Hi James,
> On 27/07/15 12:47, Sifan Naeem wrote:
> > The requested bit rate can be outside the range supported by the driver.
> > The maximum bit rate this driver supports at the moment is 400Khz.
> >
> > Return -EINVAL if the bit rate is larger than 400khz.
> >
> > Maximum speed supported by the driver can be increased to 1Mhz by
> > adding support for "fast plus mode" in the future.
> >
> > Fixes: 27bce4 ("i2c: img-scb: Add Imagination Technologies I2C SCB
> > driver")
> > Signed-off-by: Sifan Naeem <sifan.naeem@xxxxxxxxxx>
> > Cc: Stable kernel (v3.19+) <stable@xxxxxxxxxxxxxxx>
> > ---
> > drivers/i2c/busses/i2c-img-scb.c | 6 ++++++
> > 1 file changed, 6 insertions(+)
> >
> > diff --git a/drivers/i2c/busses/i2c-img-scb.c
> > b/drivers/i2c/busses/i2c-img-scb.c
> > index bbfee33..07a039c 100644
> > --- a/drivers/i2c/busses/i2c-img-scb.c
> > +++ b/drivers/i2c/busses/i2c-img-scb.c
> > @@ -1157,6 +1157,12 @@ static int img_i2c_init(struct img_i2c *i2c)
> > break;
> > }
> > }
> > + if (i2c->bitrate > timing.max_bitrate) {
> > + dev_err(i2c->adap.dev.parent,
> > + "requested bitrate (%d) is higher than the max
> bitrate supported (%d)\n",
> > + i2c->bitrate, timing.max_bitrate);
> > + return -EINVAL;
> > + }
>
> The timing is only chosen if i2c->bitrate <= timing.max_bitrate, so you'd only
> hit this case if none of the timings were valid, in which case timing ==
> timings[0], so when you print timing.max_bitrate it won't be the max bitrate
> supported, it'll be the max bitrate of the first timing in the array.
>
> Anyway, I think the original intent of the DT provided clock-frequency was
> more as a maximum bitrate. This was used with TZ1090 as a way to limit the
> bitrate of the bus if some devices on the bus don't support the full speed,
> e.g. we had an HDMI chip that would get confused at 400khz.
>
> So would it be acceptable to change it to just clamp the bitrate to the
> maximum rate supported, before the bitrate_khz calculation, such that if you
> specified 1MHz in DT, it could safely fall back to 400KHz until the driver
> supports the faster mode?
>
I'll rework this, when the requested bit rate is larger than the maximum supported, set the bitrate down to the maximum supported before bitrate_khz is calculated.
Thanks,
Sifan
> Cheers
> James
>
> >
> > /* Find the prescale that would give us that inc (approx delay = 0) */
> > prescale = SCB_OPT_INC * clk_khz / (256 * 16 * bitrate_khz);
> >
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