Re: [RFC PATCH v3 2/5] i2c: of: Introduce component probe function

[Chen-Yu Tsai]

On Wed, Nov 29, 2023 at 12:22 AM Andy Shevchenko
<andriy.shevchenko@xxxxxxxxxxxxxxx> wrote:
>
> On Tue, Nov 28, 2023 at 04:42:31PM +0800, Chen-Yu Tsai wrote:
> > Some devices are designed and manufactured with some components having
> > multiple drop-in replacement options. These components are often
> > connected to the mainboard via ribbon cables, having the same signals
> > and pin assignments across all options. These may include the display
> > panel and touchscreen on laptops and tablets, and the trackpad on
> > laptops. Sometimes which component option is used in a particular device
> > can be detected by some firmware provided identifier, other times that
> > information is not available, and the kernel has to try to probe each
> > device.
> >
> > This change attempts to make the "probe each device" case cleaner. The
> > current approach is to have all options added and enabled in the device
> > tree. The kernel would then bind each device and run each driver's probe
> > function. This works, but has been broken before due to the introduction
> > of asynchronous probing, causing multiple instances requesting "shared"
> > resources, such as pinmuxes, GPIO pins, interrupt lines, at the same
> > time, with only one instance succeeding. Work arounds for these include
> > moving the pinmux to the parent I2C controller, using GPIO hogs or
> > pinmux settings to keep the GPIO pins in some fixed configuration, and
> > requesting the interrupt line very late. Such configurations can be seen
> > on the MT8183 Krane Chromebook tablets, and the Qualcomm sc8280xp-based
> > Lenovo Thinkpad 13S.
> >
> > Instead of this delicate dance between drivers and device tree quirks,
> > this change introduces a simple I2C component probe. function For a
> > given class of devices on the same I2C bus, it will go through all of
> > them, doing a simple I2C read transfer and see which one of them responds.
> > It will then enable the device that responds.
> >
> > This requires some minor modifications in the existing device tree. The
> > status for all the device nodes for the component options must be set
> > to "failed-needs-probe". This makes it clear that some mechanism is
> > needed to enable one of them, and also prevents the prober and device
> > drivers running at the same time.
>
> ...
>
> > +/**
> > + * i2c_of_probe_component() - probe for devices of "type" on the same i2c bus
> > + * @dev: &struct device of the caller, only used for dev_* printk messages
> > + * @type: a string to match the device node name prefix to probe for
> > + *
> > + * Probe for possible I2C components of the same "type" on the same I2C bus
> > + * that have their status marked as "fail".
>
> Definitely you haven't run kernel-doc validation.

Right. Will add missing parts.

> > + */
>
> ...
>
> > +             return dev_err_probe(dev, -ENODEV, "Could not find %s device node\n", type);
>
> I haven't noticed clear statement in the description that this API is only for
> the ->probe() stages.

Will add that to the Context part of the kernel-doc.

> ...
>
> > +             if (i2c_smbus_xfer(i2c, addr, 0, I2C_SMBUS_READ, 0, I2C_SMBUS_BYTE, &data) < 0)
> > +                     continue;
>
> This will require the device to be powered on. Are you sure it will be always
> the case?

This is left as TODO. The devices I have tie the component power supplies
to an always on power rail. I guess I could get a trace of the function
calls to see if things are running as they should. Not sure if that is
enough?

ChenYu