[PATCH 3/4] i2c: added I2C_AQ_NO_CLK_STRETCH to i2c-bcm2835.c

Eric Anholt
2015-10-28T19:16:51Z

Nicola Corna <nicola@corna.info> writes:
> As reported here
> http://www.advamation.com/knowhow/raspberrypi/rpi-i2c-bug.html
> the BCM2835 has a bug in its i2c implementation which prevents a correct
> clock stretching.
>
> Signed-off-by: Nicola Corna <nicola@corna.info>
> ---
> drivers/i2c/busses/i2c-bcm2835.c | 5 +++++
> 1 file changed, 5 insertions(+)
>
> diff --git a/drivers/i2c/busses/i2c-bcm2835.c b/drivers/i2c/busses/i2c-bcm2835.c
> index 3032b89..38d7102 100644
> --- a/drivers/i2c/busses/i2c-bcm2835.c
> +++ b/drivers/i2c/busses/i2c-bcm2835.c
> @@ -222,6 +222,10 @@ static const struct i2c_algorithm bcm2835_i2c_algo = {
> .functionality = bcm2835_i2c_func,
> };
>
> +static const struct i2c_adapter_quirks bcm2835_i2c_quirks = {
> + .flags = I2C_AQ_NO_CLK_STRETCH,
> +};
> +
> static int bcm2835_i2c_probe(struct platform_device *pdev)
> {
> struct bcm2835_i2c_dev *i2c_dev;
> @@ -293,6 +297,7 @@ static int bcm2835_i2c_probe(struct platform_device *pdev)
> adap->algo = &bcm2835_i2c_algo;
> adap->dev.parent = &pdev->dev;
> adap->dev.of_node = pdev->dev.of_node;
> + adap->quirks = &bcm2835_i2c_quirks;
>
> bcm2835_i2c_writel(i2c_dev, BCM2835_I2C_C, 0);

Looking at the driver, we're not setting the REDL/FEDL fields, which are
involved in the delays following clock stretching. Looks like I2C_DEL's
low 16 bits (rising edge delay) should be I2C_DIV / 4 (wait half the
period for the slave to update its data before we sample it), and the
high 16 bits (falling edge delay) should be I2C_DIV / 16 (put our data
out before we expet the slave to sample the value).

Note: this is not a detailed analysis, nor am I asserting that this is
the bug, just something that we seem to be missing in the Linux driver.

(Added cc to linux-rpi-kernel, in case anyone on that list is interested
in I2C issues on the platform)
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