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    SubjectRE: [PATCH] ACPICA: Export mutex functions
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    Hi,

    > From: Guenter Roeck [mailto:linux@roeck-us.net]
    > Subject: Re: [PATCH] ACPICA: Export mutex functions
    >
    > On 04/17/2017 04:53 PM, Zheng, Lv wrote:
    > > Hi,
    > >
    > >> From: Guenter Roeck [mailto:linux@roeck-us.net]
    > >> Subject: Re: [PATCH] ACPICA: Export mutex functions
    > >>
    > >> On Mon, Apr 17, 2017 at 11:29:38PM +0200, Rafael J. Wysocki wrote:
    > >>> On Mon, Apr 17, 2017 at 11:03 PM, Guenter Roeck <linux@roeck-us.net> wrote:
    > >>>> On Mon, Apr 17, 2017 at 08:40:38PM +0000, Moore, Robert wrote:
    > >>>>>
    > >>>>>
    > >>>>>> From: Guenter Roeck [mailto:linux@roeck-us.net]
    > >>>>>> Subject: Re: [PATCH] ACPICA: Export mutex functions
    > >>>>>>
    > >>>>>> On Mon, Apr 17, 2017 at 07:27:37PM +0000, Moore, Robert wrote:
    > >>>>>>>
    > >>>>>>>> From: Moore, Robert
    > >>>>>>>> Subject: RE: [PATCH] ACPICA: Export mutex functions
    > >>>>>>>>
    > >>>>>>>> There is a model for the drivers to directly acquire an AML mutex
    > >>>>>>>> object. That is why the acquire/release public interfaces were added
    > >>>>>>>> to ACPICA.
    > >>>>>>>>
    > >>>>>>>> I forget all of the details, but the model was developed with MS and
    > >>>>>>>> others during the ACPI 6.0 timeframe.
    > >>>>>>>>
    > >>>>>>>>
    > >>>>>>> [Moore, Robert]
    > >>>>>>>
    > >>>>>>>
    > >>>>>>> Here is the case where the OS may need to directly acquire an AML
    > >>>>>> mutex:
    > >>>>>>>
    > >>>>>>> From the ACPI spec:
    > >>>>>>>
    > >>>>>>> 19.6.2 Acquire (Acquire a Mutex)
    > >>>>>>>
    > >>>>>>> Note: For Mutex objects referenced by a _DLM object, the host OS may
    > >>>>>> also contend for ownership.
    > >>>>>>>
    > >>>>>> From the context in the dsdt, and from description of expected use cases
    > >>>>>> for _DLM objects I can find, this is what the mutex is used for (to
    > >>>>>> serialize access to a resource on a low pin count serial interconnect,
    > >>>>>> aka LPC).
    > >>>>>>
    > >>>>>> What does that mean in practice ? That I am not supposed to use it
    > >>>>>> because it doesn't follow standard ACPI mutex declaration rules ?
    > >>>>>>
    > >>>>>> Thanks,
    > >>>>>> Guenter
    > >>>>>>
    > >>>>>>>
    > >>>>> [Moore, Robert]
    > >>>>>
    > >>>>> I'm not an expert on the _DLM method, but I would point you to the description section in the
    > >> ACPI spec, 5.7.5 _DLM (DeviceLock Mutex).
    > >>>>>
    > >>>>
    > >>>> I did. However, not being an ACPI expert, that doesn't tell me anything.
    > >>>
    > >>> Basically, if the kernel and AML need to access a device concurrently,
    > >>> there should be a _DLM object under that device in the ACPI tables.
    > >>> In that case it is expected to return a list of (AML) mutexes that can
    > >>> be acquired by the kernel in order to synchronize device access with
    > >>> respect to AML (and for each mutex it may also return a description of
    > >>> the specific resources to be protected by it).
    > >>>
    > >>> Bottom line: without _DLM, the kernel cannot synchronize things with
    > >>> respect to AML properly, because it has no information how to do that
    > >>> then.
    > >>
    > >> That is all quite interesting. I do see the mutex in question used on various
    > >> motherboards from various vendors (I checked boards from Gigabyte, MSI, and
    > >> Intel). Interestingly, the naming seems to be consistent - it is always named
    > >> "MUT0". For the most part, it seems to be available on more recent
    > >> motherboards; older motherboards tend to use the resource without locking.
    > >> However, I don't see any mention of "_DLM" in any of the DSDTs.
    > >>
    > >
    > > OK, then you might be having problems in your opregion driver.
    > >
    > >> At the same time, access to ports 0x2e/0x2f is widely used in the kernel.
    > >> As mentioned before, it is used in watchdog, hardware monitoring, and gpio
    > >> drivers, but also in parallel port and infrared driver code. Effectively
    > >> that means that all this code is inherently unsafe on systems with ACPI
    > >> support.
    > >>
    > >> I had thought about implementing a set of utility functions to make the kernel
    > >> code safer to use if the mutex is found to exist.
    > >
    > > As what you've mentioned, there are already lots of parallel accesses in kernel without enabling
    > ACPI.
    > > Are these accesses mutually exclusive (safe)?
    >
    > In-kernel, yes (using request_muxed_region). Against ACPI, no.
    >
    > > If so, why do you need to invent a new synchronization mechanism?
    > >
    >
    > Because I am seeing a problem with the current code (more specifically,
    > with the it87 hwmon driver) on new Gigabyte boards.

    I checked superio_enter()/superio_exit(), IMO, the mutual exclusion
    might be handled using 1 of the following 2 solutions:

    1. _DLM, then you can find superio related mutex from _DLM and
    acquire/release it in superio_enter()/superio_exit().
    You really need a set of new APIs to acquire the _DLM related mutex.
    If you don't have _DLM in your DSDT, directly exporting ACPICA mutex
    functions seem to be a reasonable solution.
    2. Normally, AML developer should abstract superio accesses into customized
    opregion, then you can prepare a superio opregion driver.

    >
    > >> Right now I wonder, though,
    > >> if such code would have a chance to be accepted. Any thoughts on that ?
    > >
    > > Is that possible to make it safe in the opregion driver?
    > >
    >
    > Sorry, I don't think I understand what you mean with "opregion driver".
    > Do you refer to the drivers accessing the memory region in question,
    > or in other words replicating the necessary code in every driver accessing
    > that region ? Sure, I can do that, if that is the preferred solution;
    > I have no problem with that. However, that would require exporting
    > the ACPI mutex functions. My understanding is that you are opposed to
    > exporting those, so I assume that is not what you refer to.
    > Can you clarify ?

    I mean solution 2.
    From it87_find() I really couldn't see a possibility to convert superio
    accesses into opregions. Could you paste some example superio access AML
    code from your DSDT here.

    Thanks and best regards
    Lv

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