Re: [PATCH v4 0/4] drm: sun4i: set proper TCON0 DCLK rate in DSI mode

[Frank Oltmanns <frank@oltmanns.dev>]

Hi Roman,

On 2023-05-05 at 07:21:06 +0200, Roman Beranek <me@crly.cz> wrote:
> According to Allwinner's BSP code, in DSI mode, TCON0 clock needs to be
> running at what's effectively the per-lane datarate of the DSI link.
> Given that the TCON DCLK divider is fixed to 4 (SUN6I_DSI_TCON_DIV),
> DCLK can't be set equal to the dotclock. Therefore labeling TCON DCLK
> as sun4i_dotclock or tcon-pixel-clock shall be avoided.
>
> With bpp bits per pixel transmitted over n DSI lanes, the target DCLK
> rate for a given pixel clock is obtained as follows:
>
> DCLK rate = 1/4 * bpp / n * pixel clock
>
> Effect of this change can be observed through the rate of Vblank IRQs
> which should now match refresh rate implied by set display mode. It
> was verified to do so on a A64 board with a 2-lane and a 4-lane panel.
>
> v2:
> 1. prevent reparent of tcon0 to pll-video0-2x
> 2. include pll-video0 in setting TCON0 DCLK rate
> 3. tested the whole thing also on a PinePhone
>
> v3:
> 1. accept that pll-video0 can't be included in setting TCON0 DCLK rate
> 2. reset pll-video0 to its default rate in case u-boot changed it
>
> v4:
> 1. keep pll-video0 as is
> 2. assign parent to TCON0 mux in sun50i_a64_ccu_probe (not in DT)
>
> Roman Beranek (4):
>   clk: sunxi-ng: a64: force select PLL_MIPI in TCON0 mux
>   ARM: dts: sunxi: rename tcon's clock output
>   drm: sun4i: rename sun4i_dotclock to sun4i_tcon_dclk
>   drm: sun4i: calculate proper DCLK rate for DSI
>
>  arch/arm/boot/dts/sun5i.dtsi                  |  2 +-
>  arch/arm/boot/dts/sun8i-a23-a33.dtsi          |  2 +-
>  arch/arm/boot/dts/sun8i-a83t.dtsi             |  2 +-
>  arch/arm/boot/dts/sun8i-v3s.dtsi              |  2 +-
>  arch/arm64/boot/dts/allwinner/sun50i-a64.dtsi |  2 +-
>  drivers/clk/sunxi-ng/ccu-sun50i-a64.c         | 14 +++++-
>  drivers/gpu/drm/sun4i/Makefile                |  2 +-
>  drivers/gpu/drm/sun4i/sun4i_tcon.c            | 46 +++++++++++--------
>  .../{sun4i_dotclock.c => sun4i_tcon_dclk.c}   |  2 +-
>  .../{sun4i_dotclock.h => sun4i_tcon_dclk.h}   |  0
>  10 files changed, 46 insertions(+), 28 deletions(-)
>  rename drivers/gpu/drm/sun4i/{sun4i_dotclock.c => sun4i_tcon_dclk.c} (99%)
>  rename drivers/gpu/drm/sun4i/{sun4i_dotclock.h => sun4i_tcon_dclk.h} (100%)
>
>
> base-commit: 8a91b29f1f50ce7742cdbe5cf11d17f128511f3f

I tested this on my pinephone on drm-next, using additional patches for
the pinephone's panel. [1] [2] [3]

I played back a 60 fps video (10 seconds) and recorded the panel's
output with a 240 fps camera. I noticed only 2 dropped frames, that I
account to the imperfect data rate of 107.8MHz instead of 108 MHz due to
pll-video0's rate being 294MHz instead of the 297 MHz for reasons I
described in the thread on your v2 of this patch [4]).

Tested-by: Frank Oltmanns <frank@oltmanns.dev>

Thanks,
  Frank

[1]: https://lore.kernel.org/all/20230213123238.76889-2-frank@oltmanns.dev/
[2]: https://lore.kernel.org/all/20230211171748.36692-2-frank@oltmanns.dev/
[3]: https://github.com/megous/linux/commit/e83ffbfe930562257abef1eed4abb8e2251b795a
[4]: https://lore.kernel.org/all/87cz3uzpx1.fsf@oltmanns.dev/