spi/ath79: add delay between SCK changes

The driver uses the "as fast as it can" approach
to drive the SCK signal. However this does not
work with certain low speed SPI chips (e.g. the
PCF2123 RTC chip).

The patch adds per-bit slowdowns in order to be
able to use the driver with such chips as well.
Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>

drivers/spi/spi-ath79.c

@@ -24,17 +24,24 @@
 #include <linux/spi/spi_bitbang.h>
 #include <linux/bitops.h>
 #include <linux/gpio.h>
+#include <linux/clk.h>
+#include <linux/err.h>
 
 #include <asm/mach-ath79/ar71xx_regs.h>
 #include <asm/mach-ath79/ath79_spi_platform.h>
 
 #define DRV_NAME	"ath79-spi"
 
+#define ATH79_SPI_RRW_DELAY_FACTOR	12000
+#define MHZ				(1000 * 1000)
+
 struct ath79_spi {
 	struct spi_bitbang	bitbang;
 	u32			ioc_base;
 	u32			reg_ctrl;
 	void __iomem		*base;
+	struct clk		*clk;
+	unsigned		rrw_delay;
 };
 
 static inline u32 ath79_spi_rr(struct ath79_spi *sp, unsigned reg)
@@ -52,6 +59,12 @@ static inline struct ath79_spi *ath79_spidev_to_sp(struct spi_device *spi)
 	return spi_master_get_devdata(spi->master);
 }
 
+static inline void ath79_spi_delay(struct ath79_spi *sp, unsigned nsecs)
+{
+	if (nsecs > sp->rrw_delay)
+		ndelay(nsecs - sp->rrw_delay);
+}
+
 static void ath79_spi_chipselect(struct spi_device *spi, int is_active)
 {
 	struct ath79_spi *sp = ath79_spidev_to_sp(spi);
@@ -184,7 +197,9 @@ static u32 ath79_spi_txrx_mode0(struct spi_device *spi, unsigned nsecs,
 
 		/* setup MSB (to slave) on trailing edge */
 		ath79_spi_wr(sp, AR71XX_SPI_REG_IOC, out);
+		ath79_spi_delay(sp, nsecs);
 		ath79_spi_wr(sp, AR71XX_SPI_REG_IOC, out | AR71XX_SPI_IOC_CLK);
+		ath79_spi_delay(sp, nsecs);
 
 		word <<= 1;
 	}
@@ -198,6 +213,7 @@ static int ath79_spi_probe(struct platform_device *pdev)
 	struct ath79_spi *sp;
 	struct ath79_spi_platform_data *pdata;
 	struct resource	*r;
+	unsigned long rate;
 	int ret;
 
 	master = spi_alloc_master(&pdev->dev, sizeof(*sp));
@@ -236,12 +252,36 @@ static int ath79_spi_probe(struct platform_device *pdev)
 		goto err_put_master;
 	}
 
+	sp->clk = clk_get(&pdev->dev, "ahb");
+	if (IS_ERR(sp->clk)) {
+		ret = PTR_ERR(sp->clk);
+		goto err_unmap;
+	}
+
+	ret = clk_enable(sp->clk);
+	if (ret)
+		goto err_clk_put;
+
+	rate = DIV_ROUND_UP(clk_get_rate(sp->clk), MHZ);
+	if (!rate) {
+		ret = -EINVAL;
+		goto err_clk_disable;
+	}
+
+	sp->rrw_delay = ATH79_SPI_RRW_DELAY_FACTOR / rate;
+	dev_dbg(&pdev->dev, "register read/write delay is %u nsecs\n",
+		sp->rrw_delay);
+
 	ret = spi_bitbang_start(&sp->bitbang);
 	if (ret)
-		goto err_unmap;
+		goto err_clk_disable;
 
 	return 0;
 
+err_clk_disable:
+	clk_disable(sp->clk);
+err_clk_put:
+	clk_put(sp->clk);
 err_unmap:
 	iounmap(sp->base);
 err_put_master:
@@ -256,6 +296,8 @@ static int ath79_spi_remove(struct platform_device *pdev)
 	struct ath79_spi *sp = platform_get_drvdata(pdev);
 
 	spi_bitbang_stop(&sp->bitbang);
+	clk_disable(sp->clk);
+	clk_put(sp->clk);
 	iounmap(sp->base);
 	platform_set_drvdata(pdev, NULL);
 	spi_master_put(sp->bitbang.master);