Re: [PATCH v12 4/7] drm/meson: gate px_clk when setting rate

[Neil Armstrong]

On 10/04/2024 21:34, Martin Blumenstingl wrote:
> Hi Neil,
>
> On Wed, Apr 3, 2024 at 9:46 AM Neil Armstrong <neil.armstrong@xxxxxxxxxx> wrote:
> > Disable the px_clk when setting the rate to recover a fully
> > configured and correctly reset VCLK clock tree after the rate
> > is set.
> >
> > Fixes: 77d9e1e6b846 ("drm/meson: add support for MIPI-DSI transceiver")
> > Signed-off-by: Neil Armstrong <neil.armstrong@xxxxxxxxxx>
> > ---
> >   drivers/gpu/drm/meson/meson_dw_mipi_dsi.c | 7 +++++++
> >   1 file changed, 7 insertions(+)
> >
> > diff --git a/drivers/gpu/drm/meson/meson_dw_mipi_dsi.c b/drivers/gpu/drm/meson/meson_dw_mipi_dsi.c
> > index a6bc1bdb3d0d..a10cff3ca1fe 100644
> > --- a/drivers/gpu/drm/meson/meson_dw_mipi_dsi.c
> > +++ b/drivers/gpu/drm/meson/meson_dw_mipi_dsi.c
> > @@ -95,6 +95,7 @@ static int dw_mipi_dsi_phy_init(void *priv_data)
> >                  return ret;
> >          }
> >
> > +       clk_disable_unprepare(mipi_dsi->px_clk);
> nit-pick: clk_disable(mipi_dsi->px_clk); should be enough here as my
> understanding is that we only need to {un,}prepare a clock once.
>
> >          ret = clk_set_rate(mipi_dsi->px_clk, mipi_dsi->mode->clock * 1000);
> >
> >          if (ret) {
> > @@ -103,6 +104,12 @@ static int dw_mipi_dsi_phy_init(void *priv_data)
> >                  return ret;
> >          }
> >
> > +       ret = clk_prepare_enable(mipi_dsi->px_clk);
> > +       if (ret) {
> > +               dev_err(mipi_dsi->dev, "Failed to enable DSI Pixel clock (ret %d)\n", ret);
> > +               return ret;
> If we ever hit this error case then there will be a lot of additional
> errors in the kernel log:
> - initially the clock is prepared and enabled in
> meson_dw_mipi_dsi_probe() by calling devm_clk_get_enabled()
> - we then disable the clock above (generally disabling a clock is
> expected to always succeed)
> - if the clock can NOT be re-enabled here we just log the error
> - in case a user tries to rmmod the driver (to modprobe it again) to
> try and recover from an error the automatic disabling of the pix_clk
> (based on devm_clk_get_enabled() where it was enabled initially) there
> will be a splat because the clock is already disabled (and enabled
> count is zero, so it cannot be disabled any further)
>
> For the 32-bit SoC video clocks I keep track of them being enabled or
> disabled, see [0], [1] and [2].
> In my case this is important because we can run into cases where the
> PLL doesn't lock (I am not sure how likely this is for your case).
>
> It *seems* like we need to do something similar as
> dw_mipi_dsi_phy_init() can be called when changing the display
> resolution (or whenever drm_bridge_funcs.atomic_pre_enable) is called.
> To illustrate what I have in mind I attached a diff (it's based on
> this patch) - it's compile tested only as I have no DSI hardware.
> In case dw_mipi_dsi_phy_init() is called only once per device
> lifecycle things may get easier.

Indeed your scheme looks good, I'll try it since indeed we only need
to prepare it once in the lifetime of the driver.

> PS: I'm so happy that we don't need any clock notifiers for this!
> So: good work with the clock driver bits.

Thx !

> Let me know what you think,
> Martin
>
>
> [0] https://github.com/xdarklight/linux/blob/meson-mx-integration-6.9-20240323/drivers/gpu/drm/meson/meson_vclk.c#L1177-L1179
> [1] https://github.com/xdarklight/linux/blob/meson-mx-integration-6.9-20240323/drivers/gpu/drm/meson/meson_vclk.c#L1077
> [2] https://github.com/xdarklight/linux/blob/meson-mx-integration-6.9-20240323/drivers/gpu/drm/meson/meson_vclk.c#L1053