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SubjectRe: [PATCH v5 1/1] platform/chrome: add a driver for HPS
On Wed, Oct 12, 2022 at 09:29:03PM -0700, Dan Callaghan wrote:
> Excerpts from Tzung-Bi Shih’s message of 2022-10-12 19:46:51 +1100:
> > On Wed, Oct 12, 2022 at 03:09:18PM +1100, Dan Callaghan wrote:
> > > + hps->enable_gpio = devm_gpiod_get(&client->dev, "enable", GPIOD_OUT_HIGH);
> > > + if (IS_ERR(hps->enable_gpio)) {
> > > + ret = PTR_ERR(hps->enable_gpio);
> > > + dev_err(&client->dev, "failed to get enable gpio: %d\n", ret);
> > > + return ret;
> > > + }
> > > +
> > > + ret = misc_register(&hps->misc_device);
> > > + if (ret) {
> > > + dev_err(&client->dev, "failed to initialize misc device: %d\n", ret);
> > > + return ret;
> > > + }
> > > +
> > > + hps_set_power(hps, false);
> >
> > IIUC, the GPIO will raise to HIGH in the first place, and then fall
> > to LOW until here. Is it an expected behavior? How about gpiod_get()
> > with GPIOD_OUT_LOW?
>
> It might seem a little unusual, but it is intentional. The enable line is
> already high when we enter the kernel from firmware. Acquiring the GPIO
> line with GPIOD_OUT_HIGH preserves its existing state (high) in case later
> steps fail.
>
> We power off the periphal only once the driver is successfully bound and has
> taken control of its power state.

I see. Please put some context comments before calling devm_gpiod_get().

> > > +static int hps_i2c_remove(struct i2c_client *client)
> > > +{
> > > + struct hps_drvdata *hps = i2c_get_clientdata(client);
> > > +
> > > + pm_runtime_disable(&client->dev);
> > > + misc_deregister(&hps->misc_device);
> > > + hps_set_power(hps, true);
> >
> > Why does it need to raise the GPIO again when removing the device?
>
> Similar to the above, we want to preserve the default power state
> (i.e. powered on) whenever the driver is not bound to the device.
>
> This behaviour made sense to us mainly because we were originally controlling
> the peripheral entirely from userspace, so it was always powered on by default.
>
> Do you think this behaviour is acceptable, or do we need to change it?

I think it's fine. Please put some context comments before calling
hps_set_power().

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