linux-kernel - Re: [PATCH v12 1/7] mfd: Add core driver for Nuvoton NCT6694

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Message-ID: <2025061914-sternum-factoid-4269@gregkh>
Date: Thu, 19 Jun 2025 19:18:24 +0200
From: Greg KH <gregkh@...uxfoundation.org>
To: Guenter Roeck <linux@...ck-us.net>
Cc: Ming Yu <a0282524688@...il.com>, Lee Jones <lee@...nel.org>,
	linus.walleij@...aro.org, brgl@...ev.pl, andi.shyti@...nel.org,
	mkl@...gutronix.de, mailhol.vincent@...adoo.fr,
	andrew+netdev@...n.ch, davem@...emloft.net, edumazet@...gle.com,
	kuba@...nel.org, pabeni@...hat.com, wim@...ux-watchdog.org,
	jdelvare@...e.com, alexandre.belloni@...tlin.com,
	linux-kernel@...r.kernel.org, linux-gpio@...r.kernel.org,
	linux-i2c@...r.kernel.org, linux-can@...r.kernel.org,
	netdev@...r.kernel.org, linux-watchdog@...r.kernel.org,
	linux-hwmon@...r.kernel.org, linux-rtc@...r.kernel.org,
	linux-usb@...r.kernel.org, Ming Yu <tmyu0@...oton.com>
Subject: Re: [PATCH v12 1/7] mfd: Add core driver for Nuvoton NCT6694

