linux-kernel - Re: [PATCH v4 3/4] dt-bindings: phy: Add support for QMP phy

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Date:   Tue, 24 Jan 2017 15:03:33 +0530
From:   Kishon Vijay Abraham I <kishon@...com>
To:     Stephen Boyd <sboyd@...eaurora.org>,
        Vivek Gautam <vivek.gautam@...eaurora.org>
CC:     Bjorn Andersson <bjorn.andersson@...aro.org>, <robh+dt@...nel.org>,
        <linux-kernel@...r.kernel.org>, <devicetree@...r.kernel.org>,
        <mark.rutland@....com>, <srinivas.kandagatla@...aro.org>,
        <linux-arm-msm@...r.kernel.org>
Subject: Re: [PATCH v4 3/4] dt-bindings: phy: Add support for QMP phy

Hi,

On Friday 20 January 2017 03:12 AM, Stephen Boyd wrote:
> On 01/19, Vivek Gautam wrote:
>>
>> On 01/19/2017 06:10 AM, Stephen Boyd wrote:
>>>>
>>> Didn't we already move away from subnodes for lanes in an earlier
>>> revision of these patches? I seem to recall we did that because
>>> lanes are not devices and the whole "phy as a bus" concept not
>>> making sense.
>>
>> Yea, we started out without having any sub-nodes and we
>> argued that we don't require them since the qmp device is
>> represented by the qmp node itself.
>> The lanes otoh are representative of gen_phys and related properties.
>>
>> In the driver -
>> "struct qmp_phy " represents the lanes and holds "struct phy",
>> "struct qcom_qmp" represents the qmp block as a whole and holds
>> "struct device"
>> Does this make lanes qualify to be childs of qmp ?
> 
> Hmm... maybe I was recalling the DSI phy binding. I think there
> are lanes there too but we decided to just have one node.
> 
>>
>> "phy as a bus" (just trying to understand here) -
>> let's say a usb phy controller has one HSIC phy port and one USB2 phy port.
>> So, should this phy controller be a bus providing two ports (and so
>> we will have
>> couple of child nodes to the phy controller) ?
>>
> 
> Typically in DT a subnode or collection of subnodes means there's
> some sort of bus involved. Usually each node corresponds to a
> struct device, and the parent node corresponds to the bus or
> controller for the logical bus.
> 
> In this case (only PCIe though? not UFS or USB?) it seems like we
> have multiple phys that share a common register space, but
> otherwise they have their own register space and power
> management. Would you have each PCIe controller point to a
> different subnode for their associated phy? I'm trying to
> understand the benefit of the subnodes if they aren't treated as
> struct devices.

Yes, instead of having all the controller having a phandle to the same PHY and
then using other mechanisms to differentiate between the PHYs, each controller
can have a phandle to the exact port that it is connected to.

This also gives a better representation of the hardware and can avoid lot of
boilerplate code in the driver.

Thanks
Kishon