Date:   Tue, 16 Feb 2021 22:14:21 +0000
From:   Min Li <min.li.xe@...esas.com>
To:     Arnd Bergmann <arnd@...nel.org>
CC:     Derek Kiernan <derek.kiernan@...inx.com>,
        Dragan Cvetic <dragan.cvetic@...inx.com>,
        Arnd Bergmann <arnd@...db.de>,
        gregkh <gregkh@...uxfoundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Networking <netdev@...r.kernel.org>,
        Richard Cochran <richardcochran@...il.com>
Subject: RE: [PATCH net-next] misc: Add Renesas Synchronization Management
 Unit (SMU) support

> 
> I can't help but think you are evading my question I asked. If there is no
> specific action that this pcm4l tool needs to perform, then I'd think we
> should better not provide any interface for it at all.
> 
> I also found a reference to only closed source software at
> https://www.renesas.com/us/en/software-tool/ptp-clock-manager-linux
> We don't add low-level interfaces to the kernel that are only usable by
> closed-source software.
> 
> Once you are able to describe the requirements for what pcm4l actually
> needs from the hardware, we can start discussing what a high-level
> interface would look like that can be used to replace the your current
> interface, in a way that would work across vendors and with both pcm4l and
> open-source tools that do the same job.
> 
>       Arnd

Hi Arnd

This driver is used by pcm4l to access functionalities that cannot be accessed through PHC(ptp hardware clock) interface.

All these functions are kind of specific to Renesas SMU device and I have never heard other devices offering similar functions

The 3 functions currently provided are (more to be added in the future)

- set combomode

In Telecom Boundary Clock (T-BC) and Telecom Time Slave Clock (T-TSC) applications per ITU-T G.8275.2, two DPLLs can be used:
one DPLL is configured as a DCO to synthesize PTP clocks, and the other DPLL is configured as an EEC(Ethernet Equipment Clock)
to generate physical layer clocks. Combo mode provides physical layer frequency support from the EEC/SEC to the PTP clock.

- read DPLL's FFO

Read fractional frequency offset (FFO) from a DPLL. 

For a DPLL channel, a Frequency Control Word (FCW) is used to adjust the frequency output of the DCO. A positive value will
increase the output frequency and a negative one will decrease the output frequency.

This function will read FCW first and convert it to FFO.

-read DPLL's state

The DPLLs support four primary operating modes: Free-Run, Locked, Holdover, and DCO. In Free-Run mode the DPLLs synthesize
clocks based on the system clock alone. In Locked mode the DPLLs filter reference clock jitter with the selected bandwidth. Additionally
in Locked mode, the long-term output frequency accuracy is the same as the long-term frequency accuracy of the selected input
reference. In Holdover mode, the DPLL uses frequency data acquired while in Locked mode to generate accurate frequencies when input
references are not available. In DCO mode, the DPLL control loop is opened and the DCO can be controlled by a PTP clock recovery
servo running on an external processor to synthesize PTP clocks.

Again, at the bottom, these function are just reading/writing certain registers through I2C/SPI interface.

I am making this driver to support pcm4l since my my company, Renesas, wants to abstract hw details into the kernel. But I can not figure out
how to make this universally applied interface and I find misc is the best place to hold driver like this. On the other hand, if you have better ideas,
I am all ears.

Min

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