[PATCH v1 8/9] drm/bridge: tc358768: fix THS_TRAILCNT computation

[Robert Foss]

On Thu, Apr 27, 2023 at 4:34 PM Francesco Dolcini <francesco at dolcini.it> wrote:
>
> From: Francesco Dolcini <francesco.dolcini at toradex.com>
>
> Correct computation of THS_TRAILCNT register.
>
> This register must be set to a value that ensure that
> THS_TRAIL > 60 ns + 4 x UI
>  and
> THS_TRAIL > 8 x UI
>  and
> THS_TRAIL < TEOT
>  with
> TEOT = 105 ns + (12 x UI)
>
> with the actual value of THS_TRAIL being
>
> (1 + THS_TRAILCNT) x ByteClk cycle + ((1 to 2) + 2) xHSBYTECLK cycle +
>  - (PHY output delay)
>
> with PHY output delay being about
>
> (8 + (5 to 6)) x MIPIBitClk cycle in the BitClk conversion.
>
> Fixes: ff1ca6397b1d ("drm/bridge: Add tc358768 driver")
> Signed-off-by: Francesco Dolcini <francesco.dolcini at toradex.com>
> ---
>  drivers/gpu/drm/bridge/tc358768.c | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/gpu/drm/bridge/tc358768.c b/drivers/gpu/drm/bridge/tc358768.c
> index 854fc04f08d0..947c7dca567a 100644
> --- a/drivers/gpu/drm/bridge/tc358768.c
> +++ b/drivers/gpu/drm/bridge/tc358768.c
> @@ -779,9 +779,10 @@ static void tc358768_bridge_pre_enable(struct drm_bridge *bridge)
>         dev_dbg(priv->dev, "TCLK_POSTCNT: 0x%x\n", val);
>         tc358768_write(priv, TC358768_TCLK_POSTCNT, val);
>
> -       /* 60ns + 4*UI < THS_PREPARE < 105ns + 12*UI */
> -       val = tc358768_ns_to_cnt(60 + tc358768_to_ns(15 * ui_nsk),
> -                                dsibclk_nsk) - 5;
> +       /* max(60ns + 4*UI, 8*UI) < THS_TRAILCNT < 105ns + 12*UI */
> +       raw_val = tc358768_ns_to_cnt(60 + tc358768_to_ns(18 * ui_nsk),
> +                                    dsibclk_nsk) - 4;
> +       val = clamp(raw_val, 0, 15);
>         dev_dbg(priv->dev, "THS_TRAILCNT: 0x%x\n", val);
>         tc358768_write(priv, TC358768_THS_TRAILCNT, val);
>
> --
> 2.25.1
>

Reviewed-by: Robert Foss <rfoss at kernel.org>