Re: [PATCH 2/2] mmc: sdhci-iproc: Add support for the legacy sdhci controller on the BCM7211

[Florian Fainelli <f.fainelli@gmail.com>]

On 6/15/2021 8:30 AM, Ulf Hansson wrote:
> [...]
> 
>>>
>>>>
>>>> In all honesty, I am a bit surprised that the Linux device driver model
>>>> does not try to default the absence of a ->shutdown() to a ->suspend()
>>>> call since in most cases they are functionally equivalent, or should be,
>>>> in that they need to save power and quiesce the hardware, or leave
>>>> enough running to support a wake-up event.
>>>
>>> Well, the generall assumption is that the platform is going to be
>>> entirely powered off, thus moving things into a low power state would
>>> just be a waste of execution cycles. Of course, that's not the case
>>> for your platform.
>>
>> That assumption may hold true for ACPI-enabled machines but power off is
>> offered as a general function towards other more flexible and snowflaky
>> systems (read embedded) as well.
>>
>>>
>>> As I have stated earlier, to me it looks a bit questionable to use the
>>> kernel_power_off() path to support the use case you describe. On the
>>> other hand, we may not have a better option at this point.
>>
>> Correct, there is not really anything better and I am not sure what the
>> semantics of something better could be anyway.
>>
>>>
>>> Just a few things, from the top of my head, that we certainly are
>>> missing to support your use case through kernel_power_off() path
>>> (there are certainly more):
>>> 1. In general, subsystems/drivers don't care about moving things into
>>> lower power modes from their ->shutdown() callbacks.
>>> 2. System wakeups and devices being affected in the wakeup path, needs
>>> to be respected properly. Additionally, userspace should be able to
>>> decide if system wakeups should be enabled or not.
>>> 3. PM domains don't have ->shutdown() callbacks, thus it's likely that
>>> they remain powered on.
>>> 4. Etc...
>>
>> For the particular eMMC driver being discussed here this is a no-brainer
>  > because  it is not a wake-up source, therefore there is no reason not to
>> power if off if we can. It also seems proper to have it done by the
>> kernel as opposed to firmware.
> 
> Okay, I have applied the $subject patch onto my next branch, along
> with patch 1/2 (the DT doc change).
> 
> However, I still think we should look for a proper long term solution,
> because the kernel_power_off() path does not currently support your
> use case, with system wakeups etc.

Not really, it does work fine, some drivers like gpio-keys.c or
gpio-brcmstb.c will ensure that the GPIOs that are enabled as wake-up
interrupts are configured that way during kernel_power_off() and the
various interrupt controllers like irq-brcmstb-l2.c will make sure they
don't mask wake-up interrupts.

> 
> I guess it could be a topic that is easier to bring up at the Linux
> Plumbers Conf, for example.

OK, not sure if I will be able to attend, but would definitively try to.
-- 
Florian