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

[Laurent Pinchart <laurent.pinchart@ideasonboard.com>]

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.

> > 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.

-- 
Regards,

Laurent Pinchart