Re: [PATCH v5 06/10] i2c: Introduce OF component probe function

Chen-Yu Tsai
2024-08-23T08:40:49Z

On Thu, Aug 22, 2024 at 10:02 PM Andy Shevchenko
<andriy.shevchenko@linux.intel.com> wrote:
>
> On Thu, Aug 22, 2024 at 05:19:59PM +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.
>
> ...
>
> > --- a/drivers/i2c/Makefile
> > +++ b/drivers/i2c/Makefile
> > @@ -9,6 +9,7 @@ i2c-core-objs := i2c-core-base.o i2c-core-smbus.o
> > i2c-core-$(CONFIG_ACPI) += i2c-core-acpi.o
> > i2c-core-$(CONFIG_I2C_SLAVE) += i2c-core-slave.o
> > i2c-core-$(CONFIG_OF) += i2c-core-of.o
> > +i2c-core-$(CONFIG_OF_DYNAMIC) += i2c-core-of-prober.o
>
> Seems like all the above (except ACPI) have the same issue, i.e. TABs/spaces
> mixture.

I was lazy and just copied the previous line. I'll fix the new line for now.

> ...
>
> > + ret = of_changeset_apply(ocs);
> > + if (!ret) {
>
> Why not positive conditional?

No real reason. I suppose having the error condition come first is more
common. Not sure if it makes any difference in this case though?

Thanks
ChenYu

> > + /*
> > + * ocs is intentionally kept around as it needs to
> > + * exist as long as the change is applied.
> > + */
> > + void *ptr __always_unused = no_free_ptr(ocs);
> > + } else {
> > + /* ocs needs to be explicitly cleaned up before being freed. */
> > + of_changeset_destroy(ocs);
> > + }
>
> --
> With Best Regards,
> Andy Shevchenko
>
>