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

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Message-ID: <B8A0CC86-7C24-4154-B8F3-69CD6B6C94BD@o2.pl>
Date: Fri, 25 Oct 2024 08:01:37 +0200
From: Mateusz Jończyk <mat.jonczyk@...pl>
To: Dmitry Torokhov <dmitry.torokhov@...il.com>,
 Alexandre Belloni <alexandre.belloni@...tlin.com>
CC: 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

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;

when you are at it. 

> }
> 
> static int cmos_nvram_write(void *priv, unsigned int off, void *val,
>@@ -671,23 +670,23 @@ static int cmos_nvram_write(void *priv, unsigned int off, void *val,
> 	 * NVRAM to update, updating checksums is also part of its job.
> 	 */
> 	off += NVRAM_OFFSET;
>-	spin_lock_irq(&rtc_lock);
>-	for (; count; count--, off++) {
>+	for (; count; count--, off++, buf++) {
> 		/* don't trash RTC registers */
> 		if (off == cmos->day_alrm
> 				|| off == cmos->mon_alrm
> 				|| off == cmos->century)
>-			buf++;
>-		else if (off < 128)
>-			CMOS_WRITE(*buf++, off);
>+			continue;
>+
>+		guard(spinlock_irq)(&rtc_lock);
>+		if (off < 128)
>+			CMOS_WRITE(*buf, off);
> 		else if (can_bank2)
>-			cmos_write_bank2(*buf++, off);
>+			cmos_write_bank2(*buf, off);
> 		else
>-			break;
>+			return -EIO;
> 	}
>-	spin_unlock_irq(&rtc_lock);
> 
>-	return count ? -EIO : 0;
>+	return count;

return 0;

> }
> 
> /*----------------------------------------------------------------*/