On Tue, Nov 26, 2024 at 04:45:54PM +0100, Nikolaus Voss wrote:
> LDB clock has to be a fixed multiple of the pixel clock.
> As LDB and pixel clock are derived from different clock sources
> (at least on imx8mp), this constraint cannot be satisfied for
> any pixel clock, which leads to flickering and incomplete
> lines on the attached display.
>
> To overcome this, check this condition in mode_fixup() and
> adapt the pixel clock accordingly.
>
> Cc: <stable@xxxxxxxxxxxxxxx>
>
> Signed-off-by: Nikolaus Voss <nv@xxxxxxx>
> ---
> drivers/gpu/drm/bridge/fsl-ldb.c | 40 ++++++++++++++++++++++++++++----
> 1 file changed, 36 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/gpu/drm/bridge/fsl-ldb.c b/drivers/gpu/drm/bridge/fsl-ldb.c
> index 0e4bac7dd04ff..e341341b8c600 100644
> --- a/drivers/gpu/drm/bridge/fsl-ldb.c
> +++ b/drivers/gpu/drm/bridge/fsl-ldb.c
> @@ -104,12 +104,14 @@ static inline struct fsl_ldb *to_fsl_ldb(struct drm_bridge *bridge)
> return container_of(bridge, struct fsl_ldb, bridge);
> }
>
> +static unsigned int fsl_ldb_link_freq_factor(const struct fsl_ldb *fsl_ldb)
> +{
> + return fsl_ldb_is_dual(fsl_ldb) ? 3500 : 7000;
> +}
> +
> static unsigned long fsl_ldb_link_frequency(struct fsl_ldb *fsl_ldb, int clock)
> {
> - if (fsl_ldb_is_dual(fsl_ldb))
> - return clock * 3500;
> - else
> - return clock * 7000;
> + return clock * fsl_ldb_link_freq_factor(fsl_ldb);
> }
>
> static int fsl_ldb_attach(struct drm_bridge *bridge,
> @@ -121,6 +123,35 @@ static int fsl_ldb_attach(struct drm_bridge *bridge,
> bridge, flags);
> }
>
> +static bool fsl_ldb_mode_fixup(struct drm_bridge *bridge,
> + const struct drm_display_mode *mode,
> + struct drm_display_mode *adjusted_mode)
The driver uses atomic callbacks. Please use .atomic_check() instead.
> +{
> + const struct fsl_ldb *fsl_ldb = to_fsl_ldb(bridge);
> + unsigned long requested_link_freq =
> + mode->clock * fsl_ldb_link_freq_factor(fsl_ldb);
> + unsigned long freq = clk_round_rate(fsl_ldb->clk, requested_link_freq);
> +
> + if (freq != requested_link_freq) {
> + /*
> + * this will lead to flicker and incomplete lines on
> + * the attached display, adjust the CRTC clock
> + * accordingly.
> + */
> + int pclk = freq / fsl_ldb_link_freq_factor(fsl_ldb);
> +
> + if (adjusted_mode->clock != pclk) {
> + dev_warn(fsl_ldb->dev, "Adjusted pixel clk to match LDB clk (%d kHz -> %d kHz)!\n",
> + adjusted_mode->clock, pclk);
> +
> + adjusted_mode->clock = pclk;
> + adjusted_mode->crtc_clock = pclk;
> + }
> + }
> +
> + return true;
> +}
> +
> static void fsl_ldb_atomic_enable(struct drm_bridge *bridge,
> struct drm_bridge_state *old_bridge_state)
> {
> @@ -280,6 +311,7 @@ fsl_ldb_mode_valid(struct drm_bridge *bridge,
>
> static const struct drm_bridge_funcs funcs = {
> .attach = fsl_ldb_attach,
> + .mode_fixup = fsl_ldb_mode_fixup,
> .atomic_enable = fsl_ldb_atomic_enable,
> .atomic_disable = fsl_ldb_atomic_disable,
> .atomic_duplicate_state = drm_atomic_helper_bridge_duplicate_state,
> --
> 2.43.0
>
--
With best wishes
Dmitry
