Re: [PATCH] ACPICA: Export mutex functions

[Guenter Roeck]

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@xxxxxxxxxxxx]
> > > Subject: Re: [PATCH] ACPICA: Export mutex functions
> > >
> > > On 04/17/2017 04:53 PM, Zheng, Lv wrote:
> > > > Hi,
> > > >
> > > > > From: Guenter Roeck [mailto:linux@xxxxxxxxxxxx]
> > > > > 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@xxxxxxxxxxxx> wrote:
> > > > > > > On Mon, Apr 17, 2017 at 08:40:38PM +0000, Moore, Robert wrote:
> > > > > > > > > From: Guenter Roeck [mailto:linux@xxxxxxxxxxxx]
> > > > > > > > > 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.

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