linux-kernel - Re: [PATCH v16 2/7] power: add power sequence library

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Date:   Tue, 18 Jul 2017 19:06:05 +0200
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Peter Chen <hzpeterchen@...il.com>
Cc:     "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Peter Chen <peter.chen@....com>,
        Mark Rutland <mark.rutland@....com>,
        Ulf Hansson <ulf.hansson@...aro.org>,
        Heiko Stuebner <heiko@...ech.de>,
        Stephen Boyd <stephen.boyd@...aro.org>, frank.li@....com,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Gary Bisson <gary.bisson@...ndarydevices.com>,
        Fabio Estevam <festevam@...il.com>,
        Joshua Clayton <stillcompiling@...il.com>,
        Arnd Bergmann <arnd@...db.de>,
        Dmitry Eremin-Solenikov <dbaryshkov@...il.com>,
        Vaibhav Hiremath <vaibhav.hiremath@...aro.org>,
        Krzysztof Kozlowski <krzk@...nel.org>, mka@...omium.org,
        Alan Stern <stern@...land.harvard.edu>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        mail@...iej.szmigiero.name, Pawel Moll <pawel.moll@....com>,
        Linux PM <linux-pm@...r.kernel.org>,
        Sascha Hauer <s.hauer@...gutronix.de>,
        troy.kisky@...ndarydevices.com, Rob Herring <robh+dt@...nel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>, hverkuil@...all.nl,
        oscar@...andei.net,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "open list:ULTRA-WIDEBAND (UWB) SUBSYSTEM:" 
        <linux-usb@...r.kernel.org>, Sebastian Reichel <sre@...nel.org>,
        Mark Brown <broonie@...nel.org>, p.zabel@...gutronix.de,
        Shawn Guo <shawnguo@...nel.org>, jun.li@....com
Subject: Re: [PATCH v16 2/7] power: add power sequence library

On Tue, Jul 18, 2017 at 6:29 AM, Peter Chen <hzpeterchen@...il.com> wrote:
> On Mon, Jul 17, 2017 at 03:39:07PM +0200, Rafael J. Wysocki wrote:
>> > Sorry, I should describe more.
>> >
>> > Let's take USB bus as an example, when the new USB device is at the
>> > host port, the device structure at device model is not created until
>> > it is discoverable by the USB bus. If this new USB device needs to be
>> > powered on before can be discoverable by the bus, the device structure
>> > will be not created without powering on operation. The code usb_alloc_dev
>> > (drivers/usb/core/usb.c) is only called for discoverable device.
>> >
>> > Unlike the other bus, eg, platform bus, it creates device structure
>> > according to DT node. The USB bus was designed for hot plug model, the
>> > device structure is for discoverable device. In recent years, we begin
>> > to have some hard-wired USB device, Eg, onboard USB-hub, onboard USB 4G
>> > Modem, etc at the market. It needs some board level power operation before
>> > it can be found by the USB bus. This patch set is designed primarily for
>> > fix this kind of problem. You will see at at pwrseq_generic.c, we use DT
>> > version clock API of_clk_get and DT version gpio API of_get_named_gpio_flags
>> > instead of device structure version, like devm_clk_get and
>> > devm_gpiod_get_optional.
>> >
>> > MMC system has similar use case, it creates power sequence platform
>> > device for this issue, but all those power stuffs (clock, gpio, etc)
>> > may not be suitable as a dedicated virtual device at DT, they are belonged
>> > to one physical device, so this patch set is created to see if this issue
>> > can be fixed better.
>>
>> OK, thanks for the explanation.
>>
>> The above needs to be part of your problem statement.
>
> Ok, I will add it to cover letter.
>
>> >
>> > The bus will power up all device nodes in this bus according to DT
>> > information, the device structure has not created at this time.
>>
>> OK
>>
>> I still think that the information on power resources depended on by devices
>> should be used for power management as well as for the initial power-up.
>>
>> The most straightforward way to arrange for that would be to make it possible
>> to find the DT node matching the device after the device has been discovered
>> and struct device created for it, say by USB.  That would require adding some
>> more information on the device to the DT node, probably.
>
> After the device is created, the device node structure is under struct
> device, say dev->of_node. The most difficulty for this issue is the
> device creation is dynamic and is after the physical device is
> discovered by the bus, the initial power-up is needed before the device
> can be discovered by the bus.

So you power up all devices on the bus using the information from
of_nodes upfront.

Then you scan the bus and discover devices.  For each of them, once it
has been discovered, you look up a matching of_node and initialize
power_state from there.

>>
>> Then, the DT device nodes would be used for the initial power-up and next, after
>> discovering a device, you'd do a lookup in the DT, find the node matching it
>> and read the power resuources information from there to populate the device's
>> power state structure.  From that point on you can simply use the interface I
>> suggested.
>>
>
> Just like I said above, without initial power-up, the device can't be
> discovered by the bus.

Right.

So you do the power up upfront, as per the above.

Thanks,
Rafael