kernel/spec-i2c: bugfix: add delays in bitbanging i2c

This is a long-standing bug, and completely my fault. Since our i2c is simply a bitbanging gpio-based interface, I need to add some delays into the operations. Without this, accessing eeprom worked with most mezzanines but not with all of them. The measured speed was more than 1MHz. The devices are rated at 400kHz, so delay 1.25 usecs at each output operation (thus, writing is three such delay for each bit). Addin the unpredictable software delays I see slightly more than than 200kHz in my test system. The driver now takes 2.5s to read the whole eeprom, but there's ample space to make it better by fixing the FIXME in spec-i2c.c . Signed-off-by: Alessandro Rubini <rubini@gnudd.com>

kernel/spec-i2c: bugfix: add delays in bitbanging i2c
This is a long-standing bug, and completely my fault. Since our i2c is simply a bitbanging gpio-based interface, I need to add some delays into the operations. Without this, accessing eeprom worked with most mezzanines but not with all of them. The measured speed was more than 1MHz. The devices are rated at 400kHz, so delay 1.25 usecs at each output operation (thus, writing is three such delay for each bit). Addin the unpredictable software delays I see slightly more than than 200kHz in my test system. The driver now takes 2.5s to read the whole eeprom, but there's ample space to make it better by fixing the FIXME in spec-i2c.c . Signed-off-by: Alessandro Rubini <rubini@gnudd.com>
8b7a486f · Alessandro Rubini · 210ba8b0 · 8b7a486f
Commit 8b7a486f authored May 09, 2013 by Alessandro Rubini
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spec-i2c.c kernel/spec-i2c.c +3 -0

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--- a/kernel/spec-i2c.c
+++ b/kernel/spec-i2c.c
@@ -15,6 +15,7 @@
 #include <linux/time.h>
 #include <linux/pci.h>
 #include <linux/slab.h>
+#include <linux/delay.h>
 #include <linux/fmc.h>
 #include "spec.h"
 #include "hw/wrc_syscon_regs.h"
@@ -45,6 +46,7 @@ static void set_sda(struct fmc_device *fmc, int val)
 		fmc_writel(fmc, SYSC_GPSR_FMC_SDA, SYSC_REG_GPSR);
 	else
 		fmc_writel(fmc, SYSC_GPCR_FMC_SDA, SYSC_REG_GPCR);
+	ndelay(1250); /* 400kHz -> 2.5us/loop */
 }
 static void set_scl(struct fmc_device *fmc, int val)
@@ -53,6 +55,7 @@ static void set_scl(struct fmc_device *fmc, int val)
 		fmc_writel(fmc, SYSC_GPSR_FMC_SCL, SYSC_REG_GPSR);
 	else
 		fmc_writel(fmc, SYSC_GPCR_FMC_SCL, SYSC_REG_GPCR);
+	ndelay(1250); /* 400kHz -> 2.5us/loop */
 }
 static int get_sda(struct fmc_device *fmc)