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

[Andy Shevchenko <andy.shevchenko@gmail.com>]

On Mon, Nov 16, 2020 at 02:15:01PM +0000, Dan Scally wrote:
> On 16/11/2020 14:10, Laurent Pinchart wrote:
> > 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.

If you try to get an I²C ore SPI device out of pure ACPI device (with given
APCI _HID) you will fail. So, it's not correct. You are retrieving companion
devices, while they are still in the struct acpi_device.

And don't ask me, why it's so. I wasn't designed that and didn't affect any
decision made there.

> >>> 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.

-- 
With Best Regards,
Andy Shevchenko