On Thu, Jun 19, 2025 at 09:58:04AM -0700, Guenter Roeck wrote:
> On 6/19/25 09:20, Greg KH wrote:
> > On Fri, Jun 20, 2025 at 12:03:01AM +0800, Ming Yu wrote:
> > > Lee Jones <lee@...nel.org> 於 2025年6月19日 週四 下午11:28寫道：
> > > > 
> > > > On Thu, 19 Jun 2025, Ming Yu wrote:
> > > > 
> > > > > Lee Jones <lee@...nel.org> 於 2025年6月19日 週四 下午7:53寫道：
> > > > > > 
> > > > > > On Fri, 13 Jun 2025, Ming Yu wrote:
> > > > > > 
> > > > > > > Lee Jones <lee@...nel.org> 於 2025年6月13日 週五 下午9:11寫道：
> > > > > > > > 
> > > > > > > > On Fri, 13 Jun 2025, Ming Yu wrote:
> > > > > > > > 
> > > > > > > > > Lee Jones <lee@...nel.org> 於 2025年6月12日 週四 下午11:23寫道：
> > > > > > > > > > 
> > > > > > > > > > On Thu, 12 Jun 2025, Ming Yu wrote:
> > > > > > > > > > 
> > > > > > > > > > > Dear Lee,
> > > > > > > > > > > 
> > > > > > > > > > > Thank you for reviewing,
> > > > > > > > > > > 
> > > > > > > > > > > Lee Jones <lee@...nel.org> 於 2025年6月12日 週四 下午10:00寫道：
> > > > > > > > > > > > 
> > > > > > > > > > > ...
> > > > > > > > > > > > > +static const struct mfd_cell nct6694_devs[] = {
> > > > > > > > > > > > > +     MFD_CELL_BASIC("nct6694-gpio", NULL, NULL, 0, 0),
> > > > > > > > > > > > > +     MFD_CELL_BASIC("nct6694-gpio", NULL, NULL, 0, 1),
> > > > > > > > > > > > > +     MFD_CELL_BASIC("nct6694-gpio", NULL, NULL, 0, 2),
> > > > > > > > > > > > > +     MFD_CELL_BASIC("nct6694-gpio", NULL, NULL, 0, 3),
> > > > > > > > > > > > > +     MFD_CELL_BASIC("nct6694-gpio", NULL, NULL, 0, 4),
> > > > > > > > > > > > > +     MFD_CELL_BASIC("nct6694-gpio", NULL, NULL, 0, 5),
> > > > > > > > > > > > > +     MFD_CELL_BASIC("nct6694-gpio", NULL, NULL, 0, 6),
> > > > > > > > > > > > > +     MFD_CELL_BASIC("nct6694-gpio", NULL, NULL, 0, 7),
> > > > > > > > > > > > > +     MFD_CELL_BASIC("nct6694-gpio", NULL, NULL, 0, 8),
> > > > > > > > > > > > > +     MFD_CELL_BASIC("nct6694-gpio", NULL, NULL, 0, 9),
> > > > > > > > > > > > > +     MFD_CELL_BASIC("nct6694-gpio", NULL, NULL, 0, 10),
> > > > > > > > > > > > > +     MFD_CELL_BASIC("nct6694-gpio", NULL, NULL, 0, 11),
> > > > > > > > > > > > > +     MFD_CELL_BASIC("nct6694-gpio", NULL, NULL, 0, 12),
> > > > > > > > > > > > > +     MFD_CELL_BASIC("nct6694-gpio", NULL, NULL, 0, 13),
> > > > > > > > > > > > > +     MFD_CELL_BASIC("nct6694-gpio", NULL, NULL, 0, 14),
> > > > > > > > > > > > > +     MFD_CELL_BASIC("nct6694-gpio", NULL, NULL, 0, 15),
> > > > > > > > > > > > > +
> > > > > > > > > > > > > +     MFD_CELL_BASIC("nct6694-i2c", NULL, NULL, 0, 0),
> > > > > > > > > > > > > +     MFD_CELL_BASIC("nct6694-i2c", NULL, NULL, 0, 1),
> > > > > > > > > > > > > +     MFD_CELL_BASIC("nct6694-i2c", NULL, NULL, 0, 2),
> > > > > > > > > > > > > +     MFD_CELL_BASIC("nct6694-i2c", NULL, NULL, 0, 3),
> > > > > > > > > > > > > +     MFD_CELL_BASIC("nct6694-i2c", NULL, NULL, 0, 4),
> > > > > > > > > > > > > +     MFD_CELL_BASIC("nct6694-i2c", NULL, NULL, 0, 5),
> > > > > > > > > > > > 
> > > > > > > > > > > > Why have we gone back to this silly numbering scheme?
> > > > > > > > > > > > 
> > > > > > > > > > > > What happened to using IDA in the child driver?
> > > > > > > > > > > > 
> > > > > > > > > > > 
> > > > > > > > > > > In a previous version, I tried to maintain a static IDA in each
> > > > > > > > > > > sub-driver. However, I didn’t consider the case where multiple NCT6694
> > > > > > > > > > > devices are bound to the same driver — in that case, the IDs are not
> > > > > > > > > > > fixed and become unusable for my purpose.
> > > > > > > > > > 
> > > > > > > > > > Not sure I understand.
> > > > > > > > > > 
> > > > > > > > > 
> > > > > > > > > As far as I know, if I maintain the IDA in the sub-drivers and use
> > > > > > > > > multiple MFD_CELL_NAME("nct6694-gpio") entries in the MFD, the first
> > > > > > > > > NCT6694 device bound to the GPIO driver will receive IDs 0~15.
> > > > > > > > > However, when a second NCT6694 device is connected to the system, it
> > > > > > > > > will receive IDs 16~31.
> > > > > > > > > Because of this behavior, I switched back to using platform_device->id.
> > > > > > > > 
> > > > > > > > Each of the devices will probe once.
> > > > > > > > 
> > > > > > > > The first one will be given 0, the second will be given 1, etc.
> > > > > > > > 
> > > > > > > > Why would you give multiple IDs to a single device bound to a driver?
> > > > > > > > 
> > > > > > > 
> > > > > > > The device exposes multiple peripherals — 16 GPIO controllers, 6 I2C
> > > > > > > adapters, 2 CAN FD controllers, and 2 watchdog timers. Each peripheral
> > > > > > > is independently addressable, has its own register region, and can
> > > > > > > operate in isolation. The IDs are used to distinguish between these
> > > > > > > instances.
> > > > > > > For example, the GPIO driver will be probed 16 times, allocating 16
> > > > > > > separate gpio_chip instances to control 8 GPIO lines each.
> > > > > > > 
> > > > > > > If another device binds to this driver, it is expected to expose
> > > > > > > peripherals with the same structure and behavior.
> > > > > > 
> > > > > > I still don't see why having a per-device IDA wouldn't render each
> > > > > > probed device with its own ID.  Just as you have above.
> > > > > > 
> > > > > 
> > > > > For example, when the MFD driver and the I2C sub-driver are loaded,
> > > > > connecting the first NCT6694 USB device to the system results in 6
> > > > > nct6694-i2c platform devices being created and bound to the
> > > > > i2c-nct6694 driver. These devices receive IDs 0 through 5 via the IDA.
> > > > > 
> > > > > However, when a second NCT6694 USB device is connected, its
> > > > > corresponding nct6694-i2c platform devices receive IDs 6 through 11 —
> > > > > instead of 0 through 5 as I originally expected.
> > > > > 
> > > > > If I've misunderstood something, please feel free to correct me. Thank you!
> > > > 
> > > > In the code above you register 6 I2C devices.  Each device will be
> > > > assigned a platform ID 0 through 5. The .probe() function in the I2C
> > > > driver will be executed 6 times.  In each of those calls to .probe(),
> > > > instead of pre-allocating a contiguous assignment of IDs here, you
> > > > should be able to use IDA in .probe() to allocate those same device IDs
> > > > 0 through 5.
> > > > 
> > > > What am I missing here?
> > > > 
> > > 
> > > You're absolutely right in the scenario where a single NCT6694 device
> > > is present. However, I’m wondering how we should handle the case where
> > > a second or even third NCT6694 device is bound to the same MFD driver.
> > > In that situation, the sub-drivers using a static IDA will continue
> > > allocating increasing IDs, rather than restarting from 0 for each
> > > device. How should this be handled?
> > 
> > What is wrong with increasing ids?  The id value means nothing, they
> > just have to be unique.
> > 
> 
> Unless they are used in the client driver as index into an array, as in
> "this is the Nth instance of this device for this chip". There has to be
> _some_ means to pass N to the client driver.

Ick, that should just be walking the list of child devices instead, as
obviously no one is hard coding array sizes for devices these days,
right?  :)

Anyway, sure, if you _have_ to have a specific id, then use a specific
id, but really, it should not matter.

thanks,

greg k-h