linux-kernel - Re: [PATCH v3 2/2] clk: meson: add sub MMC clock controller driver

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Date:   Mon, 30 Jul 2018 10:57:26 +0200
From:   Jerome Brunet <jbrunet@...libre.com>
To:     Stephen Boyd <sboyd@...nel.org>,
        Neil Armstrong <narmstrong@...libre.com>,
        Yixun Lan <yixun.lan@...ogic.com>
Cc:     Rob Herring <robh@...nel.org>,
        Martin Blumenstingl <martin.blumenstingl@...glemail.com>,
        Kevin Hilman <khilman@...libre.com>,
        Michael Turquette <mturquette@...libre.com>,
        linux-kernel@...r.kernel.org,
        Boris Brezillon <boris.brezillon@...tlin.com>,
        Liang Yang <liang.yang@...ogic.com>,
        Qiufang Dai <qiufang.dai@...ogic.com>,
        Miquel Raynal <miquel.raynal@...tlin.com>,
        Carlo Caione <carlo@...one.org>,
        linux-amlogic@...ts.infradead.org, linux-clk@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, Jian Hu <jian.hu@...ogic.com>
Subject: Re: [PATCH v3 2/2] clk: meson: add sub MMC clock controller driver

On Fri, 2018-07-27 at 09:45 -0700, Stephen Boyd wrote:
> Quoting Stephen Boyd (2018-07-27 09:41:40)
> > Quoting Yixun Lan (2018-07-27 07:52:23)
> > > HI Stephen:
> > > 
> > > On 07/26/2018 11:20 PM, Stephen Boyd wrote:
> > > > Quoting Yixun Lan (2018-07-12 14:12:44)
> > > > > diff --git a/drivers/clk/meson/mmc-clkc.c b/drivers/clk/meson/mmc-clkc.c
> > > > > new file mode 100644
> > > > > index 000000000000..36c4c7cd69a6
> > > > > --- /dev/null
> > > > > +++ b/drivers/clk/meson/mmc-clkc.c
> > > > > @@ -0,0 +1,367 @@
> > > > > +// SPDX-License-Identifier: (GPL-2.0+ OR MIT)
> > > > > +/*
> > > > > + * Amlogic Meson MMC Sub Clock Controller Driver
> > > > > + *
> > > > > + * Copyright (c) 2017 Baylibre SAS.
> > > > > + * Author: Jerome Brunet <jbrunet@...libre.com>
> > > > > + *
> > > > > + * Copyright (c) 2018 Amlogic, inc.
> > > > > + * Author: Yixun Lan <yixun.lan@...ogic.com>
> > > > > + */
> > > > > +
> > > > > +#include <linux/clk.h>
> > > > 
> > > > Is this include used?
> > > > 
> > > 
> > > this is needed by clk_get_rate()
> > > see drivers/clk/meson/mmc-clkc.c:204
> > 
> > Hmm ok. That's unfortunate.
> 
> You should be able to read the hardware to figure out the clk frequency?
> This may be a sign that the phase clk_ops are bad and should be passing
> in the frequency of the parent clk to the op so that phase can be
> calculated. Jerome?
> 

It could be a away to do it but:
a) if we modify the API, we would need to update every clock driver using it.
   There is not that many users of the phase API but still, it is annoying
b) This particular driver need the parent rate, other might need something else
I guess. (parent phase ??, duty cycle ??) 

I think the real problem here it that you are using the consumer API. You should
be using the provider API like clk_hw_get_rate. Look at the clk-divider.c which
use clk_hw_round_rate() on the parent clock. 

Clock drivers should deal with 'struct clk_hw', not 'struct clk'. I think it was
mentioned in the past that the 'clk' within 'struct clk_hw' might be removed
someday.

Yixun, please don't put your clock driver within the controller driver. Please
implement your 'phase-delay' clock in its own file and export the ops, like
every other clock in the amlogic directory. Also, please review your list of
'#define', some of them are unnecessary copy/paste from the MMC driver.

Regards

Jerome