linux-kernel - Re: [PATCH] ACPI / LPSS: Add device link for CHT SD card dependency on I2C

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Message-ID: <e6be8f82-c471-8b18-7011-7da71f9ddb11@intel.com>
Date:   Tue, 5 Dec 2017 16:25:20 +0200
From:   Adrian Hunter <adrian.hunter@...el.com>
To:     "Rafael J. Wysocki" <rjw@...ysocki.net>
Cc:     Hans de Goede <hdegoede@...hat.com>,
        "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
        Mika Westerberg <mika.westerberg@...ux.intel.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        linux-acpi@...r.kernel.org, linux-kernel@...r.kernel.org,
        Carlo Caione <carlo@...lessm.com>
Subject: Re: [PATCH] ACPI / LPSS: Add device link for CHT SD card dependency
 on I2C

On 04/12/17 17:00, Rafael J. Wysocki wrote:
> On Monday, December 4, 2017 3:41:45 PM CET Adrian Hunter wrote:
>> On 04/12/17 16:33, Hans de Goede wrote:
>>> Hi,
>>>
>>> On 04-12-17 15:30, Adrian Hunter wrote:
>>>> On 04/12/17 15:48, Hans de Goede wrote:
>>>>> Hi,
>>>>>
>>>>> Wouldn't it be easier to use the ACPI _DEP tracking for this, e.g.
>>>>
>>>> It is using _DEP, see acpi_lpss_dep()
>>>>
>>>>> add something like this to the the probe function:
>>>>>
>>>>>      struct acpi_device = ACPI_COMPANION(device);
>>>>>
>>>>>      if (acpi_device->dep_unmet)
>>>>>          return -EPROBE_DEFER;
>>>>>
>>>>> No idea if this will work, but if it does work, using the deps described
>>>>> in the ACPI tables seems like a better solution then hardcoding this.
>>>>
>>>> That would not work because there are other devices listed in the _DEP
>>>> method so dep_unmet is always true.  So we are left checking _DEP but only
>>>> for specific device dependencies.
>>>
>>> Ugh, understood thank you for explaining this. Perhaps it is a good idea
>>> to mention in the commit message why acpi_dev->dep_unmet cannot be used
>>> here?
>>
>> dep_unmet predates device links, but now we have device links, they are
>> better anyway.
> 
> Right (they cover PM too, for example), but it would be good to note why
> it is necessary to hardcode the links information in the code.

It isn't entirely necessary to hardcode the links information.  For example,
another possibility is to create device links for all LPSS devices with _DEP
methods that point to other LPSS devices i.e. match against
acpi_lpss_device_ids.  The assumptions would be that all LPSS devices have
drivers so it would be safe to create links between them, and that the
nature of the dependency is correctly represented by a device link.

An advantage of that approach would be that it might work for future
dependencies between LPSS devices without having to add entries to a table.
The disadvantage would be the possibility that creating a device link
somehow turns out not to be the right thing to do.