Hi Sreeram,

On Tue, Sep 10, 2019 at 2:25 AM Sreeram Veluthakkal [off-list ref] wrote:

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

quoted

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

quoted

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 <redacted>

Thanks for your patch!

quoted

quoted

--- 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 ) 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

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

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