linux-kernel - Re: [PATCH] rtc: cmos: avoid taking rtc

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Message-ID: <Zxv6ltPBWfWs957R@google.com>
Date: Fri, 25 Oct 2024 13:07:50 -0700
From: Dmitry Torokhov <dmitry.torokhov@...il.com>
To: Mateusz Jończyk <mat.jonczyk@...pl>
Cc: Alexandre Belloni <alexandre.belloni@...tlin.com>,
	Mario Limonciello <mario.limonciello@....com>,
	Dan Carpenter <dan.carpenter@...aro.org>,
	Joy Chakraborty <joychakr@...gle.com>, linux-rtc@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] rtc: cmos: avoid taking rtc_lock for extended period of
 time

On Fri, Oct 25, 2024 at 08:01:37AM +0200, Mateusz Jończyk wrote:
> Dnia 24 października 2024 22:37:08 CEST, Dmitry Torokhov <dmitry.torokhov@...il.com> napisał/a:
> >On my device reading entirety of /sys/devices/pnp0/00:03/cmos_nvram0/nvmem
> >takes about 9 msec during which time interrupts are off on the CPU that
> >does the read and the thread that performs the read can not be migrated
> >or preempted by another higher priority thread (RT or not).
> >
> >Allow readers and writers be preempted by taking and releasing rtc_lock
> >spinlock for each individual byte read or written rather than once per
> >read/write request.
> 
> Hello, 
> A nice idea! 
> (sorry for any formatting problems, I'm on a train right now) 
> 
> >
> >Signed-off-by: Dmitry Torokhov <dmitry.torokhov@...il.com>
> >---
> > drivers/rtc/rtc-cmos.c | 31 +++++++++++++++----------------
> > 1 file changed, 15 insertions(+), 16 deletions(-)
> >
> >diff --git a/drivers/rtc/rtc-cmos.c b/drivers/rtc/rtc-cmos.c
> >index 35dca2accbb8..e8f2fe0d8560 100644
> >--- a/drivers/rtc/rtc-cmos.c
> >+++ b/drivers/rtc/rtc-cmos.c
> >@@ -645,18 +645,17 @@ static int cmos_nvram_read(void *priv, unsigned int off, void *val,
> > 	unsigned char *buf = val;
> > 
> > 	off += NVRAM_OFFSET;
> >-	spin_lock_irq(&rtc_lock);
> >-	for (; count; count--, off++) {
> >+	for (; count; count--, off++, buf++) {
> >+		guard(spinlock_irq)(&rtc_lock);
> > 		if (off < 128)
> >-			*buf++ = CMOS_READ(off);
> >+			*buf = CMOS_READ(off);
> > 		else if (can_bank2)
> >-			*buf++ = cmos_read_bank2(off);
> >+			*buf = cmos_read_bank2(off);
> > 		else
> >-			break;
> >+			return -EIO;
> > 	}
> >-	spin_unlock_irq(&rtc_lock);
> > 
> >-	return count ? -EIO : 0;
> >+	return count;
> 
> return 0;

Oh, yes, of course, thank you.

-- 
Dmitry