linux-kernel - [PATCH] Revert "spi: omap2-mcspi: Switch to readl_poll

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Message-ID: <20200910122624.8769-1-a-govindraju@ti.com>
Date:   Thu, 10 Sep 2020 17:56:24 +0530
From:   Aswath Govindraju <a-govindraju@...com>
To:     unlisted-recipients:; (no To-header on input)
CC:     Aswath Govindraju <a-govindraju@...com>,
        Sekhar Nori <nsekhar@...com>, Mark Brown <broonie@...nel.org>,
        Vignesh R <vigneshr@...com>, <linux-spi@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>
Subject: [PATCH] Revert "spi: omap2-mcspi: Switch to readl_poll_timeout()"

This reverts commit 13d515c796adc49a49b0cd2212ccd7f43a37fc5a.

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>
---

 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 1c9478e6e5d9..d4c9510af393 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>

@@ -348,9 +347,19 @@ 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,

base-commit: dff9f829e5b0181d4ed9d35aa62d695292399b54
-- 
2.17.1