Hi Sreeram,

On Tue, Sep 10, 2019 at 2:25 AM Sreeram Veluthakkal srrmvlt@gmail.com wrote:

On Mon, Sep 09, 2019 at 10:56:25AM +0100, Greg KH wrote:

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On Sun, Sep 08, 2019 at 08:26:05PM -0500, Sreeram Veluthakkal wrote:

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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@gmail.com

Thanks for your patch!

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--- 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

https://github.com/torvalds/linux/blob/master/kernel/time/timer.c#L1993 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 message.

Gr{oetje,eeting}s,

Geert