linux-kernel - Re: [resend PATCH 1/3] pwm: mediatek: drop flag 'has

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Date:   Thu, 15 Nov 2018 10:16:16 +0800
From:   Ryder Lee <ryder.lee@...iatek.com>
To:     John Crispin <john@...ozen.org>
CC:     Thierry Reding <thierry.reding@...il.com>,
        Rob Herring <robh+dt@...nel.org>, <linux-pwm@...r.kernel.org>,
        Weijie Gao <weijie.gao@...iatek.com>,
        Roy Luo <cheng-hao.luo@...iatek.com>,
        <devicetree@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <linux-arm-kernel@...ts.infradead.org>,
        <linux-mediatek@...ts.infradead.org>
Subject: Re: [resend PATCH 1/3] pwm: mediatek: drop flag 'has_clks'

On Wed, 2018-11-14 at 14:27 +0100, John Crispin wrote:
> On 14/11/2018 13:47, Thierry Reding wrote:
> > On Tue, Nov 13, 2018 at 10:08:22AM +0800, Ryder Lee wrote:
> >> The flag 'has_clks' and related checks are superfluous as the CCF
> >> subsystem does this for you.
> > Both of these mechanisms aren't equivalent. While CCF can deal with
> > optional clocks, what the has_clks flag actually means is that the
> > device doesn't need a clock (or doesn't have a clock input) on the
> > devices where it is cleared.
> >
> > So I'd actually be in favor of keeping the has_clks property because it
> > serves as an additional sanity check. For example if you run this driver
> > on an SoC that "has clocks" but if you don't list them in DT, then after
> > this patch the driver will happily continue without clocks, even though
> > it may break completely without those clocks. I've seen SoCs respond to
> > disabled clocks for a hardware block in different ways, in many cases an
> > access to any of the registers will completely hang the CPU. In other
> > cases it may just crash in some other way or give you some sort of
> > machine exception. None of those are good, and make the tiny bit of
> > additional code required to support the has_clks flag very attractive.
> >
> > But that's just my opinion. If you prefer to throw away that safety
> > barrier, be my guest. But if you do, please move this functionality into
> > the clock framework first and then make the driver use it.
> >
> > Thierry
> 
> Hi,
> 
> sorry for my late response. I added the flag for the legacy MIPS 
> silicon. These SoCs only have a single clock register with a few on/off 
> bits. there is no complex clocktree or scaling. Hence COMMON_CLK is not 
> supported by those SoCs. I fully agree with Thierry, that the flag makes 
> this explicit and the intent was indeed to make sure that on silicon 
> where clocks are required, that they really are listed in OF. This is 
> indeed an extra sanity check and hiding it in an implicit check inside 
> CCF does not feel right.
> 
>      John
> 

Thanks for the detailed information:)  I will drop this patch in v1.

Ryder