Wyatt <jbwyatt4@...il.com>,
        outreachy-kernel@...glegroups.com,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Payal Kshirsagar <payal.s.kshirsagar.98@...il.com>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        linux-fbdev@...r.kernel.org,
        "open list:STAGING SUBSYSTEM" <devel@...verdev.osuosl.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [Outreachy kernel] [PATCH] staging: fbtft: Replace udelay with
 preferred usleep_range

On Sun, Mar 29, 2020 at 2:23 PM Sam Muhammed <jane.pnx9@...il.com> wrote:
> On Sun, 2020-03-29 at 12:37 +0200, Julia Lawall wrote:
> > On Sun, 29 Mar 2020, Soumyajit Deb wrote:
> >

First of all, let's stop topposting.

> > > 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.  Just because checkpatch
> > suggests something doesn't mean that it is easy to address the problem.

> Hi all, i think when it comes to a significant change in the code, we
> should at least be familiar with the driver or be able to test the
> change.
>
> In the very beginning of the Documentation/timers/timers-howto.rst
> there is the question:
> "Is my code in an atomic context?"
> It's not just about the range, it's more of at which context this code
> runs, for atomic-context -> udelay must be used.
> for non-atomic context -> usleep-range is better for power-management.
>
> unless we are familiar with the driver we wouldn't really know in what
> context this code is run at.
>
> This thread though had the same conversation about this change, for the
> same driver.
> https://patchwork.kernel.org/patch/11137125/

While it's a good discussion it reminds me that this entire function,
i.e. reset(), repeats the on provided by fbtft core.
Yes, the only question if it's atomic or not. IIRC ->reset() is being
called only in non-atomic contexts and keeping reset signal longer is
fine (but better to check with datasheet).

So, I would rather to drop the function completely in order to use
fbtft's core one.

--
With Best Regards,
Andy Shevchenko