Re: [PATCH 0/7] ACPI: scan: Split root scanning into 2 steps

[Hans de Goede <hdegoede@redhat.com>]

Hi,

On 12/2/20 8:57 PM, Rafael J. Wysocki wrote:
> On Wed, Dec 2, 2020 at 8:39 PM Hans de Goede <hdegoede@redhat.com> wrote:
>>
>> Hi,
>>
>> On 12/2/20 2:49 PM, Rafael J. Wysocki wrote:
>>> On Sat, Nov 21, 2020 at 9:30 PM Hans de Goede <hdegoede@redhat.com> wrote:
>>>>
>>>> Hi Rafael,
>>>>
>>>> A while ago (almost 2 years ago) I discussed an issue with you about
>>>> some devices, where some of the methods used during device-addition
>>>> (such as _HID) may rely on OpRegions of other devices:
>>>>
>>>> https://www.spinics.net/lists/linux-acpi/msg86303.html
>>>>
>>>> An example of this is the Acer Switch 10E SW3-016 model. The _HID method
>>>> of the ACPI node for the UART attached Bluetooth, reads GPIOs to detect
>>>> the installed wifi chip and update the _HID for the Bluetooth's ACPI node
>>>> accordingly. The current ACPI scan code calls _HID before the GPIO
>>>> controller's OpRegions are available, leading to the wrong _HID being
>>>> used and Bluetooth not working.
>>>>
>>>> Last week I bought a second hand Acer device, not knowing it was this
>>>> exact model. Since I now have access to affected hardware I decided to
>>>> take a shot at fixing this.
>>>>
>>>> In the discussion you suggested to split the acpi_bus_scan of the root
>>>> into 2 steps, first scan devices with an empty _DEP, putting other
>>>> acpi_handle-s on a list of deferred devices and then in step 2 scan the
>>>> rest.
>>>>
>>>> I'm happy to report that, at least on the affected device, this works
>>>> nicely. While working on this I came up with a less drastic way to
>>>> deal with this. As you will see in patch 4 of this series, I decided
>>>> to first add a more KISS method of deciding which devices to defer
>>>> to the second scan step by matching by HID. This has the disadvantage
>>>> of not being a generic solution. But it has the advantage of being a
>>>> solution which does not potentially regress other devices.
>>>>
>>>> Then in patch 5 I actually do add the option to defer or not based on
>>>> _DEP being empty. I've put this behind a kernel commandline option as
>>>> I'm not sure we should do this everywhere by default. At least no without
>>>> a lot more testing.
>>>>
>>>> Patch 6 fixes an issue with patch 5 which causes battery devices to stop
>>>> working.
>>>>
>>>> And patch 7 adds some extra HIDs to the list of HIDs which should be
>>>> ignored when checking if the _DEP list is empty from Linux' pov, iow
>>>> some extra HIDs which Linux does not bind to.
>>>>
>>>> Please let me know what you think about this patch-set. I would be happy
>>>> to see just patches 1-4 merged.
>>>
>>> I took patches 1 and 2, because IMO they are generally useful (I
>>> rewrote the changelogs to avoid mentioning the rest of the series
>>> though),
>>
>> That is fine. Thanks for taking those.
>>
>>> but I have some reservations regarding the rest.
>>>
>>> First off, I'm not really sure if failing acpi_add_single_object() for
>>> devices with missing dependencies is a good idea.  IIRC there is
>>> nothing in there that should depend on any opregions supplied by the
>>> other devices, so it should be safe to allow it to complete.
>>
>> Actually acpi_add_single_object() does depend on ACPI methods
>> which may depend on opregions, that is the whole reason why
>> this series is necessary. Otherwise we could just delay the
>> binding of the driver based in dep_unmet which would be easier.
>>
>> Here are 2 actual examples of acpi_add_single_object() calling
>> ACPI methods which may depend on opregions:
>>
>> 1. acpi_add_single_object() calls acpi_init_device_object() which
>> calls acpi_set_pnp_ids() which fills a bunch if fields of
>> struct acpi_device with info returned by the acpi_get_object_info()
>> call.
>>
>> Specifically it stores the value returned by the _HID method in
>> the acpi_device_pnp array for the device and that _HID method is
>> actually the problem in the example device which started this
>> series. The _HID method of the bluetooth device reads 2 GPIOs
>> to get a hw-id (0-3) and then translates the hwid to a _HID
>> string. If the GPIO opregion's handlers have not registered yet
>> then the reading of the GPIOs is correctly skipped, and hwid
>> 0 is assumed, which is wrong in this case.
>>
>> 2. I've also seen examples where _STA depends on GPIOs in a similar
>> manner; and acpi_add_single_object() calls acpi_bus_get_power_flags()
>> which calls acpi_bus_init_power() which calls acpi_device_is_present()
>> which depends on _STA results.
> 
> Well, this means that there is a bug in acpi_bus_attach() which
> shouldn't call acpi_bus_init_power() which has been called already.

I'm afraid we have a bit of a misunderstanding here, the problem is
not that acpi_bus_attach() calls acpi_bus_init_power(), the problem is
that acpi_bus_init_power() (which is called from acpi_add_single_object())
depends on the value returned by _STA and that in turn may depend on
some OpRegions being available. IOW it is the same problem as the _HID
problem.

> And it all means that either deferring acpi_add_single_object() is
> needed and so there need to be 2 passes in acpi_bus_attach() overall,
> or acpi_add_single_object() needs to avoid calling methods that may
> depend on supplied opregions.  I guess the latter is rather
> unrealistic, so the only practical choice is the former.

I agree.

> However, I still don't think that the extra list of "dependent
> devices" is needed.

I'm not sure what you are trying to say here? Do you mean this list:

+/*
+ * List of HIDs for which we defer adding them to the second step of the
+ * scanning of the root, because some of their methods used during addition
+ * depend on OpRegions registered by the drivers for other ACPI devices.
+ */
+static const char * const acpi_defer_add_hids[] = {
+	"BCM2E5B", /* Acer SW3-016 bluetooth HID when GPIO OpRegs or not available yet */
+	NULL
+};
+

?

That indeed is not necessary if you take the entire set and always enable the
new behavior instead of using the module option. I guess we could go that route
for 5.12, and get it into next as soon as 5.11-rc1 is available for maximum
testing.

Do you want me to do a new version of the series which drops the acpi_defer_add_hids[]
thing and the module option and simply always uses the new behavior?

Regards,

Hans