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SubjectRe: [PATCH v2 1/3] i3c: fix i2c and i3c scl rate by bus mode
On Thu, 6 Jun 2019 17:16:55 +0000
Vitor Soares <Vitor.Soares@synopsys.com> wrote:

> From: Boris Brezillon <boris.brezillon@collabora.com>
> Date: Thu, Jun 06, 2019 at 15:18:44
>
> > On Thu, 6 Jun 2019 16:00:01 +0200
> > Vitor Soares <Vitor.Soares@synopsys.com> wrote:
> >
> > > Currently the I3C framework limits SCL frequency to FM speed when
> > > dealing with a mixed slow bus, even if all I2C devices are FM+ capable.
> > >
> > > The core was also not accounting for I3C speed limitations when
> > > operating in mixed slow mode and was erroneously using FM+ speed as the
> > > max I2C speed when operating in mixed fast mode.
> > >
> > > Fixes: 3a379bbcea0a ("i3c: Add core I3C infrastructure")
> > > Signed-off-by: Vitor Soares <vitor.soares@synopsys.com>
> > > Cc: Boris Brezillon <bbrezillon@kernel.org>
> > > Cc: <stable@vger.kernel.org>
> > > Cc: <linux-kernel@vger.kernel.org>
> > > ---
> > > Changes in v2:
> > > Enhance commit message
> > > Add dev_warn() in case user-defined i2c rate doesn't match LVR constraint
> > > Add dev_warn() in case user-defined i3c rate lower than i2c rate.
> > >
> > > drivers/i3c/master.c | 61 +++++++++++++++++++++++++++++++++++++++++-----------
> > > 1 file changed, 48 insertions(+), 13 deletions(-)
> > >
> > > diff --git a/drivers/i3c/master.c b/drivers/i3c/master.c
> > > index 5f4bd52..8cd5824 100644
> > > --- a/drivers/i3c/master.c
> > > +++ b/drivers/i3c/master.c
> > > @@ -91,6 +91,12 @@ void i3c_bus_normaluse_unlock(struct i3c_bus *bus)
> > > up_read(&bus->lock);
> > > }
> > >
> > > +static struct i3c_master_controller *
> > > +i3c_bus_to_i3c_master(struct i3c_bus *i3cbus)
> > > +{
> > > + return container_of(i3cbus, struct i3c_master_controller, bus);
> > > +}
> > > +
> > > static struct i3c_master_controller *dev_to_i3cmaster(struct device *dev)
> > > {
> > > return container_of(dev, struct i3c_master_controller, dev);
> > > @@ -565,20 +571,48 @@ static const struct device_type i3c_masterdev_type = {
> > > .groups = i3c_masterdev_groups,
> > > };
> > >
> > > -int i3c_bus_set_mode(struct i3c_bus *i3cbus, enum i3c_bus_mode mode)
> > > +int i3c_bus_set_mode(struct i3c_bus *i3cbus, enum i3c_bus_mode mode,
> > > + unsigned long max_i2c_scl_rate)
> > > {
> > > - i3cbus->mode = mode;
> > >
> > > - if (!i3cbus->scl_rate.i3c)
> > > - i3cbus->scl_rate.i3c = I3C_BUS_TYP_I3C_SCL_RATE;
> > > + struct i3c_master_controller *master = i3c_bus_to_i3c_master(i3cbus);
> > >
> > > - if (!i3cbus->scl_rate.i2c) {
> > > - if (i3cbus->mode == I3C_BUS_MODE_MIXED_SLOW)
> > > - i3cbus->scl_rate.i2c = I3C_BUS_I2C_FM_SCL_RATE;
> > > - else
> > > - i3cbus->scl_rate.i2c = I3C_BUS_I2C_FM_PLUS_SCL_RATE;
> > > + i3cbus->mode = mode;
> > > +
> > > + switch (i3cbus->mode) {
> > > + case I3C_BUS_MODE_PURE:
> > > + if (!i3cbus->scl_rate.i3c)
> > > + i3cbus->scl_rate.i3c = I3C_BUS_TYP_I3C_SCL_RATE;
> > > + break;
> > > + case I3C_BUS_MODE_MIXED_FAST:
> > > + if (!i3cbus->scl_rate.i3c)
> > > + i3cbus->scl_rate.i3c = I3C_BUS_TYP_I3C_SCL_RATE;
> > > + if (!i3cbus->scl_rate.i2c)
> > > + i3cbus->scl_rate.i2c = max_i2c_scl_rate;
> > > + break;
> > > + case I3C_BUS_MODE_MIXED_SLOW:
> > > + if (!i3cbus->scl_rate.i2c)
> > > + i3cbus->scl_rate.i2c = max_i2c_scl_rate;
> > > + if (!i3cbus->scl_rate.i3c ||
> > > + i3cbus->scl_rate.i3c > i3cbus->scl_rate.i2c)
> > > + i3cbus->scl_rate.i3c = i3cbus->scl_rate.i2c;
> > > + break;
> > > + default:
> > > + return -EINVAL;
> > > }
> > >
> > > + if (i3cbus->scl_rate.i3c < i3cbus->scl_rate.i2c)
> > > + dev_warn(&master->dev,
> > > + "i3c-scl-hz=%ld lower than i2c-scl-hz=%ld\n",
> > > + i3cbus->scl_rate.i3c, i3cbus->scl_rate.i2c);
> > > +
> > > + if (i3cbus->scl_rate.i2c != I3C_BUS_I2C_FM_SCL_RATE &&
> > > + i3cbus->scl_rate.i2c != I3C_BUS_I2C_FM_PLUS_SCL_RATE &&
> > > + i3cbus->mode != I3C_BUS_MODE_PURE)
> >
> > If you are so strict, there's clearly no point exposing an i2c-scl-hz
> > property. I'm still not convinced having an i2c rate that's slower than
> > what the I2C/I3C spec defines as the *typical* rate is a bad thing,
>
> I'm not been strictive, I just inform the user about that case.

Then use dev_debug() and don't make the trace conditional on
i2c_rate != typical_rate. The only case where we should warn users
is i2c_rate > typical_rate, because that might lead to malfunctions.

>
> > just
> > like I'm not convinced having an I3C rate that's slower than the I2C
> > one is a problem (it's definitely a weird situation, but there's nothing
> > preventing that in the spec).
>
> You agree that there is no point for case where i3c rate < i2c rate yet
> you are not convinced.

I didn't say that, there might be use cases where one wants to slow
down the I3C bus to be able to probe it or use a slower rate when
things do not work properly. It's rather unlikely to happen, but I
don't think it deserves a warning message when that's the case.

> Do you thing that will be users for this case?
>
> Anyway, this isn't a high requirement for me. The all point of this patch
> is to introduce the limited bus configuration.

And yet, you keep insisting (and ignoring my feedback) on that point :P.

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