linux-kernel - [patch 04/11] ALSA: pcsp: Make hrtimer forwarding more robust

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Message-ID: <20210923153339.623208460@linutronix.de>
Date:   Thu, 23 Sep 2021 18:04:25 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     LKML <linux-kernel@...r.kernel.org>
Cc:     Peter Zijlstra <peterz@...radead.org>, alsa-devel@...a-project.org,
        Takashi Iwai <tiwai@...e.com>, Jaroslav Kysela <perex@...ex.cz>
Subject: [patch 04/11] ALSA: pcsp: Make hrtimer forwarding more robust

The hrtimer callback pcsp_do_timer() prepares rearming of the timer with
hrtimer_forward(). hrtimer_forward() is intended to provide a mechanism to
forward the expiry time of the hrtimer by a multiple of the period argument
so that the expiry time greater than the time provided in the 'now'
argument.

pcsp_do_timer() invokes hrtimer_forward() with the current timer expiry
time as 'now' argument. That's providing a periodic timer expiry, but is
not really robust when the timer callback is delayed so that the resulting
new expiry time is already in the past which causes the callback to be
invoked immediately again. If the timer is delayed then the back to back
invocation is not really making it better than skipping the missed
periods. Sound is distorted in any case.

Use hrtimer_forward_now() which ensures that the next expiry is in the
future. This prevents hogging the CPU in the timer expiry code and allows
later on to remove hrtimer_forward() from the public interfaces.

Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
Cc: alsa-devel@...a-project.org
Cc: Takashi Iwai <tiwai@...e.com>
Cc: Jaroslav Kysela <perex@...ex.cz>
---
 sound/drivers/pcsp/pcsp_lib.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/sound/drivers/pcsp/pcsp_lib.c
+++ b/sound/drivers/pcsp/pcsp_lib.c
@@ -143,7 +143,7 @@ enum hrtimer_restart pcsp_do_timer(struc
 	if (pointer_update)
 		pcsp_pointer_update(chip);

-	hrtimer_forward(handle, hrtimer_get_expires(handle), ns_to_ktime(ns));
+	hrtimer_forward_now(handle, ns_to_ktime(ns));

 	return HRTIMER_RESTART;
 }