Re: [PATCHv3 8/9] ARM: OMAP2+: AM33XX: Basic suspend resume support

[Kevin Hilman <khilman@linaro.org>]

Dave Gerlach <d-gerlach@ti.com> writes:

> On 08/27/2013 04:45 PM, Kevin Hilman wrote:
>> Dave Gerlach <d-gerlach@ti.com> writes:
>>
>>> From: Vaibhav Bedia <vaibhav.bedia@ti.com>
>>>
>>> AM335x supports various low power modes as documented
>>> in section 8.1.4.3 of the AM335x TRM which is available
>>> @ http://www.ti.com/litv/pdf/spruh73f
>>>
>>> DeepSleep0 mode offers the lowest power mode with limited
>>> wakeup sources without a system reboot and is mapped as
>>> the suspend state in the kernel. In this state, MPU and
>>> PER domains are turned off with the internal RAM held in
>>> retention to facilitate resume process. As part of the boot
>>> process, the assembly code is copied over to OCMCRAM using
>>> the OMAP SRAM code.
>>>
>>> AM335x has a Cortex-M3 (WKUP_M3) which assists the MPU
>>> in DeepSleep0 entry and exit. WKUP_M3 takes care of the
>>> clockdomain and powerdomain transitions based on the
>>> intended low power state. MPU needs to load the appropriate
>>> WKUP_M3 binary onto the WKUP_M3 memory space before it can
>>> leverage any of the PM features like DeepSleep.
>>>
>>> The IPC mechanism between MPU and WKUP_M3 uses a mailbox
>>> sub-module and 8 IPC registers in the Control module. MPU
>>> uses the assigned Mailbox for issuing an interrupt to
>>> WKUP_M3 which then goes and checks the IPC registers for
>>> the payload. WKUP_M3 has the ability to trigger on interrupt
>>
>> s/trigger on interrupt/trigger an interrupt/  ??
>
> Oops I will fix that.
>
>>
>>> to MPU by executing the "sev" instruction.
>>>
>>> In the current implementation when the suspend process
>>> is initiated MPU interrupts the WKUP_M3 to let it know about
>>> the intent of entering DeepSleep0 and waits for an ACK. When
>>> the ACK is received MPU continues with its suspend process
>>> to suspend all the drivers and then jumps to assembly in
>>> OCMC RAM. The assembly code puts the PLLs in bypass, puts the
>>> external RAM in self-refresh mode and then finally execute the
>>> WFI instruction. Execution of the WFI instruction triggers another
>>> interrupt to the WKUP_M3 which then continues wiht the power down
>>> sequence wherein the clockdomain and powerdomain transition takes
>>> place. As part of the sleep sequence, WKUP_M3 unmasks the interrupt
>>> lines for the wakeup sources. WFI execution on WKUP_M3 causes the
>>> hardware to disable the main oscillator of the SoC.
>>>
>>> When a wakeup event occurs, WKUP_M3 starts the power-up
>>> sequence by switching on the power domains and finally
>>> enabling the clock to MPU. Since the MPU gets powered down
>>> as part of the sleep sequence in the resume path ROM code
>>> starts executing. The ROM code detects a wakeup from sleep
>>> and then jumps to the resume location in OCMC which was
>>> populated in one of the IPC registers as part of the suspend
>>> sequence.
>>>
>>> The low level code in OCMC relocks the PLLs, enables access
>>> to external RAM and then jumps to the cpu_resume code of
>>> the kernel to finish the resume process.
>>
>> [...]
>>
>>>   arch/arm/mach-omap2/pm33xx.c  |  474 +++++++++++++++++++++++++++++++++++++++++
>>>   arch/arm/mach-omap2/pm33xx.h  |   77 +++++++
>>>   arch/arm/mach-omap2/wkup_m3.c |  183 ++++++++++++++++
>>
>>
>> Looking closer at this code as I'm trying to fully get my head around
>> all the IPC, I have some more comments.
>>
>> I think the split between pm33xx.c and the M3 driver is still confusing
>> here.  For example, am33xx_ping_wkup_m3(),
>> am33xx_m3_state_machine_reset() and the guts of am33xx_pm_begin() all
>> belong inside the M3 driver, along with all the wakeup_src stuff, which
>> is info coming from the M3.
>>
>> IOW, the communication with M3 should be abstracted from pm33xx by the
>> M3 driver (or possibly an eventual remoteproc/rpmsg implementation) with
>> a well defined API.  In this implementation, the interface is pretty
>> fuzzy and mixed between pm33xx.c and wkup_m3.c.
>
> I have since moved much more of the m3 functionality, including the
> ping and wakeup src code, into the wkup_m3 driver to make the split
> more clear but I haven't yet moved the state machine portion into the
> wkup_m3 driver. I feel that this is the portion of the IPC that could
> potentially be the most variant between different SoC implementations
> so leaving this in the pm code should allow for more flexibility.

I still think this belogs in the M3 driver, because AFAICT, it's
specific to the M3 firmware, not to the SoC.

>>
>> Kevin
>>
>> P.S. I'd also suggest renaming wakeup_src to something else since
>> it's close to wakeup_source which has a rather different meaning in the
>> kernel (c.f. linux/pm_wakeup.h)
>>
>
> I will rename this to tie it into the M3 code.

Great, thanks.

Kevin