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

[Mike Frysinger]

On Wednesday, July 07, 2010 00:45:42 Andrew Dyer wrote:
> On Tue, Jul 6, 2010 at 1:14 AM, Thomas Chou <thomas at wytron.com.tw> wrote:
> > We should not set SDA after TRISTATE, as it results in contention.
> > 
> > Signed-off-by: Thomas Chou <thomas at wytron.com.tw>
> > ---
> >  drivers/i2c/soft_i2c.c |    2 +-
> >  1 files changed, 1 insertions(+), 1 deletions(-)
> > 
> > diff --git a/drivers/i2c/soft_i2c.c b/drivers/i2c/soft_i2c.c
> > index 847db76..344b7f8 100644
> > --- a/drivers/i2c/soft_i2c.c
> > +++ b/drivers/i2c/soft_i2c.c
> > @@ -305,8 +305,8 @@ static uchar read_byte(int ack)
> >        /*
> >         * Read 8 bits, MSB first.
> >         */
> > -       I2C_TRISTATE;
> >        I2C_SDA(1);
> > +       I2C_TRISTATE;
> >        data = 0;
> >        for(j = 0; j < 8; j++) {
> >                I2C_SCL(0);
> > --
> 
> I2C_TRISTATE is supposed to be persistent until I2C_ACTIVE is called,
> so in the original code it should still be in effect when I2C_SDA(1)
> is executed and there should be no contention.  This patch causes the
> code to actively drive SDA high at the same time the addressed device
> might be driving it low, causing contention until the I2C_TRISTATE
> takes effect.
> 
> In some sense the code is misleadingly written, as it is not allowed
> in the spec to actively drive a '1' on the bus, a chip is only
> supposed to drive 'z' or '0', and the platform is supposed to provide
> the pullup current.  This comes more into play if the i2c bus supports
> clock stretching or arbitration among multiple masters.

how do you propose we get i2c gpio working ?  it works fine under Linux.  we 
cannot tristate a pin (set the gpio to an input) and then turn around and 
attempt to drive it (set the gpio to an output with a specific value).  that 
is what the code currently does.

our end goal is simple: have i2c gpio bitbanging work under u-boot like under 
linux.  we dont really care about the exact way we get there.  this patch was 
just one idea.
-mike
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