On 08/09/2013 03:34 PM, Kevin Hilman wrote:
Nishanth Menon <nm@xxxxxx> writes:
On 08/09/2013 11:12 AM, Kevin Hilman wrote:
Nishanth Menon <nm@xxxxxx> writes:
On 08/08/2013 06:04 PM, Kevin Hilman wrote:
Nishanth Menon <nm@xxxxxx> writes:
On 08/08/2013 04:14 PM, Kevin Hilman wrote:
Dave Gerlach <d-gerlach@xxxxxx> writes:
On 08/08/2013 10:03 AM, Santosh Shilimkar wrote:
$subject and patch don't match.
On Thursday 08 August 2013 08:26 AM, Nishanth Menon wrote:
On 08/08/2013 03:45 AM, Russ Dill wrote:
In reference to
the M3 handling it, the M3 wouldn't know which devices have a driver
bound and which don't.
Does it need to? M3 firmware can pretty much define "I will force
the device into low power state, and if the drivers dont handle
things properly, fix the darned driver". M3 behavior should be
considered as a "hardware" as far as Linux running on MPU is
concerned, and firmware helps change the behavior by accounting for
SoC quirks. *if* we have ability to handle this in the firmware,
there is no need to carry this in Linux.
I agree with Nishant. I don't like this patch and IIRC, I gave same
comment in the last version. Linux need not know about all such firmware
quirks. Also all these M3 specific stuff, should be done somewhere
else. Probably having a small M3 driver won't be a bad idea.
I am not opposed to doing it this way and letting the M3 firmware
handle idling these modules, however the one concern raised in the
last series is that an approach that does not acknowledge drivers will
hide driver PM bugs. I suppose as long as I make sure to document that
the devices are being idled by the M3 firmware this may not be an
issue. I will look into implementing this.
No, please don't start idling devices in firmware that are otherwise
managed by Linux. Keep the firmware simple and dumb. Linux is managing
these devices, it should manage their bugs too.
This is not just about idling devices. This is about handling broken IP
blocks whose power-on reset state does not allow the the powerdomain to
reach its target state. That's just bad hardware design.
Right, this is where M3 can help -> provide a consistent state for
linux kernel to work with. by the fact that we want to keep majority
of the power code inside master CPU, we are just letting M3 help us
with nothing major at all..
heh, I would say HW design bugs like this are more than "nothing major
at all." :)
tiny stuff like these can help "fix" the hardware design quirks by
hiding it behind the firmware and modifying the hardware behavior.
I disagree here. I'm a firmware minimalist, and hiding bugs like this
in the firmware is wrong when Linux is otherwise managing these devices.
It also imposes criteria on the firmware of future SoCs that doesn't
belong there either. IMO, the only stuff the firmware should do is what
Linux *cannot* do.
Remember, this only needs to happen when there isn't a driver for these
devices. Should we communicate to the firmware that the OS has no
driver, so please enable the hack? I think not.
My view is that the M3 should *ignore* the presence/existence of MPU's
drivers. M3 will do whatever to force the system to go to suspend once
notified - this saves us the prehistoric perpetual trouble when
drivers have bugs (which get exposed in weird usage scenarios) in
production systems, we dont get any hardware help to fix them up while
attempting low power states and system never really hits low power
state. This was always because OMAP and it's derivatives have been
"democratic" in power management - if every hardware block achieves
proper state, then we achieve a system-wide low power state.
I know it breaks the purity of role, but as the
next evolution, we might want to consider M3 something like an
"accelerator" for power management activity.. (not saying it is that
fast.. but conceptually).
Yes, it breaks the purity of role, and makes it hard to maintain and
extend to future SoCs. As a maintainer, that's a red flag. IMO, the
roles need to be kept clear. The M3 manages some devices and the
interconnect that MPU/Linux cannot, the rest are managed by Linux.
suspend is a very controlled state as against cpuidle where driver
knowledge is necessary and in fact mandatory. drivers are supposed to
release their resources - and even though we test the hell out of
them, we do have paths untrodden when it comes to production systems.
Since folks don't seem to care about idle for AM33xx (starting with the
hw designers, from what I can tell), you have the luxury of thinking
only about suspend, where firmware can be heavy handed and force things
into submission. Unfortunately, with cpuidle, life is not that easy and
you have to have cooperation of the device drivers. Coordinating that
with firmware is not so simple, to put it mildly.
Any SW/firmware design that does not account for *both* static PM
(suspend/resume) and dynamic PM (runtime PM + CPUidle) is not long-term
maintainable, and thus ready for mainline IMO. (BTW, this is another
theme from previous reviews of this series.)
I completely agree with you. But is'nt the specific suspend state we
are attempting to achieve on AM335x just tooo expensive latency wise
for being even considered for cpuidle?
I am sure you recollect the latencies involved in OMAP3 OFF mode Vs
OMAP4+ OFF mode - which basically kicked out OFF mode from OMAP4
cpuidle C states? - it was practically useless
in this *specific* power state we are attempting, we do a bunch of i2c
operations, etc, in short something that cannot even be considered for
cpuidle.
Considering this, we can consider the same only for suspend path -
hence allowing firmware to do more here.
This does not conflict with cpuidle (which controls MPU) or runtime PM
(which kicks in once you have drivers active, but if drivers get
active, we dont need to deal with this crap).
Dont you think this helps the specific case to move this into firmware
rather than into omap_device?
No, I don't.
That means the firmware design is based on several assumptions about
what Linux can and can't do in idle, and then imposing that on future
Linux designs as well. I dont' buy it.
OK, I back out my proposal. Will let Dave try out a generic solution in
kernel and comment back.
--
Regards,
Nishanth Menon
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