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