netdev - Re: [RFC PATCH v4 0/4] Create common DPLL/clock configuration API

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Message-ID: <Y77QEajGlJewGKy1@nanopsycho>
Date:   Wed, 11 Jan 2023 16:04:49 +0100
From:   Jiri Pirko <jiri@...nulli.us>
To:     "Kubalewski, Arkadiusz" <arkadiusz.kubalewski@...el.com>
Cc:     Jakub Kicinski <kuba@...nel.org>,
        Maciek Machnikowski <maciek@...hnikowski.net>,
        'Vadim Fedorenko' <vfedorenko@...ek.ru>,
        'Jonathan Lemon' <jonathan.lemon@...il.com>,
        'Paolo Abeni' <pabeni@...hat.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-clk@...r.kernel.org" <linux-clk@...r.kernel.org>
Subject: Re: [RFC PATCH v4 0/4] Create common DPLL/clock configuration API

Wed, Jan 11, 2023 at 03:16:59PM CET, arkadiusz.kubalewski@...el.com wrote:
>>From: Jiri Pirko <jiri@...nulli.us>
>>Sent: Wednesday, January 11, 2023 9:20 AM
>>
>>Tue, Jan 10, 2023 at 09:05:49PM CET, kuba@...nel.org wrote:
>>>On Mon, 9 Jan 2023 14:43:01 +0000 Kubalewski, Arkadiusz wrote:
>>>> This is a simplified network switch board example.
>>>> It has 2 synchronization channels, where each channel:
>>>> - provides clk to 8 PHYs driven by separated MAC chips,
>>>> - controls 2 DPLLs.
>>>>
>>>> Basically only given FW has control over its PHYs, so also a control
>>over it's
>>>> MUX inputs.
>>>> All external sources are shared between the channels.
>>>>
>>>> This is why we believe it is not best idea to enclose multiple DPLLs
>>with one
>>>> object:
>>>> - sources are shared even if DPLLs are not a single synchronizer chip,
>>>> - control over specific MUX type input shall be controllable from
>>different
>>>> driver/firmware instances.
>>>>
>>>> As we know the proposal of having multiple DPLLs in one object was a try
>>to
>>>> simplify currently implemented shared pins. We fully support idea of
>>having
>>>> interfaces as simple as possible, but at the same time they shall be
>>flexible
>>>> enough to serve many use cases.
>>>
>>>I must be missing context from other discussions but what is this
>>>proposal trying to solve? Well implemented shared pins is all we need.
>>
>>There is an entity containing the pins. The synchronizer chip. One
>>synchronizer chip contains 1-n DPLLs. The source pins are connected
>>to each DPLL (usually). What we missed in the original model was the
>>synchronizer entity. If we have it, we don't need any notion of somehow
>>floating pins as independent entities being attached to one or many
>>DPLL refcounted, etc. The synchronizer device holds them in
>>straightforward way.
>>
>>Example of a synchronizer chip:
>>https://www.renesas.com/us/en/products/clocks-timing/jitter-attenuators-
>>frequency-translation/8a34044-multichannel-dpll-dco-four-eight-
>>channels#overview
>
>Not really, as explained above, multiple separated synchronizer chips can be
>connected to the same external sources.
>This is why I wrote this email, to better explain need for references between
>DPLLs and shared pins.
>Synchronizer chip object with multiple DPLLs would have sense if the pins would
>only belong to that single chip, but this is not true.

I don't understand how it is physically possible that 2 pins belong to 2
chips. Could you draw this to me?


>As the pins are shared between multiple DPLLs (both inside 1 integrated circuit
>and between multiple integrated circuits), all of them shall have current state
>of the source or output.
>Pins still need to be shared same as they would be inside of one synchronizer
>chip.

Do I understand correctly that you connect one synchronizer output to
the input of the second synchronizer chip?

>
>BR,
>Arkadiusz