[PATCH v1 4/9] drm/bridge: tc358768: fix TCLK_ZEROCNT computation

[Robert Foss]

On Thu, Apr 27, 2023 at 4:35 PM Francesco Dolcini <francesco at dolcini.it> wrote:
>
> From: Francesco Dolcini <francesco.dolcini at toradex.com>
>
> Correct computation of TCLK_ZEROCNT register.
>
> This register must be set to a value that ensure that
> (TCLK-PREPARECNT + TCLK-ZERO) > 300ns
>
> with the actual value of (TCLK-PREPARECNT + TCLK-ZERO) being
>
> (1 to 2) + (TCLK_ZEROCNT + 1)) x HSByteClkCycle + (PHY output delay)
>
> with PHY output delay being about
>
> (2 to 3) 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 | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/gpu/drm/bridge/tc358768.c b/drivers/gpu/drm/bridge/tc358768.c
> index dba1bf3912f1..aff400c36066 100644
> --- a/drivers/gpu/drm/bridge/tc358768.c
> +++ b/drivers/gpu/drm/bridge/tc358768.c
> @@ -742,10 +742,10 @@ static void tc358768_bridge_pre_enable(struct drm_bridge *bridge)
>
>         /* 38ns < TCLK_PREPARE < 95ns */
>         val = tc358768_ns_to_cnt(65, dsibclk_nsk) - 1;
> -       /* TCLK_PREPARE > 300ns */
> -       val2 = tc358768_ns_to_cnt(300 + tc358768_to_ns(3 * ui_nsk),
> -                                 dsibclk_nsk);
> -       val |= (val2 - tc358768_to_ns(phy_delay_nsk - dsibclk_nsk)) << 8;
> +       /* TCLK_PREPARE + TCLK_ZERO > 300ns */
> +       val2 = tc358768_ns_to_cnt(300 - tc358768_to_ns(2 * ui_nsk),
> +                                 dsibclk_nsk) - 2;
> +       val |= val2 << 8;
>         dev_dbg(priv->dev, "TCLK_HEADERCNT: 0x%x\n", val);
>         tc358768_write(priv, TC358768_TCLK_HEADERCNT, val);
>
> --
> 2.25.1
>

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