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From: Luca Ceresoli <>
To: Sakari Ailus <>
Cc:, Wolfram Sang <>,
	"Rafael J. Wysocki" <>,,,
	Greg Kroah-Hartman <>,, Tomasz Figa <>,
	Bartosz Golaszewski <>,
	Bingbu Cao <>,
	Chiranjeevi Rapolu <>,
	Hyungwoo Yang <>,
Subject: Re: [PATCH v8 0/6] Support running driver's probe for a device powered off
Date: Mon, 14 Sep 2020 09:58:24 +0200
Message-ID: <> (raw)
In-Reply-To: <>

Hi Sakari,

On 11/09/20 15:01, Sakari Ailus wrote:
> Hi Luca,
> On Fri, Sep 11, 2020 at 02:49:26PM +0200, Luca Ceresoli wrote:
>> Hi Sakari,
>> On 03/09/20 10:15, Sakari Ailus wrote:
>>> Hi all,
>>> These patches enable calling (and finishing) a driver's probe function
>>> without powering on the respective device on busses where the practice is
>>> to power on the device for probe. While it generally is a driver's job to
>>> check the that the device is there, there are cases where it might be
>>> undesirable. (In this case it stems from a combination of hardware design
>>> and user expectations; see below.) The downside with this change is that
>>> if there is something wrong with the device, it will only be found at the
>>> time the device is used. In this case (the camera sensors + EEPROM in a
>>> sensor) I don't see any tangible harm from that though.
>>> An indication both from the driver and the firmware is required to allow
>>> the device's power state to remain off during probe (see the first patch).
>>> The use case is such that there is a privacy LED next to an integrated
>>> user-facing laptop camera, and this LED is there to signal the user that
>>> the camera is recording a video or capturing images. That LED also happens
>>> to be wired to one of the power supplies of the camera, so whenever you
>>> power on the camera, the LED will be lit, whether images are captured from
>>> the camera --- or not. There's no way to implement this differently
>>> without additional software control (allowing of which is itself a
>>> hardware design decision) on most CSI-2-connected camera sensors as they
>>> simply have no pin to signal the camera streaming state.
>>> This is also what happens during driver probe: the camera will be powered
>>> on by the I²C subsystem calling dev_pm_domain_attach() and the device is
>>> already powered on when the driver's own probe function is called. To the
>>> user this visible during the boot process as a blink of the privacy LED,
>>> suggesting that the camera is recording without the user having used an
>>> application to do that. From the end user's point of view the behaviour is
>>> not expected and for someone unfamiliar with internal workings of a
>>> computer surely seems quite suspicious --- even if images are not being
>>> actually captured.
>>> I've tested these on linux-next master. They also apply to Wolfram's
>>> i2c/for-next branch, there's a patch that affects the I²C core changes
>>> here (see below). The patches apart from that apply to Bartosz's
>>> at24/for-next as well as Mauro's linux-media master branch.
>> Apologies for having joined this discussion this late.
> No worries. But thanks for the comments.
>> This patchset seems a good base to cover a different use case, where I
>> also cannot access the physical device at probe time.
>> I'm going to try these patches, but in my case there are a few
>> differences that need a better understanding.
>> First, I'm using device tree, not ACPI. In addition to adding OF support
>> similar to the work you've done for ACPI, I think instead of
>> acpi_dev_state_low_power() we should have a function that works for both
>> ACPI and DT.
> acpi_dev_state_low_power() is really ACPI specific: it does tell the ACPI
> power state of the device during probe or remove. It is not needed on DT
> since the power state of the device is controlled directly by the driver.
> On I²C ACPI devices, it's the framework that powers them on for probe.

I see, thanks for clarifying. I'm not used to ACPI so I didn't get that.

> You could have a helper function on DT to tell a driver what to do in
> probe, but the functionality in that case is unrelated.

So in case of DT we might think of a function that just tells whether
the device is marked to allow low-power probe, but it's just an info
from DT:

int mydriver_probe(struct i2c_client *client)
	low_power = of_dev_state_low_power(&client->dev);
	if (!low_power) {
		mydriver_initialize(); /* power+clocks, write regs */

...and, if (low_power), call mydriver_initialize() at first usage.

I'm wondering whether this might make sense in mainline.


  reply index

Thread overview: 28+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-09-03  8:15 Sakari Ailus
2020-09-03  8:15 ` [PATCH v8 1/6] i2c: Allow an ACPI driver to manage the device's power state during probe Sakari Ailus
2020-09-03  8:15 ` [PATCH v8 2/6] Documentation: ACPI: Document i2c-allow-low-power-probe _DSD property Sakari Ailus
2020-09-03  8:15 ` [PATCH v8 3/6] ACPI: Add a convenience function to tell a device is in low power state Sakari Ailus
     [not found]   ` <>
2020-09-11 13:01     ` Sakari Ailus
2020-09-03  8:15 ` [PATCH v8 4/6] ov5670: Support probe whilst the device is in a " Sakari Ailus
2020-09-03  8:15 ` [PATCH v8 5/6] media: i2c: imx319: Support probe while the device is off Sakari Ailus
2020-09-03  8:15 ` [PATCH v8 6/6] at24: Support probing while off Sakari Ailus
2020-09-09  9:35   ` Bartosz Golaszewski
2020-09-09 11:11     ` Wolfram Sang
2020-09-09 11:56       ` Bartosz Golaszewski
2020-09-09 12:16         ` Sakari Ailus
2020-09-11 12:11         ` Sakari Ailus
2020-10-06 11:20   ` Tomasz Figa
2020-10-06 11:23     ` Tomasz Figa
2020-09-11 12:49 ` [PATCH v8 0/6] Support running driver's probe for a device powered off Luca Ceresoli
2020-09-11 13:01   ` Sakari Ailus
2020-09-14  7:58     ` Luca Ceresoli [this message]
     [not found]       ` <>
2020-09-14 16:49         ` Luca Ceresoli
2020-09-23 11:08           ` Sakari Ailus
2020-09-23 15:37             ` Luca Ceresoli
2020-09-26 12:38         ` Tomasz Figa
2020-09-27 19:39           ` Wolfram Sang
2020-09-27 19:44             ` Tomasz Figa
2020-09-28 14:17               ` Rafael J. Wysocki
2020-09-28 16:49                 ` Tomasz Figa
2020-09-28 20:49                   ` Sakari Ailus
2020-09-26 12:46 ` Tomasz Figa

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