Re: [PATCH RFC 3/5] dt-bindings: Add PDC timer bindings for Qualcomm SoCs

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On 1/4/2019 2:49 AM, Stephen Boyd wrote:
Quoting Raju P L S S S N (2019-01-03 04:22:58)

On 12/29/2018 3:08 AM, Stephen Boyd wrote:
Quoting Raju P L S S S N (2018-12-26 01:44:43)

There are two RSC devices in SoC one for application processor subsystem
& other display subsystem. Both RSC contain registers for PDC timers
(one for each subsystem).

When is the timer programmed on the display subsystem's RSC? It's hard
to give advice without all the information.

For display subsystem RSC, hardware sleep solver takes care of timer
programming for wakeup when the subsystem goes to Power collapse.

So the display subsystem doesn't need to program their PDC in their RSC
from software?

I would think that it would make sense for the application processor's
RSC timer to be programmed from the broadcast timer mechanism in the
kernel so that timers during idle work and suspend turns off the timer
appropriately with a shutdown hook. I guess the PDC can't tell you the
time though? It looks like a shadow (and limited) version of the ARM
architected MMIO timer that we already program for the broadcast timer
mechanism. Is that because even the MMIO timer can't wakeup the system
in deep idle?  Assuming that's true, it means the ARM MMIO timer can't
always be used as the system wide broadcast mechanism because we need to
augment it with the PDC timer to get the actual wakeup.

Yes. this is correct.

Maybe we should be adding hooks into the broadcast timer mechanism to
program this wakeup event hardware in addition to the ARM MMIO timer. Or
we should stop using the ARM MMIO timer on these systems and read the
system register based physical time in the RSC timer driver and register
this 64-bit PDC register as the broadcast timer. So the time reading
would be through sysreg and the wakeup programming would be done by
writing the PDC timer. The assumption would be that we have access to
the physical time registers (which sounds like the assumption we have to

There are no physical timer registers available in RSC for this purpose.

Do we get an interrupt somewhere from the RSC hardware when the timer
fires? Or does that just cause a system wakeup event without any pending
irq and then another irq (like the ARM architected timer) just happens
to be pending around the same time? If we get an interrupt somehow then
I would prefer to drop the ARM MMIO timer and do this hybrid broadcast
timer approach.

There is no interrupt for PDC timeout. It just causes system wakeup
without a pending irq. ARM MMIO is necessary for irq.

Does that system wakeup cause the CPUs to be enabled? So the sysreg
based timer in the CPU would start working? Or does it only make the
system come out of a deep enough idle state to make an always on domain
work that only contains the MMIO based ARM architected timer?

It only makes the system come out of deep sleep state for MMIO timer to function.

I'd hope that each RSC's PDC timer wakes up the owner of the RSC so that
we can use the sysreg based timers and ignore the MMIO based timers
here. This isn't a very important distinction to make though, so if you
have to use the MMIO timers then it just means some extra DT things need
to be done to relate the MMIO timers with the RSC that has the timer
that needs to be programmed too.

Either way we would need a way to either hook the ARM architected timer
driver in the kernel, or reimplement the bit of code needed to implement
the clockevent based on the ARM architected timer that programs the ARM
timer and the PDC timer together.

Could you please provide some more details on the implementation?

How the RSC is used in general by other devices, like display, is not
clear to me. We don't have a "wakeup event" framework in the kernel that
device drivers like the display driver can grab a reference to and
program some system wide wakeup for. That sounds like something new that
could be handled entirely in the display driver with direct register
writes, or it could be some qcom specific API/framework that eventually
calls down into the same RSC driver that knows what offsets to write
into in the display RSC's register space, or it could be an entirely
generic framework like clk or regulator frameworks that could be used by
anything. BTW, are we using the display RSC yet?

Only display subsystem RSC is programmed along with CPU RSC in Linux.
display RSC instance is not present in upstream but it is present in
downstream and used for resource communication purpose only.

 From the comment above it sounds like the display RSC handles the wakeup
programming in hardware? So there isn't a need to add a 'wakeup event'
framework or anything like that. Please correct me if I'm wrong.

Yes. There is no need for any framework. From RSC driver point of view there is a need to differentiate apps_rsc vs display_rsc as SW programs PDC timer only in apps_rsc case. How about having a dt property to capture this?


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