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From:   Ulf Hansson <ulf.hansson@linaro.org>
Date:   Tue, 12 Jan 2021 15:16:44 +0100
Message-ID: <CAPDyKFpQUFTYJ8K2hPMeQCCbQTF_TCE_B+xkXpcL4Fi7h2ReHw@mail.gmail.com>
Subject: Re: [PATCH v2 35/48] drm/tegra: dc: Support OPP and SoC core voltage scaling
To:     Dmitry Osipenko <digetx@gmail.com>
Cc:     Thierry Reding <thierry.reding@gmail.com>,
        Jonathan Hunter <jonathanh@nvidia.com>,
        Mauro Carvalho Chehab <mchehab@kernel.org>,
        Krzysztof Kozlowski <krzk@kernel.org>,
        "Rafael J. Wysocki" <rjw@rjwysocki.net>,
        Kevin Hilman <khilman@kernel.org>,
        Peter De Schrijver <pdeschrijver@nvidia.com>,
        Viresh Kumar <vireshk@kernel.org>,
        Stephen Boyd <sboyd@kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Linux Media Mailing List <linux-media@vger.kernel.org>,
        linux-tegra <linux-tegra@vger.kernel.org>
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- trimmed cc-list

On Thu, 17 Dec 2020 at 19:08, Dmitry Osipenko <digetx@gmail.com> wrote:
>
> Add OPP and SoC core voltage scaling support to the display controller
> driver. This is required for enabling system-wide DVFS on pre-Tegra186
> SoCs.
>
> Tested-by: Peter Geis <pgwipeout@gmail.com>
> Tested-by: Nicolas Chauvet <kwizart@gmail.com>
> Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
> ---
>  drivers/gpu/drm/tegra/dc.c | 66 +++++++++++++++++++++++++++++++++++++-
>  1 file changed, 65 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/gpu/drm/tegra/dc.c b/drivers/gpu/drm/tegra/dc.c
> index b6676f1fe358..105ad786e432 100644
> --- a/drivers/gpu/drm/tegra/dc.c
> +++ b/drivers/gpu/drm/tegra/dc.c
> @@ -11,9 +11,12 @@
>  #include <linux/interconnect.h>
>  #include <linux/module.h>
>  #include <linux/of_device.h>
> +#include <linux/pm_domain.h>
> +#include <linux/pm_opp.h>
>  #include <linux/pm_runtime.h>
>  #include <linux/reset.h>
>
> +#include <soc/tegra/common.h>
>  #include <soc/tegra/pmc.h>
>
>  #include <drm/drm_atomic.h>
> @@ -1699,6 +1702,48 @@ int tegra_dc_state_setup_clock(struct tegra_dc *dc,
>         return 0;
>  }
>
> +static void tegra_dc_update_voltage_state(struct tegra_dc *dc,
> +                                         struct tegra_dc_state *state)
> +{
> +       unsigned long rate, pstate;
> +       struct dev_pm_opp *opp;
> +       int err;
> +
> +       /* calculate actual pixel clock rate which depends on internal divider */
> +       rate = DIV_ROUND_UP(clk_get_rate(dc->clk) * 2, state->div + 2);
> +
> +       /* find suitable OPP for the rate */
> +       opp = dev_pm_opp_find_freq_ceil(dc->dev, &rate);
> +
> +       if (opp == ERR_PTR(-ERANGE))
> +               opp = dev_pm_opp_find_freq_floor(dc->dev, &rate);
> +
> +       /* -ENOENT means that this device-tree doesn't have OPP table */
> +       if (opp == ERR_PTR(-ENOENT))
> +               return;
> +
> +       if (IS_ERR(opp)) {
> +               dev_err(dc->dev, "failed to find OPP for %luHz: %pe\n",
> +                       rate, opp);
> +               return;
> +       }
> +
> +       pstate = dev_pm_opp_get_voltage(opp);
> +       dev_pm_opp_put(opp);
> +
> +       /*
> +        * The minimum core voltage depends on the pixel clock rate (which
> +        * depends on internal clock divider of the CRTC) and not on the
> +        * rate of the display controller clock. This is why we're not using
> +        * dev_pm_opp_set_rate() API and instead controlling the power domain
> +        * directly.
> +        */
> +       err = dev_pm_genpd_set_performance_state(dc->dev, pstate);

As you state above, in general we should not need to call the
dev_pm_genpd_set_performance_state() directly for the consumer driver.

Even if this looks like a special case to me, I would appreciate a
confirmation from Viresh that this is the way he also would like to
move forward from the opp library perspective.

> +       if (err)
> +               dev_err(dc->dev, "failed to set power domain state to %lu: %d\n",
> +                       pstate, err);
> +}
> +

[...]

Kind regards
Uffe