Date:   Tue, 30 Aug 2016 14:50:58 -0600
From:   Stephen Warren <swarren@...dotorg.org>
To:     Rob Herring <robh@...nel.org>
Cc:     Lars Persson <lars.persson@...s.com>,
        Mark Rutland <mark.rutland@....com>,
        devicetree@...r.kernel.org, netdev@...r.kernel.org,
        linux-tegra@...r.kernel.org, Stephen Warren <swarren@...dia.com>
Subject: Re: [PATCH V2] dt: net: enhance DWC EQoS binding to support Tegra186

On 08/30/2016 01:01 PM, Rob Herring wrote:
> On Wed, Aug 24, 2016 at 03:20:46PM -0600, Stephen Warren wrote:
>> From: Stephen Warren <swarren@...dia.com>
>>
>> The Synopsys DWC EQoS is a configurable IP block which supports multiple
>> options for bus type, clocking and reset structure, and feature list.
>> Extend the DT binding to define a "compatible value" for the configuration
>> contained in NVIDIA's Tegra186 SoC, and define some new properties and
>> list property entries required by that configuration.

>> diff --git a/Documentation/devicetree/bindings/net/snps,dwc-qos-ethernet.txt b/Documentation/devicetree/bindings/net/snps,dwc-qos-ethernet.txt

>>  Required properties:

>> -- clocks: Phandles to the reference clock and the bus clock
>> -- clock-names: Should be "phy_ref_clk" for the reference clock and "apb_pclk"
>> -  for the bus clock.
>> +- clocks: Phandle and clock specifiers for each entry in clock-names, in the
>> +  same order. See ../clock/clock-bindings.txt.
>> +- clock-names: May contain any/all of the following depending on the IP
>> +  configuration, in any order:
>
> No, they should be in a defined order.

If the binding only defines "clocks", then yes the order must be specified.

If the binding defines clock-names too, then the order is arbitrary 
since all clocks must be looked up via clock-names. That's the entire 
point of having a clock-names property.

...
>> +    The EQOS transmit path clock. The HW signal name is clk_tx_i.
>> +    In some configurations (e.g. GMII/RGMII), this clock also drives the PHY TX
>> +    path. In other configurations, other clocks (such as tx_125, rmii) may
>> +    drive the PHY TX path.
>> +  - "rx"
>> +    The EQOS receive path clock. The HW signal name is clk_rx_i.
>> +    In some configurations (e.g. GMII/RGMII), this clock also drives the PHY RX
>> +    path. In other configurations, other clocks (such as rx_125, pmarx_0,
>> +    pmarx_1, rmii) may drive the PHY RX path.
>> +  - "slave_bus"
>> +    (Alternate name "apb_pclk"; only one alternate must appear.)
>> +    The CPU/slave-bus (CSR) interface clock. Despite the name, this applies to
>> +    any bus type; APB, AHB, AXI, etc. The HW signal name is hclk_i (AHB) or
>> +    clk_csr_i (other buses).
>
> Sounds like 2 clocks here.
>
>> +  - "master_bus"
>> +    The master bus interface clock. Only required in configurations that use a
>> +    separate clock for the master and slave bus interfaces. The HW signal name
>> +    is hclk_i (AHB) or aclk_i (AXI).
>
> Sounds like 2 clocks. I'm guessing these are mutually exclusive based on
> whether you configure the IP for AHB or AXI?

Yes, my understanding is that the two clocks are mutually exclusive in 
both cases.

It seems simpler to have an entry in clocks/clock-names for each logical 
purpose, so that the driver can always retrieve a "slave bus clock" and 
a "master bus clock". That way, there's never any conditional code in 
the driver; it just gets two fixed clock names and enables them, no 
matter what the HW configuration.

If the binding specifies 3 clocks, hclk_i, clk_csr_i, and aclk_i, then 
the driver needs to know which subset of clocks to retrieve based on 
compatible value or HW configuration. That seems like unnecessary 
complexity. I suppose the driver could just attempt to retrieve all 3 
clocks, and ignore any missing clocks, but that would allow malformed 
DTs not to be noticed since the driver wouldn't validate the set of 
clocks present, and could lead to the driver touching the HW without all 
required clocks active, which at least in Tegra can lead to a HW hang.

>> +  Note: Support for additional IP configurations may require adding the
>> +  following clocks to this list in the future: clk_rx_125_i, clk_tx_125_i,
>> +  clk_pmarx_0_i, clk_pmarx1_i, clk_rmii_i, clk_revmii_rx_i, clk_revmii_tx_i.
>> +
>> +  The following compatible values require the following set of clocks:
>> +  - "nvidia,tegra186-eqos", "snps,dwc-qos-ethernet-4.10":
>> +    - "slave_bus"
>> +    - "master_bus"
>> +    - "rx"
>> +    - "tx"
>> +    - "ptp_ref"
>> +  - "axis,artpec6-eqos", "snps,dwc-qos-ethernet-4.10":
>> +    - "phy_ref_clk"
>> +    - "apb_clk"
>
> It would be good if this was marked deprecated and the full set of
> clocks could be described and supported. Not sure if you can figure that
> out. Is it really only 2 clocks, or these have multiple connections to
> the same source.

Lars, can you answer here?

I deliberately didn't attempt to change the binding definition for the 
existing use-case, since I'm not familiar with that SoC, and don't 
relish changing DTs for a platform I can't test.

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