On Mon, Apr 17, 2017 at 11:03 PM, Guenter Roeck <[email protected]> wrote: > On Mon, Apr 17, 2017 at 08:40:38PM +0000, Moore, Robert wrote: >> >> >> > -----Original Message----- >> > From: Guenter Roeck [mailto:[email protected]] >> > Sent: Monday, April 17, 2017 12:45 PM >> > To: Moore, Robert <[email protected]> >> > Cc: Zheng, Lv <[email protected]>; Wysocki, Rafael J >> > <[email protected]>; 'Len Brown' <[email protected]>; 'linux- >> > [email protected]' <[email protected]>; '[email protected]' >> > <[email protected]>; '[email protected]' <linux- >> > [email protected]>; Box, David E <[email protected]> >> > Subject: Re: [PATCH] ACPICA: Export mutex functions >> > >> > On Mon, Apr 17, 2017 at 07:27:37PM +0000, Moore, Robert wrote: >> > > >> > > > -----Original Message----- >> > > > From: Moore, Robert >> > > > Sent: Monday, April 17, 2017 10:13 AM >> > > > To: Guenter Roeck <[email protected]>; Zheng, Lv >> > > > <[email protected]> >> > > > Cc: Wysocki, Rafael J <[email protected]>; Len Brown >> > > > <[email protected]>; [email protected]; [email protected]; >> > > > linux- [email protected] >> > > > 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. Thanks, Rafael
