linux-kernel - [PATCH] spi-gpio: Implement spidelay() for busses that need it.

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Message-Id: <200902151923.00120.mb@bu3sch.de>
Date:	Sun, 15 Feb 2009 19:22:59 +0100
From:	Michael Buesch <mb@...sch.de>
To:	David Brownell <dbrownell@...rs.sourceforge.net>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, openwrt-devel@...ts.openwrt.org
Subject: [PATCH] spi-gpio: Implement spidelay() for busses that need it.

Implement the SPI-GPIO delay function for busses that need speed limitation.
If the special all-inline API is used, the delay is disabled at compiletime.
Otherwise it's an unlikely condition that can be disabled via max_speed_hz.
So it should have minimal or zero impact on the speed of implementations without delay.

Signed-off-by: Michael Buesch <mb@...sch.de>

---

I need this delay when using a slow microcontroller attached via SPI-GPIO
to a fast embedded router device.

Index: linux-2.6/drivers/spi/spi_gpio.c
===================================================================
--- linux-2.6.orig/drivers/spi/spi_gpio.c	2009-02-15 16:27:16.000000000 +0100
+++ linux-2.6/drivers/spi/spi_gpio.c	2009-02-15 19:19:36.000000000 +0100
@@ -21,6 +21,7 @@
 #include <linux/init.h>
 #include <linux/platform_device.h>
 #include <linux/gpio.h>
+#include <linux/delay.h>
 
 #include <linux/spi/spi.h>
 #include <linux/spi/spi_bitbang.h>
@@ -69,6 +70,7 @@ struct spi_gpio {
  *		#define	SPI_MOSI_GPIO	120
  *		#define	SPI_SCK_GPIO	121
  *		#define	SPI_N_CHIPSEL	4
+ *		#undef NEED_SPIDELAY
  *		#include "spi_gpio.c"
  */
 
@@ -76,6 +78,7 @@ struct spi_gpio {
 #define DRIVER_NAME	"spi_gpio"
 
 #define GENERIC_BITBANG	/* vs tight inlines */
+#define NEED_SPIDELAY	1
 
 /* all functions referencing these symbols must define pdata */
 #define SPI_MISO_GPIO	((pdata)->miso)
@@ -120,12 +123,20 @@ static inline int getmiso(const struct s
 #undef pdata
 
 /*
- * NOTE:  this clocks "as fast as we can".  It "should" be a function of the
- * requested device clock.  Software overhead means we usually have trouble
- * reaching even one Mbit/sec (except when we can inline bitops), so for now
- * we'll just assume we never need additional per-bit slowdowns.
+ * NOTE:  to clock "as fast as we can", set spi_device.max_speed_hz
+ * and spi_transfer.speed_hz to 0.
+ * Otherwise this is a function of the requested device clock.
+ * Software overhead means we usually have trouble
+ * reaching even one Mbit/sec (except when we can inline bitops). So on small
+ * embedded devices with fast SPI slaves you usually don't need a delay.
  */
-#define spidelay(nsecs)	do {} while (0)
+static inline void spidelay(unsigned nsecs)
+{
+#ifdef NEED_SPIDELAY
+	if (unlikely(nsecs))
+		ndelay(nsecs);
+#endif /* NEED_SPIDELAY */
+}
 
 #define	EXPAND_BITBANG_TXRX
 #include <linux/spi/spi_bitbang.h>

-- 
Greetings, Michael.
--
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