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

> From: Guenter Roeck [mailto:linux@roeck-us.net]
> Subject: Re: [PATCH] ACPICA: Export mutex functions
>
> On 04/18/2017 12:14 AM, Zheng, Lv wrote:
> > Hi,
> >
> >> From: Zheng, Lv
> >> Subject: RE: [PATCH] ACPICA: Export mutex functions
> >>
> >> 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.
> >
> > Maybe I should provide more detailed examples for this solution.
> >
> > For example:
> > OperationRegion (SIOT, SuperIOAddressSpace, Zero, 100)
> > Field (SIOT, ByteAcc, Lock, Preserve)
> > {
> > FNC1, 8,
> > FNC2, 8,
> > ...
> > }
> >
> > Acquire (MUX0)
> > Store (0, FNC1)
> > Release (MUX0)
> >
> > Then you can call (let me use casual pseudo code)
> > acpi_install_operation_region(SuperIOAddressSpace, superio_opregion_handler) from OS side.
> > In its callback superio_opregion_handler(), you can:
> >
> > superio_enter();
> > If (address == 0) {
> > /* mean FNC1 */
> > Perform the locked superior accesses
> > } else if (address == 1) {
> > /* mean FNC2 */
> > Perform the locked superior accesses
> > }
> > superio_exit();
> >
> > Are there similar approach in your DSDT?
> >
>
> Some snippets from the DSDT:
>
> Device (SIO1)
> {
> Name (_HID, EisaId ("PNP0C02") /* PNP Motherboard Resources */) // _HID: Hardware ID
> Name (_UID, Zero) // _UID: Unique ID
> ...
> Mutex (MUT0, 0x00)
> Method (ENFG, 1, NotSerialized)
> {
> Acquire (MUT0, 0x0FFF)
> INDX = 0x87
> INDX = One
> INDX = 0x55
> If ((SP1O == 0x2E))
> {
> INDX = 0x55
> }
> Else
> {
> INDX = 0xAA
> }
>
> LDN = Arg0
> }
>
> Method (EXFG, 0, NotSerialized)
> {
> INDX = 0x02
> DATA = 0x02
> Release (MUT0)
> }
>
> OperationRegion (IOID, SystemIO, SP1O, 0x02) /* SP1O is 0x2e */
> Field (IOID, ByteAcc, NoLock, Preserve)
> {
> INDX, 8,
> DATA, 8
> }
> ...
>
> Example for use:
> Method (DCNT, 2, NotSerialized)
> {
> ENFG (CGLD (Arg0))
> If (((DMCH < 0x04) && ((Local1 = (DMCH & 0x03)) != Zero)))
> {
> RDMA (Arg0, Arg1, Local1++)
> }
>
> ACTR = Arg1
> Local1 = (IOAH << 0x08)
> Local1 |= IOAL
> RRIO (Arg0, Arg1, Local1, 0x08)
> EXFG ()
> }
>
> Can there be more than one address space handler for a given region ?
> Each driver accessing the resource would need that handler.

From the above AML code, the solution 2 is not possible for ensuring the
mutual exclusion of accessing the resources.
Because the mutual exclusion must be ensured for the entire transaction
including ENFG() and EXFG() rather than a single SystemIo operation
region field access.

So you really need the solution 1 and the new API.

Thanks and best regards
Lv

>
> Thanks,
> Guenter
>
> > Thanks and best regards
> > Lv
> >
> >> 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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