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

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Message-ID: <20170718042955.GA12080@b29397-desktop>
Date:   Tue, 18 Jul 2017 12:29:55 +0800
From:   Peter Chen <hzpeterchen@...il.com>
To:     "Rafael J. Wysocki" <rjw@...ysocki.net>
CC:     Peter Chen <peter.chen@....com>, <mark.rutland@....com>,
        <ulf.hansson@...aro.org>, <heiko@...ech.de>,
        <stephen.boyd@...aro.org>, <frank.li@....com>,
        <linux-kernel@...r.kernel.org>, <gary.bisson@...ndarydevices.com>,
        <festevam@...il.com>, <stillcompiling@...il.com>, <arnd@...db.de>,
        <dbaryshkov@...il.com>, <vaibhav.hiremath@...aro.org>,
        <krzk@...nel.org>, <mka@...omium.org>, <stern@...land.harvard.edu>,
        <devicetree@...r.kernel.org>, <mail@...iej.szmigiero.name>,
        <pawel.moll@....com>, <linux-pm@...r.kernel.org>,
        <s.hauer@...gutronix.de>, <troy.kisky@...ndarydevices.com>,
        <robh+dt@...nel.org>, <linux-arm-kernel@...ts.infradead.org>,
        <hverkuil@...all.nl>, <oscar@...andei.net>,
        <gregkh@...uxfoundation.org>, <linux-usb@...r.kernel.org>,
        <sre@...nel.org>, <broonie@...nel.org>, <p.zabel@...gutronix.de>,
        <shawnguo@...nel.org>, <jun.li@....com>
Subject: Re: [PATCH v16 2/7] power: add power sequence library

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.

> 
> 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.

-- 

Best Regards,
Peter Chen