[PATCH v2 00/17] Make rpmsg a framework

[Jeffrey Hugo]

On 9/12/2016 12:49 PM, Bjorn Andersson wrote:
> On Mon 12 Sep 11:13 PDT 2016, Jeffrey Hugo wrote:
>
>> On 9/12/2016 12:00 PM, Bjorn Andersson wrote:
>>> On Mon 12 Sep 09:52 PDT 2016, Lina Iyer wrote:
>>>
>>>> Hi Bjorn,
>>>>
>>>> On Thu, Sep 01 2016 at 16:28 -0600, Bjorn Andersson wrote:
>>>>> This series splits the virtio rpmsg bus driver into a rpmsg bus and a virtio
>>>>> backend/wireformat.
>>>>>
>>>>>
>>>>> As we discussed the Qualcomm SMD implementation a couple of years back people
>>>>> suggested that I should make it "a rpmsg thingie". With the introduction of the
>>>>> Qualcomm 8996 platform, we must support a variant of the communication
>>>>> mechanism that share many of the characteristics of SMD, but are different
>>>>> enough that it can't be done in a single implementation. As such there is
>>>>> enough benefit to do the necessary work and being able to make SMD a "rpmsg
>>>>> thingie".
>>>>>
>>>>> On-top of this series I have patches to switch the current smd clients over to
>>>>> rpmsg (and by that drop the existing SMD implementation).
>>>>>
>>>>> All this allows me to implement the new backend and reuse all existing SMD
>>>>> drivers with the new mechanism.
>>>>>
>>>>
>>>> RPM Communication has to supported even when IRQs are disabled. The most
>>>> important use of this communication is to set the wake up time for the
>>>> CPU subsystem when all the CPUs are powered off.
>>>
>>> Can you point me to the downstream code where this is implemented so I
>>> can have a look? Do you expect to get the response on that request?
>>
>> Have a look at -
>> smd_mask_receive_interrupt()
>> smd_is_pkt_avail()
>>
>
> In msm-3.18 these still seems to only come from either
> msm_rpm_enter_sleep() and the rpm-clock driver, related to flushing
> cached sleep state requests.
>
>> Every request to the RPM generates a response.  The Linux RPM driver may
>> decide to let the response sit in the fifo, or it may need to read and
>> process it.
>>
>
> Right, I presume we save some time by not waiting for these responses as
> we want to reach sleep as soon as possible. The answer I got last time
> this was discussed was that it was an optimization, not a functional
> requirement.

Two optimizations in play here.

First, disabling interrupts prevents an immediate wakeup.  When the 
system is entering sleep, IRQs are disabled.  The sleep request to RPM 
will trigger a response, and the IRQ for that response will be queued. 
Once the sleep processing is done, IRQs get enabled, so the pending IRQ 
from RPM will cause an immediate wakeup.  The system will process the 
wakeup, and then go back to sleep (sans request because nothing has 
changed).  This down-up-down processing burns a lot of power.

Second is not waiting for the response.  Linux doesn't really do 
anything with the sleep request response, so we can enter sleep faster 
by not waiting for the response and processing (discarding) it when the 
system wakes up as scheduled.  However, Linux needs to ensure there is 
enough fifo space to hold that response while asleep, otherwise the RPM 
will panic and crash the system.  Therefore, if there are a number of 
outstanding requests that would fill the fifo, then the RPM driver on 
Linux needs to spin and drain requests from the fifo until a minimum 
free space buffer to hold additional expected pending responses is 
established.  This has to occur with IRQs disabled.

>
>
> I'm not at all against having the rpm driver cache the state
> information and the smd driver process read/writes from the rpm driver
> in IRQ context. I do however not know how to trigger the flush in a sane
> way.
>
>>>
>>>> In addition to that,
>>>> "sleep" votes that are sent by the application processor subsystem to
>>>> allow system to go into deep sleep modes can only be triggered when the
>>>> CPU PM domains are power collapsed, drivers do not have a knowledge of
>>>> when that happens.
>>>
>>> Do you mean the actual sleep votes can only be with the CPU PM domains
>>> collapsed?
>>>
>>> It's been a while since I dug through that code, but there was several
>>> cases where sleep votes would be sent out during normal execution as
>>> well, and then there's the optimization of flushing out all cached sleep
>>> votes when we're on the way down.
>>>
>>>> This has to be done by a platform code that registers
>>>> for CPU PM domain power_off/on callbacks.
>>>>
>>>
>>> Ok, sounds like we have a legit use case for improving this.
>>>
>>>> Using rpmsg may be nice for RPM SMD communication, but mutexes need to
>>>> go away for this driver to be any useful than bare bones active mode
>>>> resource requests for QCOM SoCs. By not doing that now, we lock
>>>> ourselves out of using this SMD driver in the near future when CPU PM
>>>> domains are available in the kernel with an ability to do system low
>>>> power modes.
>>>>
>>>
>>> The last time I looked at this there where no cases when it was
>>> _required_ to support transmitting requests to the rpm from IRQ context.
>>
>> I no longer work on SMD, but when I did this was in fact a strict
>> requirement.
>
> When I dissected all the users of the API I came to the conclusion that
> this requirement (on the SMD driver) came from above mentioned
> optimization.
>
>> If I recall correctly, there was a parameter in the RPM driver
>> for the transmit function that indicated if the request was being made in
>> atomic context or not, which would change the behavior of how the transmit
>> was handled.
>>
>
> You're correct, the question is still which of these code paths are
> actually needed and to motivate the endless maintenance of the extra
> code.

If we are just talking about transmitting in atomic context (not 
necessarily related to sleep), if I recall correctly, some bus requests 
are sent to RPM in atomic context, some APR requests to the Audio DSP 
are done in atomic context, and I think IPC Router uses atomic context 
in some cases.  As a generic framework that should support usecases to 
all processors/subsystems, I don't think transmitting in atomic context 
is a special case for RPM/sleep.

Lina et al would probably know better about the usecase details than I 
at this point however.

>
>
> Nice to see you on the mailing list again Jeff.
>
> Regards,
> Bjorn
>


-- 
Jeffrey Hugo
Qualcomm Datacenter Technologies as an affiliate of Qualcomm 
Technologies, Inc.
Qualcomm Technologies, Inc. is a member of the
Code Aurora Forum, a Linux Foundation Collaborative Project.