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

[Dan Scally <djrscally@gmail.com>]

On 16/11/2020 14:10, Laurent Pinchart wrote:
> Hi Andy,
>
> On Mon, Nov 16, 2020 at 03:57:17PM +0200, Andy Shevchenko wrote:
>> On Mon, Nov 16, 2020 at 10:53 AM Laurent Pinchart wrote:
>>> On Fri, Nov 13, 2020 at 09:45:00PM +0200, Andy Shevchenko wrote:
>>>> On Fri, Nov 13, 2020 at 6:22 PM Laurent Pinchart wrote:
>>>>> On Fri, Nov 13, 2020 at 10:02:30AM +0000, Dan Scally wrote:
>>>>>> On 29/10/2020 22:51, Laurent Pinchart wrote:
>>>>>>> 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:
>>>> ...
>>>>
>>>>>>>> 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?
>>>>> I like simple options :-) A patch to export acpi_bus_type, with enough
>>>>> context in the commit message (and in the cover latter of the series),
>>>>> should be enough to provide all the information the ACPI maintainers
>>>>> need to decide which option is best. With a bit of luck that patch will
>>>>> be considered the best option and no extra work will be needed.
>>>> The problem with ACPI bus is that it is not as simple as other buses,
>>>> i.e. it may have purely ACPI devices along with *companion* devices,
>>>> which are usually represented by platform bus. On top of that for
>>>> several ACPI devices there can be one physical node and it will be not
>>>> so clear what you are exactly looking for by traversing acpi_bus_type.
>>>> I believe it's hidden on purpose.
>>> Maybe there's something I don't get, as I'm not very familiar with the
>>> ACPI implementation in the kernel, but the code in the cio2-bridge,
>>> unless I'm mistaken, is really interested in ACPI devices on the ACPI
>>> bus, not companions or other devices related to the ACPI devices.
>> AFAICS cio2-bridge wants to find ACPI companion devices which are
>> enumerated as platform ones (or I²C or SPI).
> I thought we were looking for ACPI devices, not companion devices, in
> order to extract information from the DSDT and store it in a software
> node. I could very well be wrong though.
This is correct - the code to fetch the various resources we're looking
at all uses acpi_device. Whether using Andy's iterator suggestions or
previous bus_for_each_dev(&acpi_bus_type...) I'm just getting the
acpi_device via to_acpi_dev() and using that.
>>> The
>>> iterators you have proposed above use bus_find_device() on
>>> acpi_bus_type, so I don't really see how they make a difference from a
>>> cio2-bridge point of view.
>> This seems to be true. The iterators have no means to distinguish
>> between companion devices and pure ACPI, for example.
>> For this one needs to call something like 'get first physical node'
>> followed by 'let's check that it has a companion and that the one we
>> have already got from ACPI bus iterator'.
>>
>>> Is your point that acpi_bus_type shouldn't be
>>> exported because it could then be misused by *other* drivers ? Couldn't
>>> those drivers then equally misuse the iterators ?
>> My point is that the ACPI bus type here is not homogenous.
>> And thus I think it was the reason behind hiding it. I might be
>> mistaken and you may ask ACPI maintainers for the clarification.
>>
>> In summary I think we are trying to solve a problem that has not yet
>> existed (devices with several same sensors). Do we have a DSDT of such
>> to look into?
> Not to my knowledge.
>