linux-kernel - Re: [PATCH v2 2/7] dt-bindings: clock: meson: a1: pll: introduce new syspll bindings

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Date: Wed, 15 May 2024 15:15:35 +0200
From: Jerome Brunet <jbrunet@...libre.com>
To: Dmitry Rokosov <ddrokosov@...utedevices.com>
Cc: Conor Dooley <conor@...nel.org>, neil.armstrong@...aro.org,
 jbrunet@...libre.com, mturquette@...libre.com, sboyd@...nel.org,
 robh+dt@...nel.org, krzysztof.kozlowski+dt@...aro.org,
 khilman@...libre.com, martin.blumenstingl@...glemail.com,
 jian.hu@...ogic.com, kernel@...rdevices.ru, rockosov@...il.com,
 linux-amlogic@...ts.infradead.org, linux-clk@...r.kernel.org,
 devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
 linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v2 2/7] dt-bindings: clock: meson: a1: pll: introduce
 new syspll bindings


On Mon 13 May 2024 at 21:30, Dmitry Rokosov <ddrokosov@...utedevices.com> wrote:

> On Mon, May 13, 2024 at 04:48:33PM +0100, Conor Dooley wrote:
>> On Mon, May 13, 2024 at 12:18:02PM +0300, Dmitry Rokosov wrote:
>> > Hello Conor,
>> > 
>> > Thank you for quick review!
>> > 
>> > On Sat, May 11, 2024 at 02:08:03PM +0100, Conor Dooley wrote:
>> > > On Fri, May 10, 2024 at 12:08:54PM +0300, Dmitry Rokosov wrote:
>> > > > The 'syspll' PLL is a general-purpose PLL designed specifically for the
>> > > > CPU clock. It is capable of producing output frequencies within the
>> > > > range of 768MHz to 1536MHz.
>> > > > 
>> > > > The clock source sys_pll_div16, being one of the GEN clock parents,
>> > > > plays a crucial role and cannot be tagged as "optional". Unfortunately,
>> > > > it was not implemented earlier due to the cpu clock ctrl driver's
>> > > > pending status on the TODO list.
>> > > 
>> > > It's fine to not mark it optional in the binding, but it should be
>> > > optional in the driver as otherwise backwards compatibility will be
>> > > broken. Given this is an integral clock driver, sounds like it would
>> > > quite likely break booting on these devices if the driver doesn't treat
>> > > syspll_in as optional.
>> > > A lesson perhaps in describing the hardware entirely, even if the
>> > > drivers don't make use of all the information yet?
>> > 
>> > Yes, it's definitely the right lesson for me. However, without syspll or
>> > syspll_in, we cannot utilize CPU power management at all.
>> 
>> That's the status-quo, right? The incorrect dts would continue to not
>> support CPU power management and the new one with the correct description
>> would?
>
> Hmmm, correct. Okay, I see, I will support sys_pll as optional
> connection :)

Again, the way controller is written, all inputs are actually optional.
The controller does not error out if an input is missing, it behave as
if the input is disconnected

-- 
Jerome