Re: [RFC PATCH v3 9/9] ipu3-cio2: Add functionality allowing software_node connections to sensors on platforms designed for Windows

[Dan Scally]

On 29/10/2020 22:51, Laurent Pinchart wrote:
> Hi Andy,
>
> On Fri, Oct 30, 2020 at 12:22:15AM +0200, Andy Shevchenko wrote:
>> On Thu, Oct 29, 2020 at 11:29:30PM +0200, Laurent Pinchart wrote:
>>> On Thu, Oct 29, 2020 at 10:26:56PM +0200, Andy Shevchenko wrote:
>>>> On Thu, Oct 29, 2020 at 10:21 PM Laurent Pinchart wrote:
>>>>> On Mon, Oct 26, 2020 at 06:10:50PM +0200, Andy Shevchenko wrote:
>>>>>> On Sat, Oct 24, 2020 at 12:37:02PM +0300, Laurent Pinchart wrote:
>>>>>>> On Sat, Oct 24, 2020 at 09:50:07AM +0100, Dan Scally wrote:
>>>>>>>> On 24/10/2020 02:24, Laurent Pinchart wrote:
>>>>>>>>> On Mon, Oct 19, 2020 at 11:59:03PM +0100, Daniel Scally wrote:
>>>>>>>>>> +              adev = acpi_dev_get_first_match_dev(supported_devices[i], NULL, -1);
>>>>>>>>> What if there are multiple sensor of the same model ?
>>>>>>>> Hmm, yeah, that would be a bit of a pickle. I guess the newer
>>>>>>>> smartphones have multiple sensors on the back, which I presume are the
>>>>>>>> same model. So that will probably crop up at some point. How about
>>>>>>>> instead I use bus_for_each_dev() and in the applied function check if
>>>>>>>> the _HID is in the supported list?
>>>>>>> Sounds good to me.
>>>>>>>
>>>>>>>>>> +              if (!adev)
>>>>>>>>>> +                      continue;
>>>>>> Please, don't.
>>>>>>
>>>>>> If we have so weird ACPI tables it must be w/a differently. The all, even badly
>>>>>> formed, ACPI tables I have seen so far are using _UID to distinguish instance
>>>>>> of the device (see second parameter to the above function).
>>>>>>
>>>>>> If we meet the very broken table I would like rather to know about, then
>>>>>> silently think ahead what could be best.
>>>>>>
>>>>>> I.o.w. don't change this until we will have a real example of the problematic
>>>>>> firmware.
>>>>> I'm not sure to follow you. Daniel's current code loops over all the
>>>>> supported HID (as stored in the supported_devices table), and then gets
>>>>> the first ACPI device for each of them. If multiple ACPI devices exist
>>>>> with the same HID, we need to handle them all, so enumerating all ACPI
>>>>> devices and checking whether their HID is one we handle seems to be the
>>>>> right option to me.
>>>> Devices with the same HID should be still different by another
>>>> parameter in ACPI. The above mentioned call just uses the rough
>>>> estimation for relaxed conditions. If you expect more than one device
>>>> with the same HID how do you expect to distinguish them? The correct
>>>> way is to use _UID. It may be absent, or set to a value. And this
>>>> value should be unique (as per U letter in UID abbreviation). That
>>>> said, the above is good enough till we find the firmware with the
>>>> above true (several devices with the same HID). Until then the code is
>>>> fine.
>>> I expect those devices with the same _HID to have different _UID values,
>>> yes. On the systems I've seen so far, that assumption is not violated,
>>> and I don't think we need to already plan how we will support systems
>>> where multiple devices would have the same _HID and _UID (within the
>>> same scope). There's no disagreement there.
>>>
>>> My point is that supported_devices stores HID values, and doesn't care
>>> about UID. The code loops over supported_devices, and for each entry,
>>> calls acpi_dev_get_first_match_dev() and process the ACPI devices
>>> returned by that call. We thus process at most one ACPI device per HID,
>>> which isn't right.
>> In this case we probably need something like
>>
>> struct acpi_device *
>> acpi_dev_get_next_match_dev(struct acpi_device *adev,
>> 			    const char *hid, const char *uid, s64 hrv)
>> {
>> 	struct device *start = adev ? &adev->dev : NULL;
>> 	...
>> 	dev = bus_find_device(&acpi_bus_type, start, &match, acpi_dev_match_cb);
>> 	...
>> }
>>
>> in drivers/acpi/utils.c and
>>
>> static inline struct acpi_device *
>> acpi_dev_get_first_match_dev(const char *hid, const char *uid, s64 hrv)
>> {
>> 	return acpi_dev_get_next_match_dev(NULL, hid, uid, hrv);
>> }
>>
>> in include/linux/acpi.h.
>>
>> Then we may add
>>
>> #define for_each_acpi_dev_match(adev, hid, uid, hrv)			\
>> 	for (adev = acpi_dev_get_first_match_dev(hid, uid, hrv);	\
>> 	     adev;							\
>> 	     adev = acpi_dev_get_next_match_dev(adev, hid, uid, hrv))
> What the cio2-bridge code needs is indeed
>
> 	for each hid in supported hids:
> 		for each acpi device that is compatible with hid:
> 			...
>
> which could also be expressed as
>
> 	for each acpi device:
> 		if acpi device hid is in supported hids:
> 			...
>
> I don't mind either option, I'll happily follow the preference of the
> ACPI maintainers.
>
Does this need raising elsewhere then? The original idea of just
bus_for_each_dev(&acpi_bus_type...) I have now tested and it works fine,
but it does mean that I need to export acpi_bus_type (currently that
symbol's not available)...that seems much simpler to me but I'm not sure
whether that's something to avoid, and if so whether Andy's approach is
better.


Thoughts?