Re: [PATCH 8/9] PM / Domains: Add support for multi PM domains per device to genpd
From: Jon Hunter
Date: Wed May 23 2018 - 05:28:47 EST
On 23/05/18 10:47, Ulf Hansson wrote:
On 23 May 2018 at 11:45, Jon Hunter <jonathanh@xxxxxxxxxx> wrote:
On 23/05/18 10:33, Ulf Hansson wrote:
On 23 May 2018 at 11:27, Rajendra Nayak <rnayak@xxxxxxxxxxxxxx> wrote:
On 05/23/2018 02:37 PM, Jon Hunter wrote:
On 23/05/18 07:12, Ulf Hansson wrote:
...
Thanks for sending this. Believe it or not this has still been on
my to-do list
and so we definitely need a solution for Tegra.
Looking at the above it appears that additional power-domains
exposed as devices
to the client device. So I assume that this means that the drivers
for devices
with multiple power-domains will need to call RPM APIs for each of
these
additional power-domains. Is that correct?
They can, but should not!
Instead, the driver shall use device_link_add() and
device_link_del(),
dynamically, depending on what PM domain that their original device
needs for the current running use case.
In that way, they keep existing runtime PM deployment, operating on
its original device.
OK, sounds good. Any reason why the linking cannot be handled by the
above API? Is there a use-case where you would not want it linked?
I am guessing the linking is what would give the driver the ability to
decide which subset of powerdomains it actually wants to control
at any point using runtime PM. If we have cases wherein the driver
would want to turn on/off _all_ its associated powerdomains _always_
then a default linking of all would help.
First, I think we need to decide on *where* the linking should be
done, not at both places, as that would just mess up synchronization
of who is responsible for calling the device_link_del() at detach.
Second, It would in principle be fine to call device_link_add() and
device_link_del() as a part of the attach/detach APIs. However, there
is a downside to such solution, which would be that the driver then
needs call the detach API, just to do device_link_del(). Of course
then it would also needs to call the attach API later if/when needed.
Doing this adds unnecessary overhead - comparing to just let the
driver call device_link_add|del() when needed. On the upside, yes, it
would put less burden on the drivers as it then only needs to care
about using one set of functions.
Which solution do you prefer?
Any reason why we could not add a 'boolean' argument to the API to
indicate whether the new device should be linked? I think that I prefer the
API handles it, but I can see there could be instances where drivers may
wish to handle it themselves.
Rajendra, do you have a use-case right now where the driver would want
to handle the linking?
So if I understand this right, any driver which does want to control
individual powerdomain state would
need to do the linking itself right?
What I am saying is, if I have device A, with powerdomains X and Y, and
if I want to turn on only X,
then I would want only X to be linked with A, and at a later point if I
want both X and Y to be turned on,
I would then go ahead and link both X and Y to A? Is that correct or did
I get it all wrong?
Correct!
I know atleast Camera on msm8996 would need to do this since it has 2 vfe
powerdoamins, which can be
turned on one at a time (depending on what resolution needs to be
supported) or both together if we
really need very high resolution using both vfe modules.
I think this is also the case for the Tegra XUSB subsystem.
The usb device is always attached to one PM domain, but depending on
if super-speed mode is used, another PM domain for that logic needs to
be powered on as well.
Jon, please correct me if I am wrong!
Yes this is technically correct, however, in reality I think we are always
going to enable the superspeed domain if either the host or device domain is
enabled. So we would probably always link the superspeed with the host and
device devices.
Why? Wouldn't that waste power if the superspeed mode isn't used?
Simply to reduce complexity.
Jon
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