linux-kernel - Re: [PATCH v5 1/1] platform/chrome: add a driver for HPS

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Message-ID: <Y0em/bnh3uk/zw6b@google.com>
Date:   Thu, 13 Oct 2022 13:49:49 +0800
From:   Tzung-Bi Shih <tzungbi@...nel.org>
To:     Dan Callaghan <dcallagh@...omium.org>
Cc:     chrome-platform@...ts.linux.dev,
        LKML <linux-kernel@...r.kernel.org>,
        Sami Kyöstilä <skyostil@...omium.org>,
        Benson Leung <bleung@...omium.org>
Subject: Re: [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().