RFC: representing sdio devices oob interrupt, clks, etc. in device tree

[Ulf Hansson]

On 3 June 2014 13:07, Hans de Goede <hdegoede at redhat.com> wrote:
> Hi,
>
> On 06/03/2014 12:14 PM, Ulf Hansson wrote:
>> On 28 May 2014 11:42, Hans de Goede <hdegoede at redhat.com> wrote:
>
> <mega snip>
>
>>>> If the mmc_of_parse() returns -EPROBE_DEFER, the mmc host driver will
>>>> return the same error code from it's ->probe(). This provides us with
>>>> the ability of waiting for the "powerup driver" to be probed.
>>>
>>> Ack. Note though that mmc_of_parse will likely not do the probe itself,
>>> the way I see it it will do a platform_device_register() and let the
>>> platform bus code do its thing. Downside of this is that
>>> platform_device_register() will not propagate probe errors such as
>>> -EPROBE_DEFER, so we need to check afterwards that a driver is actually
>>> bound, see above.
>>
>> Just to confirm your ideas, this is how I see the instantiation of the
>> device and probe of the "powerup driver" as well.
>
> Ok, so given that in another mail thread we've just decided to not use
> slot subnodes in the devicetree hierarchy, how are we going to represent
> the powerup-bits in devicetree? I suggest that we represent this with
> a separate subnode under the mmc host, with its own compatible string.
>
> Since reg == 0 is for the card device, and reg 1-7 is for the sdio function
> devices, I suggest that we use reg = <8> for the powerup subnode. Then
> the mmc-core can check for such a child subnode, and if it is there
> instantiate a platform device for it, and then handle the probe as
> described above.

Why do we need to put the sdio functions devices in DT?

>
>>
>>>
>>>> If the mmc_of_parse() returns another error code, due to that the
>>>> "powerup driver" failed to be probed, the mmc host driver's ->probe()
>>>> will return the same error code and consequentially no power up of the
>>>> card will be performed at all.
>>>
>>> Ack.
>>>
>>>> Powerup driver's ->probe():
>>>> Typically the "powerup driver" will need to register a few callback
>>>> functions towards the mmc core. Typically at mmc_of_parse(), those
>>>> callbacks will have to be connected to a particular mmc host.
>>>>
>>>> I would like to see three different callbacks, mirroring each of the
>>>> mmc_ios power_mode states MMC_POWER_OFF|UP|ON.
>>>
>>> Hmm, can't we do something with runtime pm here instead? I would be
>>> nice if we could use the platform bus for this instead of inventing
>>> a new bus for this.
>>
>> We don't need another bus. The driver only have to register some mmc
>> specific callbacks, that's all I am saying. Of course these parts
>> can't be re-used for other subsystems, unless we find it useful to
>> have similar callbacks for all subsystems.
>>
>> Still, using runtime PM might work.
>>
>> I see these important things that follow if we decide to use runtime
>> PM to trigger the power up/off sequence.
>> 1) In cases of !CONFIG_PM_RUNTIME, it means the "powerup driver" once
>> probed, will keep it's resources enabled forever.
>
> Ack.

So, the consequence is that for CONFIG_PM_SLEEP systems not using
CONFIG_PM_RUNTIME - we don't have a good solution.

Is that acceptable?

>
>> 2) If we want to use runtime PM to control fine grained power
>> management of the "powerup driver", now this can't be done.
>
> We can always add something more elaborate later if needed, the advantage
> of sticking with a platform-dev represented by its own dt subnode +
> runtime PM, is that powerup drivers can be used with other busses too,
> all the other busses will need is to specify the subnode location + address
> inside the tree, and add code to their subsys core to instantiate the
> platform device.
>
>> 3) The "powerup driver" must be able to cope with two states (on/off),
>> instead the three MMC_POWER_OFF|UP|ON states.
>
> Since we need to powerup before probing, I think this is fine,
> we will want to do the power-up before we do the OFF -> UP -> ON
> sequence in mmc_power_up(), and we will want to do the power-down
> after transitioning to OFF.
>
>> 4) The system suspend/resume sequence for the SDIO card, will be more
>> tricky to handle.
>
> See below.
>
>> In principle we need to decide what runtime PM should be used for in
>> this context.
>
> I think we should add a powerup_dev pdev pointer to the mmc-card struct,
> so that sdio drivers which want to shutdown the device to save power can
> do so (by making the relevant runtime pm calls on the pdev).

Makes sense.

>
> The mmc core will never know if it is safe to actually power down the
> device again as even if the sdio driver indicates it is ok to shutdown
> the mmc-host, it may still need the sdio device to stay powered so as to
> not loose state. Or maybe even for system-wakeup through an oob irq.

That should already be handled through the flags MMC_PM_KEEP_POWER and
MMC_PM_WAKE_SDIO_IRQ.

>
> Regards,
>
> Hans

Kind regards
Ulf Hansson