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

Ming Yu
2025-06-25T10:54:36Z

Dear Greg and Lee,

Thank you for your comments.
I've reviewed your suggestions, but would appreciate your feedback on
a few remaining points.

Lee Jones <lee@kernel.org> 於 2025年6月25日 週三 下午5:01寫道：
>
> On Fri, 20 Jun 2025, Ming Yu wrote:
>
> > Lee Jones <lee@kernel.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@kernel.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@kernel.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@kernel.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@kernel.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?
>
> I'd like to see the implementation of this before advising.
>
> In such a case, I assume there would be a differentiating factor between
> the two (or three) devices. You would then use that to decide which IDA
> would need to be incremented.
>
> However, Greg is correct. Hard-coding look-ups for userspace to use
> sounds like a terrible idea.
>

I understand.
Do you think it would be better to pass the index via platform_data
and use PLATFORM_DEVID_AUTO together with mfd_add_hotplug_devices()
instead?
For example:
struct nct6694_platform_data {
int index;
};

static struct nct6694_platform_data i2c_data[] = {
{ .index = 0 }, { .index = 1 }, { .index = 2 }, { .index = 3 }, {
.index = 4 }, { .index = 5 },
};

static const struct mfd_cell nct6694_devs[] = {
MFD_CELL_BASIC("nct6694-i2c", NULL, &i2c_data[0], sizeof(struct
nct6694_platform_data), 0),
MFD_CELL_BASIC("nct6694-i2c", NULL, &i2c_data[1], sizeof(struct
nct6694_platform_data), 0),
MFD_CELL_BASIC("nct6694-i2c", NULL, &i2c_data[2], sizeof(struct
nct6694_platform_data), 0),
MFD_CELL_BASIC("nct6694-i2c", NULL, &i2c_data[3], sizeof(struct
nct6694_platform_data), 0),
MFD_CELL_BASIC("nct6694-i2c", NULL, &i2c_data[4], sizeof(struct
nct6694_platform_data), 0),
MFD_CELL_BASIC("nct6694-i2c", NULL, &i2c_data[5], sizeof(struct
nct6694_platform_data), 0),
};
...
mfd_add_hotplug_devices(dev, nct6694_devs, ARRAY_SIZE(nct6694_devs));
...

Thank you again for your support.

Best regards,
Ming