Re: [PATCH v2 00/17] Make rpmsg a framework

From: Karthikeyan Ramasubramanian
Date: Mon Sep 12 2016 - 17:36:33 EST

On 9/12/2016 1:58 PM, Bjorn Andersson wrote:
On Mon 12 Sep 12:21 PDT 2016, Jeffrey Hugo wrote:

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:
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 -

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

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.

But which "sleep request" is this? The only one I can find is the
flushing of sleep state values from the rpm resource tables.

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

Right, as long as the RPM code doesn't consider it a timeout don't have
a problem if those ack's are handled after the resume.

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.

Right. Which means that the RPM driver needs to know how large the rx
fifo is, what overhead the underlaying transport mechanism has and then
calculate how many responses it should leave room for.

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

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

I have not looked through all of APR yet and don't know where msm_bus is
heading, but for IPC-router your correct that the downstream driver does
indeed require this; but that's a side effect of the downstream
ipcrouter implementation, not the problem itself.

APR does send messages in atomic context in addition to the RPM Driver, but IPC Router does not to the extent of my knowledge.
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