Re: [PATCH 09/18] drm/sun4i: tcon: Adjust dotclock dividers range

Chen-Yu Tsai
2017-07-14T04:14:37Z

On Thu, Jul 13, 2017 at 10:13 PM, Maxime Ripard
<maxime.ripard-wi1+55ScJUtKEb57/3fJTNBPR1lH4CV8@public.gmane.org> wrote:
> It seems like the dotclock dividers are a bit less strict range, and can
> operate even with a smaller than 6 divider. Loose the boundaries a bit.
>
> Signed-off-by: Maxime Ripard <maxime.ripard-wi1+55ScJUtKEb57/3fJTNBPR1lH4CV8@public.gmane.org>
> ---
> drivers/gpu/drm/sun4i/sun4i_dotclock.c | 20 +++++++++++++++++++-
> 1 file changed, 19 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/gpu/drm/sun4i/sun4i_dotclock.c b/drivers/gpu/drm/sun4i/sun4i_dotclock.c
> index d401156490f3..0b844c0dd102 100644
> --- a/drivers/gpu/drm/sun4i/sun4i_dotclock.c
> +++ b/drivers/gpu/drm/sun4i/sun4i_dotclock.c
> @@ -77,7 +77,25 @@ static long sun4i_dclk_round_rate(struct clk_hw *hw, unsigned long rate,
> u8 best_div = 1;
> int i;
>
> - for (i = 6; i <= 127; i++) {
> + /*
> + * There's something odd here.
> + *
> + * In the A13 user manual, this is stated to be >= 6 when
> + * dclk1 and dclk2 are used (without any hint on how to use
> + * them), and >= 4 when only dclk is used.

You set it in TCON0_IO_POL_REG, which sets the clock phase delay.
I think we were setting this before, but you removed it as part of
the previous TCON clean up patches?

In the A33, there are even more options, like DCLK / 2 (with 0 or 90
degree phase delay).

> + *
> + * In the A33 user manual, when only dclk is used, it is set
> + * to be >= 6 in the former case, and >= 1 in the
> + * latter. There's also some (obscure) explanations about the
> + * dclk1 and dclk2 vs dclk that seems to be in the upper 4
> + * bits. What those clocks are and what bit does what is not
> + * really clear.

Looks like mux bits to me. How they differ from TCON0_IO_POL_REG is
beyond me ATM.

ChenYu

> + *
> + * On the A33 however, while something lower than 4 works, it
> + * does have a few artifacts. Let's not use those values, and
> + * see how it goes.
> + */
> + for (i = 4; i <= 127; i++) {
> unsigned long ideal = rate * i;
> unsigned long rounded;
>
> --
> git-series 0.9.1
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