Re: [Outreachy kernel] [PATCH] staging: fbtft: Replace udelay with preferred usleep_range

[Julia Lawall <julia.lawall@inria.fr>]

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

julia

>
> More clarification on this issue will be helpful.
>
> On Sun, 29 Mar 2020, 15:17 Julia Lawall, <julia.lawall@inria.fr> wrote:
>
>
>       On Sun, 29 Mar 2020, John Wyatt wrote:
>
>       > On Sun, 2020-03-29 at 11:28 +0200, Julia Lawall wrote:
>       > >
>       > > On Sun, 29 Mar 2020, John B. Wyatt IV wrote:
>       > >
>       > > > Fix style issue with usleep_range being reported as
>       preferred over
>       > > > udelay.
>       > > >
>       > > > Issue reported by checkpatch.
>       > > >
>       > > > Please review.
>       > > >
>       > > > As written in Documentation/timers/timers-howto.rst udelay
>       is the
>       > > > generally preferred API. hrtimers, as noted in the docs,
>       may be too
>       > > > expensive for this short timer.
>       > > >
>       > > > Are the docs out of date, or, is this a checkpatch issue?
>       > > >
>       > > > Signed-off-by: John B. Wyatt IV <jbwyatt4@gmail.com>
>       > > > ---
>       > > >  drivers/staging/fbtft/fb_agm1264k-fl.c | 2 +-
>       > > >  1 file changed, 1 insertion(+), 1 deletion(-)
>       > > >
>       > > > diff --git a/drivers/staging/fbtft/fb_agm1264k-fl.c
>       > > > b/drivers/staging/fbtft/fb_agm1264k-fl.c
>       > > > index eeeeec97ad27..019c8cce6bab 100644
>       > > > --- a/drivers/staging/fbtft/fb_agm1264k-fl.c
>       > > > +++ b/drivers/staging/fbtft/fb_agm1264k-fl.c
>       > > > @@ -85,7 +85,7 @@ static void reset(struct fbtft_par *par)
>       > > >   dev_dbg(par->info->device, "%s()\n", __func__);
>       > > >
>       > > >   gpiod_set_value(par->gpio.reset, 0);
>       > > > - udelay(20);
>       > > > + usleep_range(20, 20);
>       > >
>       > > usleep_range should have a range, eg usleep_range(50,
>       100);.  But it
>       > > is
>       > > hard to know a priori what the range should be.  So it is
>       probably
>       > > better
>       > > to leave the code alone.
>       >
>       > Understood.
>       >
>       > With the question I wrote in the commit message:
>       >
>       > "As written in Documentation/timers/timers-howto.rst udelay is
>       the
>       > generally preferred API. hrtimers, as noted in the docs, may
>       be too
>       > expensive for this short timer.
>       >
>       > Are the docs out of date, or, is this a checkpatch issue?"
>       >
>       > Is usleep_range too expensive for this operation?
>       >
>       > Why does checkpatch favor usleep_range while the docs favor
>       udelay?
>
>       I don't know the answer in detail, but it is quite possible that
>       checkpatch doesn't pay any attention to the delay argument. 
>       Checkpatch is
>       a perl script that highlights things that may be of concern.  It
>       is not a
>       precise static analsis tool.
>
>       As a matter of form, all of your Please review comments should
>       have been
>       put below the ---.  Currently, if someone had wanted to apply
>       the patch,
>       you would make them do extra work to remove this information.
>
>       julia
>
>       >
>       > >
>       > > julia
>       > >
>       > > >   gpiod_set_value(par->gpio.reset, 1);
>       > > >   mdelay(120);
>       > > >  }
>       > > > --
>       > > > 2.25.1
>       > > >
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>       > > > .
>       > > >
>       >
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