linux-kernel - [PATCH 5.8 254/633] spi: omap2-mcspi: Improve performance waiting for CHSTAT

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Date:   Tue, 27 Oct 2020 14:49:57 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Aswath Govindraju <a-govindraju@...com>,
        Mark Brown <broonie@...nel.org>,
        Sasha Levin <sashal@...nel.org>
Subject: [PATCH 5.8 254/633] spi: omap2-mcspi: Improve performance waiting for CHSTAT

From: Aswath Govindraju <a-govindraju@...com>

[ Upstream commit 7b1d96813317358312440d0d07abbfbeb0ef8d22 ]

This reverts commit 13d515c796 (spi: omap2-mcspi: Switch to
readl_poll_timeout()).

The amount of time spent polling for the MCSPI_CHSTAT bits to be set on
AM335x-icev2 platform is less than 1us (about 0.6us) in most cases, with
or without using DMA. So, in most cases the function need not sleep.
Also, setting the sleep_usecs to zero would not be optimal here because
ktime_add_us() used in readl_poll_timeout() is slower compared to the
direct addition used after the revert. So, it is sub-optimal to use
readl_poll_timeout in this case.

When DMA is not enabled, this revert results in an increase of about 27%
in throughput and decrease of about 20% in CPU usage. However, the CPU
usage and throughput are almost the same when used with DMA.

Therefore, fix this by reverting the commit which switched to using
readl_poll_timeout().

Fixes: 13d515c796ad ("spi: omap2-mcspi: Switch to readl_poll_timeout()")
Signed-off-by: Aswath Govindraju <a-govindraju@...com>
Link: https://lore.kernel.org/r/20200910122624.8769-1-a-govindraju@ti.com
Signed-off-by: Mark Brown <broonie@...nel.org>
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 drivers/spi/spi-omap2-mcspi.c | 17 +++++++++++++----
 1 file changed, 13 insertions(+), 4 deletions(-)

diff --git a/drivers/spi/spi-omap2-mcspi.c b/drivers/spi/spi-omap2-mcspi.c
index e9e256718ef4a..10d8a722b0833 100644
--- a/drivers/spi/spi-omap2-mcspi.c
+++ b/drivers/spi/spi-omap2-mcspi.c
@@ -24,7 +24,6 @@
 #include <linux/of.h>
 #include <linux/of_device.h>
 #include <linux/gcd.h>
-#include <linux/iopoll.h>

 #include <linux/spi/spi.h>
 #include <linux/gpio.h>
@@ -349,9 +348,19 @@ static void omap2_mcspi_set_fifo(const struct spi_device *spi,

 static int mcspi_wait_for_reg_bit(void __iomem *reg, unsigned long bit)
 {
-	u32 val;
-
-	return readl_poll_timeout(reg, val, val & bit, 1, MSEC_PER_SEC);
+	unsigned long timeout;
+
+	timeout = jiffies + msecs_to_jiffies(1000);
+	while (!(readl_relaxed(reg) & bit)) {
+		if (time_after(jiffies, timeout)) {
+			if (!(readl_relaxed(reg) & bit))
+				return -ETIMEDOUT;
+			else
+				return 0;
+		}
+		cpu_relax();
+	}
+	return 0;
 }

 static int mcspi_wait_for_completion(struct  omap2_mcspi *mcspi,
-- 
2.25.1