V4L/DVB (4609): Improved i2c performance on software bitbang algoritm

Software I2C were using a very conservative value of udelay=16, meaning about
20Kbps. According with Philips I2C datasheet, the i2c should answer well for
times at the order of 4.7 us. So, using udelay=5 should work for all devices.
After this patch, the speed should be close to 66,67 Kbps, with the current
kernel software bitbang, with 30/60 duty cycle.
Anyway, added a new parameter (i2c_udelay) that would allow using conservative
values, if eventually a hardware doesn't support the datasheet values.
Thanks to Jean Delvare <khali@linux-fr.org> for pointing this improvement.

Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>

Documentation/video4linux/bttv/Insmod-options

@@ -54,6 +54,12 @@ bttv.o
 				dropouts.
 		chroma_agc=0/1	AGC of chroma signal, off by default.
 		adc_crush=0/1	Luminance ADC crush, on by default.
+		i2c_udelay=     Allow reduce I2C speed. Default is 5 usecs
+				(meaning 66,67 Kbps). The default is the
+				maximum supported speed by kernel bitbang
+				algoritm. You may use lower numbers, if I2C
+				messages are lost (16 is known to work on
+				all supported cards).
 
 		bttv_gpio=0/1
 		gpiomask=

drivers/media/video/bt8xx/bttv-i2c.c

@@ -8,6 +8,9 @@
 			   & Marcus Metzler (mocm@thp.uni-koeln.de)
     (c) 1999-2003 Gerd Knorr <kraxel@bytesex.org>
 
+    (c) 2005 Mauro Carvalho Chehab <mchehab@infradead.org>
+	- Multituner support and i2c address binding
+
     This program is free software; you can redistribute it and/or modify
     it under the terms of the GNU General Public License as published by
     the Free Software Foundation; either version 2 of the License, or
@@ -45,10 +48,18 @@ static int i2c_debug;
 static int i2c_hw;
 static int i2c_scan;
 module_param(i2c_debug, int, 0644);
+MODULE_PARM_DESC(i2c_hw,"configure i2c debug level");
 module_param(i2c_hw,    int, 0444);
+MODULE_PARM_DESC(i2c_hw,"force use of hardware i2c support, "
+			"instead of software bitbang");
 module_param(i2c_scan,  int, 0444);
 MODULE_PARM_DESC(i2c_scan,"scan i2c bus at insmod time");
 
+static unsigned int i2c_udelay = 5;
+module_param(i2c_udelay, int, 0444);
+MODULE_PARM_DESC(i2c_udelay,"soft i2c delay at insmod time, in usecs "
+		"(should be 5 or higher). Lower value means higher bus speed.");
+
 /* ----------------------------------------------------------------------- */
 /* I2C functions - bitbanging adapter (software i2c)                       */
 
@@ -426,6 +437,11 @@ int __devinit init_bttv_i2c(struct bttv *btv)
 		       sizeof(bttv_i2c_adap_hw_template));
 	} else {
 		/* bt848 */
+	/* Prevents usage of invalid delay values */
+		if (i2c_udelay<5)
+			i2c_udelay=5;
+		bttv_i2c_algo_bit_template.udelay=i2c_udelay;
+
 		memcpy(&btv->c.i2c_adap, &bttv_i2c_adap_sw_template,
 		       sizeof(bttv_i2c_adap_sw_template));
 		memcpy(&btv->i2c_algo, &bttv_i2c_algo_bit_template,

drivers/media/video/cx88/cx88-i2c.c

@@ -43,6 +43,11 @@ static unsigned int i2c_scan = 0;
 module_param(i2c_scan, int, 0444);
 MODULE_PARM_DESC(i2c_scan,"scan i2c bus at insmod time");
 
+static unsigned int i2c_udelay = 5;
+module_param(i2c_udelay, int, 0644);
+MODULE_PARM_DESC(i2c_udelay,"i2c delay at insmod time, in usecs "
+		"(should be 5 or higher). Lower value means higher bus speed.");
+
 #define dprintk(level,fmt, arg...)	if (i2c_debug >= level) \
 	printk(KERN_DEBUG "%s: " fmt, core->name , ## arg)
 
@@ -202,6 +207,11 @@ static void do_i2c_scan(char *name, struct i2c_client *c)
 /* init + register i2c algo-bit adapter */
 int cx88_i2c_init(struct cx88_core *core, struct pci_dev *pci)
 {
+	/* Prevents usage of invalid delay values */
+	if (i2c_udelay<5)
+		i2c_udelay=5;
+	cx8800_i2c_algo_template.udelay=i2c_udelay;
+
 	memcpy(&core->i2c_adap, &cx8800_i2c_adap_template,
 	       sizeof(core->i2c_adap));
 	memcpy(&core->i2c_algo, &cx8800_i2c_algo_template,