linux-kernel - Re: [PATCH v4 5/6] platform/chrome: Introduce device tree hardware prober

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Message-ID: <CAGXv+5EgqdziyheOt7wzkbe036fqPcw_UpSHiMsB3W_nTB3NWQ@mail.gmail.com>
Date: Wed, 14 Aug 2024 18:10:03 +0800
From: Chen-Yu Tsai <wenst@...omium.org>
To: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Cc: Rob Herring <robh@...nel.org>, Saravana Kannan <saravanak@...gle.com>, 
	Matthias Brugger <matthias.bgg@...il.com>, 
	AngeloGioacchino Del Regno <angelogioacchino.delregno@...labora.com>, Wolfram Sang <wsa@...nel.org>, 
	Benson Leung <bleung@...omium.org>, Tzung-Bi Shih <tzungbi@...nel.org>, 
	Mark Brown <broonie@...nel.org>, Liam Girdwood <lgirdwood@...il.com>, 
	chrome-platform@...ts.linux.dev, devicetree@...r.kernel.org, 
	linux-arm-kernel@...ts.infradead.org, linux-mediatek@...ts.infradead.org, 
	linux-kernel@...r.kernel.org, Douglas Anderson <dianders@...omium.org>, 
	Johan Hovold <johan@...nel.org>, Jiri Kosina <jikos@...nel.org>, linux-i2c@...r.kernel.org
Subject: Re: [PATCH v4 5/6] platform/chrome: Introduce device tree hardware prober

On Tue, Aug 13, 2024 at 7:46 PM Andy Shevchenko
<andriy.shevchenko@...ux.intel.com> wrote:
>
> On Thu, Aug 08, 2024 at 05:59:28PM +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 prober. For any 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.
>
> ...
>
> > + * Copyright (c) 2023 Google LLC
>
> At bare minimum we are in 2024 now.

Ack.

> ...
>
> > +#include <linux/array_size.h>
> > +#include <linux/i2c.h>
> > +#include <linux/module.h>
>
> > +#include <linux/of.h>
>
> Why?

Might have been left over from previous work and squashed into the wrong
commit. Will remove.

> > +#include <linux/platform_device.h>
>
> ...
>
> > +     for (size_t i = 0; i < ARRAY_SIZE(hw_prober_platforms); i++) {
> > +             if (!of_machine_is_compatible(hw_prober_platforms[i].compatible))
> > +                     continue;
>
> > +             int ret;
>
> I didn't know we allow this kind of definition mix besides for-loop and
> __free()... Can you point me out where this style change was discussed?

Will move to the top of the for loop block.

> > +             ret = hw_prober_platforms[i].prober(&pdev->dev, hw_prober_platforms[i].data);
> > +             /* Ignore unrecoverable errors and keep going through other probers */
> > +             if (ret == -EPROBE_DEFER)
> > +                     return ret;
> > +     }
>
> ...
>
> > +static void chromeos_of_hw_prober_driver_exit(void)
> > +{
> > +     if (!chromeos_of_hw_prober_pdev)
> > +             return;
>
> First of all, this is dup for the next call, second, when may this conditional
> be true?

When the module is loaded on a machine that doesn't match any entry,
neither the driver nor the device are registered. Hence the check.

Or maybe the proper way to handle it is to return -ENODEV or something?
I'll work towards that.

Thanks
ChenYu

> > +     platform_device_unregister(chromeos_of_hw_prober_pdev);
> > +     platform_driver_unregister(&chromeos_of_hw_prober_driver);
> > +}
> > +module_exit(chromeos_of_hw_prober_driver_exit);
>
> --
> With Best Regards,
> Andy Shevchenko
>
>