linux-kernel - Re: [PATCH v2 3/4] dt-bindings: phy: ti: Add dt binding documentation for SERDES in AM654x SoC

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Date:   Mon, 18 Feb 2019 13:46:06 -0600
From:   Rob Herring <robh@...nel.org>
To:     Kishon Vijay Abraham I <kishon@...com>
Cc:     Roger Quadros <rogerq@...com>, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 3/4] dt-bindings: phy: ti: Add dt binding
 documentation for SERDES in AM654x SoC

On Fri, Feb 08, 2019 at 10:35:44AM +0530, Kishon Vijay Abraham I wrote:
> Hi Roger,
> 
> On 07/02/19 7:52 PM, Roger Quadros wrote:
> > 
> > 
> > On 07/02/19 14:19, Kishon Vijay Abraham I wrote:
> >> Hi Roger,
> >>
> >> On 07/02/19 4:56 PM, Roger Quadros wrote:
> >>>
> >>>
> >>> On 06/02/19 13:07, Kishon Vijay Abraham I wrote:
> >>>> AM654x has two SERDES instances. Each instance has three input clocks
> >>>> (left input, externel reference clock and right input) and two output
> >>>> clocks (left output and right output) in addition to a PLL mux clock
> >>>> which the SERDES uses for Clock Multiplier Unit (CMU refclock).
> >>>> The PLL mux clock can select from one of the three input clocks.
> >>>> The right output can select between left input and external reference
> >>>> clock while the left output can select between the right input and
> >>>> external reference clock.
> >>>>
> >>>> The left and right input reference clock of SERDES0 and SERDES1
> >>>> respectively are connected to the SoC clock. In the case of two lane
> >>>> SERDES personality card, the left input of SERDES1 is connected to
> >>>> the right output of SERDES0 in a chained fashion.
> >>>>
> >>>> See section "Reference Clock Distribution" of AM65x Sitara Processors
> >>>> TRM (SPRUID7 – April 2018) for more details.
> >>>>
> >>>> Add dt-binding documentation in order to represent all these different
> >>>> configurations in device tree.
> >>>>
> >>>> Signed-off-by: Kishon Vijay Abraham I <kishon@...com>
> >>>> ---
> >>>>  .../devicetree/bindings/phy/ti-phy.txt        | 77 +++++++++++++++++++
> >>>>  include/dt-bindings/phy/phy-am654-serdes.h    | 13 ++++
> >>>>  2 files changed, 90 insertions(+)
> >>>>  create mode 100644 include/dt-bindings/phy/phy-am654-serdes.h
> >>>>
> >>>> diff --git a/Documentation/devicetree/bindings/phy/ti-phy.txt b/Documentation/devicetree/bindings/phy/ti-phy.txt
> >>>> index 57dfda8a7a1d..fc2fff6b2c37 100644
> >>>> --- a/Documentation/devicetree/bindings/phy/ti-phy.txt
> >>>> +++ b/Documentation/devicetree/bindings/phy/ti-phy.txt
> >>>> @@ -132,3 +132,80 @@ sata_phy: phy@...96000 {
> >>>>  	syscon-pllreset = <&scm_conf 0x3fc>;
> >>>>  	#phy-cells = <0>;
> >>>>  };
> >>>> +
> >>>> +
> >>>> +TI AM654 SERDES
> >>>> +
> >>>> +Required properties:
> >>>> + - compatible: Should be "ti,phy-am654-serdes"
> >>>> + - reg : Address and length of the register set for the device.
> >>>> + - reg-names: Should be "serdes" which corresponds to the register space
> >>>> +	populated in "reg".
> >>>> + - #phy-cells: determine the number of cells that should be given in the
> >>>> +	phandle while referencing this phy. Should be "2". The 1st cell
> >>>> +	corresponds to the phy type (should be one of the types specified in
> >>>> +	include/dt-bindings/phy/phy.h) and the 2nd cell should be the serdes
> >>>> +	lane function.
> >>>> +	If SERDES0 is referenced 2nd cell should be:
> >>>> +		0 - USB3
> >>>> +		1 - PCIe0 Lane0
> >>>> +		2 - ICSS2 SGMII Lane0
> >>>> +	If SERDES1 is referenced 2nd cell should be:
> >>>> +		0 - PCIe1 Lane0
> >>>> +		1 - PCIe0 Lane1
> >>>> +		2 - ICSS2 SGMII Lane1
> >>>
> >>> Instead of these magic numbers and expecting a human to decipher this
> >>> which is prone to error, is it better to create the following defines and
> >>> check for valid configuration in the driver?
> >>>
> >>> AM654_SERDES_LANE_USB3,
> >>> AM654_SERDES_LANE_PCIE_LANE0,
> >>> AM654_SERDES_LANE_PCIE_LANE1,
> >>> AM654_SERDES_LANE_SGMII,
> >>>
> >>> So if you pass AM654_SERDES_LANE_PCIE_LANE0 to SERDES1, driver can easily
> >>> figure out that it should be 1 if it is serdes0 and 0 if serdes1
> >>>
> >>> Which means the DT must contain something so that you can identify
> >>> if it is serdes0 or serdes1.
> >>
> >> Generally I'd like to avoid drivers having to know instance numbers. That gets
> >> more complicated to handle when the same IP is used in different platforms.

Yes. No indexes please.

> > 
> > But this PHY driver is for AM654 platform. Are you saying that variants of this
> > platform have different lane configurations?
> 
> It can have different lane configurations (we've already seen that in dra72).
> Nothing prevents from having the same SERDES IP in a future platform with
> different lane configuration.

Sounds like this should all be implied by the compatible if it is per 
SoC.

> This is more of a system configuration which might get complicated if we try to
> check all the valid configurations in driver.
> 
> 
> > 
> > You have already specified of the possibilities that can be in 2nd cell in
> > the binding document.
> > 
> > Is it better to create a directory ti/ and put this binding in ti,phy-am654-serdes.txt
> > instead of the already so large ti,phy.txt?
> 
> sure.

+1