[U-Boot] [PATCH] i2c: fix SDA contention in read_byte()

[Mike Frysinger]

On Monday, July 12, 2010 00:14:39 Thomas Chou wrote:
> Reinhard Meyer wrote:
> > Whenever possible by the hardware, I make I2C_SDA/SCL(1) do a tri-state
> > and I2C_TRISTATE and I2C_ACTIVE are empty.
> 
> Dear Mike,
> 
> I traced the i2c-gpio.c of linux and realized that there are potential
> bus contention with the current soft_i2c.c if the ports are not
> open-drained.
> 
> Reinhard suggested a solution, which was similar to what linux driver
> does. So I would withdraw my SDA patch.
> 
> For our i2c gpio framework, I added these changes and tested on my
> boards. Please check if it works on yours.
> 
> # ifndef I2C_ACTIVE
> #  define I2C_ACTIVE do {} while (0)
> # endif
> 
> # ifndef I2C_TRISTATE
> #  define I2C_TRISTATE do {} while (0)
> # endif
> 
> # ifndef I2C_SDA
> #  define I2C_SDA(bit) \
> 	if (bit) {						\
> 		gpio_direction_input(CONFIG_SOFT_I2C_GPIO_SDA);	\
> 	} else {						\
> 		gpio_direction_output(CONFIG_SOFT_I2C_GPIO_SDA, 0);\
> 	}
> # endif
> 
> I didn't tristate SCL(1) because it cannot be tristated on some nios2
> boards. As soft_i2c of u-boot didn't support clock stretching, it
> shouldn't matter.

this sort of fixed things for me.  the problem i was having is that repeated 
start is not the default behavior for the soft i2c bus master.  i thought that 
this would be the default, but there doesnt seem to be any documentation on 
the expected standard behavior out of the box.

once i enabled that for all Blackfin boards, soft i2c worked for me.  so i 
posted an updated patch ... if you need some open drain behavior, i think it 
best to post a patch on top of that.
-mike
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