linux-kernel - [PATCH MANUALSEL 4.4] ALSA: pcsp: Make hrtimer forwarding more robust

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Message-Id: <20211006111313.264538-1-sashal@kernel.org>
Date:   Wed,  6 Oct 2021 07:13:12 -0400
From:   Sasha Levin <sashal@...nel.org>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:     Thomas Gleixner <tglx@...utronix.de>, alsa-devel@...a-project.org,
        Takashi Iwai <tiwai@...e.com>,
        Jaroslav Kysela <perex@...ex.cz>, Takashi Iwai <tiwai@...e.de>,
        Sasha Levin <sashal@...nel.org>
Subject: [PATCH MANUALSEL 4.4] ALSA: pcsp: Make hrtimer forwarding more robust

From: Thomas Gleixner <tglx@...utronix.de>

[ Upstream commit f2ff7147c6834f244b8ce636b12e71a3bd044629 ]

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>
Link: https://lore.kernel.org/r/20210923153339.623208460@linutronix.de
Signed-off-by: Takashi Iwai <tiwai@...e.de>
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 sound/drivers/pcsp/pcsp_lib.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/sound/drivers/pcsp/pcsp_lib.c b/sound/drivers/pcsp/pcsp_lib.c
index 3689f5f6be64..3766624c018f 100644
--- a/sound/drivers/pcsp/pcsp_lib.c
+++ b/sound/drivers/pcsp/pcsp_lib.c
@@ -144,7 +144,7 @@ enum hrtimer_restart pcsp_do_timer(struct hrtimer *handle)
 	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;
 }
-- 
2.33.0