Re: [PATCH v5 3/5] drm/msm/dp: set stream_pixel rate directly

[Stephen Boyd <swboyd@chromium.org>]

Quoting Dmitry Baryshkov (2022-03-03 20:23:06)
> On Fri, 4 Mar 2022 at 01:32, Stephen Boyd <swboyd@chromium.org> wrote:
> >
> > Quoting Dmitry Baryshkov (2022-02-16 21:55:27)
> > > The only clock for which we set the rate is the "stream_pixel". Rather
> > > than storing the rate and then setting it by looping over all the
> > > clocks, set the clock rate directly.
> > >
> > > Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
> > [...]
> > > diff --git a/drivers/gpu/drm/msm/dp/dp_ctrl.c b/drivers/gpu/drm/msm/dp/dp_ctrl.c
> > > index 07f6bf7e1acb..8e6361dedd77 100644
> > > --- a/drivers/gpu/drm/msm/dp/dp_ctrl.c
> > > +++ b/drivers/gpu/drm/msm/dp/dp_ctrl.c
> > > @@ -1315,7 +1315,7 @@ static void dp_ctrl_set_clock_rate(struct dp_ctrl_private *ctrl,
> > >         DRM_DEBUG_DP("setting rate=%lu on clk=%s\n", rate, name);
> > >
> > >         if (num)
> > > -               cfg->rate = rate;
> > > +               clk_set_rate(cfg->clk, rate);
> >
> > This looks bad. From what I can tell we set the rate of the pixel clk
> > after enabling the phy and configuring it. See the order of operations
> > in dp_ctrl_enable_mainlink_clocks() and note how dp_power_clk_enable()
> > is the one that eventually sets a rate through dp_power_clk_set_rate()
> >
> >         dp_ctrl_set_clock_rate(ctrl, DP_CTRL_PM, "ctrl_link",
> >                                         ctrl->link->link_params.rate * 1000);
> >
> >         phy_configure(phy, &dp_io->phy_opts);
> >         phy_power_on(phy);
> >
> >         ret = dp_power_clk_enable(ctrl->power, DP_CTRL_PM, true);
>
> This code has been changed in the previous patch.
>
> Let's get back a bit.
> Currently dp_ctrl_set_clock_rate() doesn't change the clock rate. It
> just stores the rate in the config so that later the sequence of
> dp_power_clk_enable() -> dp_power_clk_set_rate() ->
> [dp_power_clk_set_link_rate() -> dev_pm_opp_set_rate() or
> msm_dss_clk_set_rate() -> clk_set_rate()] will use that.
>
> There are only two users of dp_ctrl_set_clock_rate():
> - dp_ctrl_enable_mainlink_clocks(), which you have quoted above.
>   This case is handled in the patch 1 from this series. It makes

Patch 1 form this series says DP is unaffected. Huh?

> dp_ctrl_enable_mainlink_clocks() call dev_pm_opp_set_rate() directly
> without storing (!) the rate in the config, calling
> phy_configure()/phy_power_on() and then setting the opp via the
> sequence of calls specified above
>
> - dp_ctrl_enable_stream_clocks(), which calls dp_power_clk_enable()
> immediately afterwards. This call would set the stream_pixel rate
> while enabling stream clocks. As far as I can see, the stream_pixel is
> the only stream clock. So this patch sets the clock rate without
> storing in the interim configuration data.
>
> Could you please clarify, what exactly looks bad to you?
>

I'm concerned about the order of operations changing between the
phy being powered on and the pixel clk frequency being set. From what I
recall the pixel clk rate operations depend on the phy frequency being
set (which is done through phy_configure?) so if we call clk_set_rate()
on the pixel clk before the phy is set then the clk frequency will be
calculated badly and probably be incorrect.