Re: [PATCH] FBTFT: fb_agm1264k: usleep_range is preferred over udelay
From: Geert Uytterhoeven
Date: Tue Sep 10 2019 - 04:00:04 EST
On Tue, Sep 10, 2019 at 2:25 AM Sreeram Veluthakkal <srrmvlt@xxxxxxxxx> wrote:
> On Mon, Sep 09, 2019 at 10:56:25AM +0100, Greg KH wrote:
> > On Sun, Sep 08, 2019 at 08:26:05PM -0500, Sreeram Veluthakkal wrote:
> > > This patch fixes the issue:
> > > FILE: drivers/staging/fbtft/fb_agm1264k-fl.c:88:
> > > CHECK: usleep_range is preferred over udelay; see Documentation/timers/timers-howto.rst
> > > + udelay(20);
> > >
> > > Signed-off-by: Sreeram Veluthakkal <srrmvlt@xxxxxxxxx>
Thanks for your patch!
> > > --- 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, 40);
> > Is it "safe" to wait 40? This kind of change you can only do if you
> > know this is correct. Have you tested this with hardware?
> > thanks,
> > greg k-h
> Hi Greg, No I haven't tested it, I don't have the hw. I dug depeer in to the usleep_range
> u64 delta = (u64)(max - min) * NSEC_PER_USEC;
> * The @delta argument gives the kernel the freedom to schedule the
> * actual wakeup to a time that is both power and performance friendly.
> * The kernel give the normal best effort behavior for "@expires+@delta",
> * but may decide to fire the timer earlier, but no earlier than @expires.
> My understanding is that keeping delta 0 (min=max=20) would be equivalent.
> I can revise the patch to usleep_range(20, 20) or usleep_range(20, 21) for a 1 usec delta.
> What do you suggest?
Please read the comment above the line you're referring to:
* In non-atomic context where the exact wakeup time is flexible, use
* usleep_range() instead of udelay(). The sleep improves responsiveness
* by avoiding the CPU-hogging busy-wait of udelay(), and the range reduces
* power usage by allowing hrtimers to take advantage of an already-
* scheduled interrupt instead of scheduling a new one just for this sleep.
Is this function always called in non-atomic context?
If it may be called in atomic context, replacing the udelay() call by a
usleep*() call will break the driver.
See also "the first and most important question" in
Documentation/timers/timers-howto.rst, as referred to by the checkpatch.pl
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx
In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
-- Linus Torvalds