linux-kernel - [PATCH 1/2] spi: pxa2xx: fix SCR (divisor) calculation

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Date:   Wed, 10 Apr 2019 14:51:35 +0200
From:   Flavio Suligoi <f.suligoi@...m.it>
To:     Daniel Mack <daniel@...que.org>,
        Haojian Zhuang <haojian.zhuang@...il.com>,
        Robert Jarzmik <robert.jarzmik@...e.fr>,
        Mark Brown <broonie@...nel.org>
CC:     <linux-arm-kernel@...ts.infradead.org>,
        <linux-spi@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        Flavio Suligoi <f.suligoi@...m.it>
Subject: [PATCH 1/2] spi: pxa2xx: fix SCR (divisor) calculation

Calculate the divisor for the SCR (Serial Clock Rate), avoiding
that the SSP transmission rate can be greater than the device rate.

When the division between the SSP clock and the device rate generates
a reminder, we have to increment by one the divisor.
In this way the resulting SSP clock will never be greater than the
device SPI max frequency.

For example, with:

 - ssp_clk  = 50 MHz
 - dev freq = 15 MHz

without this patch the SSP clock will be greater than 15 MHz:

 - 25 MHz for PXA25x_SSP and CE4100_SSP
 - 16,56 MHz for the others

Instead, with this patch, we have in both case an SSP clock of 12.5MHz,
so the max rate of the SPI device clock is respected.

Signed-off-by: Flavio Suligoi <f.suligoi@...m.it>
---
 drivers/spi/spi-pxa2xx.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/drivers/spi/spi-pxa2xx.c b/drivers/spi/spi-pxa2xx.c
index f7068cc..c9560a1 100644
--- a/drivers/spi/spi-pxa2xx.c
+++ b/drivers/spi/spi-pxa2xx.c
@@ -884,10 +884,15 @@ static unsigned int ssp_get_clk_div(struct driver_data *drv_data, int rate)
 
 	rate = min_t(int, ssp_clk, rate);
 
+	/*
+	 * Calculate the divisor for the SCR (Serial Clock Rate), avoiding
+	 * that the SSP transmission rate can be greater than the device rate
+	 */
 	if (ssp->type == PXA25x_SSP || ssp->type == CE4100_SSP)
-		return (ssp_clk / (2 * rate) - 1) & 0xff;
+		return (ssp_clk / (2 * rate) - 1 +
+			(ssp_clk % (2 * rate) ? 1 : 0)) & 0xff;
 	else
-		return (ssp_clk / rate - 1) & 0xfff;
+		return (ssp_clk / rate - 1 + (ssp_clk % rate ? 1 : 0)) & 0xfff;
 }
 
 static unsigned int pxa2xx_ssp_get_clk_div(struct driver_data *drv_data,
-- 
2.7.4