Re: [PATCH v8 0/6] Support running driver's probe for a device powered off

[Luca Ceresoli]

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.

-- 
Luca