linux-kernel - Re: [linux-sunxi] [PATCH v2 5/9] drm/sun4i: tcon

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Date:   Tue, 18 Jun 2019 15:19:43 +0800
From:   Chen-Yu Tsai <wens@...e.org>
To:     Jagan Teki <jagan@...rulasolutions.com>
Cc:     Maxime Ripard <maxime.ripard@...tlin.com>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        devicetree <devicetree@...r.kernel.org>,
        Jernej Skrabec <jernej.skrabec@...l.net>,
        Michael Trimarchi <michael@...rulasolutions.com>,
        linux-sunxi <linux-sunxi@...glegroups.com>,
        linux-amarula <linux-amarula@...rulasolutions.com>
Subject: Re: [linux-sunxi] [PATCH v2 5/9] drm/sun4i: tcon_top: Register clock
 gates in probe

On Mon, Jun 17, 2019 at 6:30 PM Jagan Teki <jagan@...rulasolutions.com> wrote:
>
> On Sun, Jun 16, 2019 at 11:01 AM Chen-Yu Tsai <wens@...e.org> wrote:
> >
> > On Sat, Jun 15, 2019 at 12:44 AM Jagan Teki <jagan@...rulasolutions.com> wrote:
> > >
> > > TCON TOP have clock gates for TV0, TV1, dsi and right
> > > now these are register during bind call.
> > >
> > > Of which, dsi clock gate would required during DPHY probe
> > > but same can miss to get since tcon top is not bound at
> > > that time.
> > >
> > > To solve, this circular dependency move the clock gate
> > > registration from bind to probe so-that DPHY can get the
> > > dsi gate clock on time.
> > >
> > > Signed-off-by: Jagan Teki <jagan@...rulasolutions.com>
> > > ---
> > >  drivers/gpu/drm/sun4i/sun8i_tcon_top.c | 94 ++++++++++++++------------
> > >  1 file changed, 49 insertions(+), 45 deletions(-)
> > >
> > > diff --git a/drivers/gpu/drm/sun4i/sun8i_tcon_top.c b/drivers/gpu/drm/sun4i/sun8i_tcon_top.c
> > > index 465e9b0cdfee..a8978b3fe851 100644
> > > --- a/drivers/gpu/drm/sun4i/sun8i_tcon_top.c
> > > +++ b/drivers/gpu/drm/sun4i/sun8i_tcon_top.c
> > > @@ -124,7 +124,53 @@ static struct clk_hw *sun8i_tcon_top_register_gate(struct device *dev,
> > >  static int sun8i_tcon_top_bind(struct device *dev, struct device *master,
> > >                                void *data)
> > >  {
> > > -       struct platform_device *pdev = to_platform_device(dev);
> > > +       struct sun8i_tcon_top *tcon_top = dev_get_drvdata(dev);
> > > +       int ret;
> > > +
> > > +       ret = reset_control_deassert(tcon_top->rst);
> > > +       if (ret) {
> > > +               dev_err(dev, "Could not deassert ctrl reset control\n");
> > > +               return ret;
> > > +       }
> > > +
> > > +       ret = clk_prepare_enable(tcon_top->bus);
> > > +       if (ret) {
> > > +               dev_err(dev, "Could not enable bus clock\n");
> > > +               goto err_assert_reset;
> > > +       }
> >
> > You have to de-assert the reset control and enable the clock before the
> > clocks it provides are registered. Otherwise a consumer may come in and
> > ask for the provided clock to be enabled, but since the TCON TOP's own
> > reset and clock are still disabled, you can't actually access the registers
> > that controls the provided clock.
>
> These rst and bus are common reset and bus clocks not tcon top clocks
> that are trying to register here. ie reason I have not moved it in
> top.

And you're sure that toggling bits in the TCON TOP block doesn't require
the reset to be de-asserted and the bus clock enabled?

Somehow I doubt that.

Once the driver register the clocks it provides, they absolutely must work.
They can't only work after the bind phase when the reset gets de-asserted
and the bus clock enabled. Or you should provide proper error reporting
in the clock ops. I doubt you want to go that way either.

ChenYu