Re: [PATCH 5/5] drm/panel: st7703: Drive XBD599 panel at higher clock rate

["Jernej Škrabec" <jernej.skrabec@gmail.com>]

Dne sreda, 20. december 2023 ob 08:14:27 CET je Frank Oltmanns napisal(a):
> 
> On 2023-12-19 at 18:04:29 +0100, Jernej Škrabec <jernej.skrabec@gmail.com> wrote:
> > Dne ponedeljek, 18. december 2023 ob 14:35:23 CET je Frank Oltmanns napisal(a):
> >> This panel is used in the pinephone that runs on a Allwinner A64 SOC.
> >> Acoording to it's datasheet, the SOC requires PLL-MIPI to run at more
> >> than 500 MHz.
> >>
> >> Therefore, change [hv]sync_(start|end) so that we reach a clock rate
> >> that is high enough to drive PLL-MIPI within its limits.
> >>
> >> Signed-off-by: Frank Oltmanns <frank@oltmanns.dev>
> >
> > I'm not too sure about this patch. I see that PLL_MIPI doesn't have set
> > minimum frequency limit in clock driver. If you add it, clock framework
> > should find rate that is high enough and divisible with target rate.
> 
> This one is really a tough nut. Unfortunately, the PLL_MIPI clock for
> this panel has to run exactly at 6 * panel clock. Let me start by
> showing the relevant part of the clock tree (this is on the pinephone
> after applying the patches):
>     pll-video0                 393600000
>        pll-mipi                500945454
>           tcon0                500945454
>              tcon-data-clock   125236363
> 
> To elaborate, tcon-data-clock has to run at 1/4 the DSI per-lane bit
> rate [1]. It's a fixed divisor
> 
> The panel I'm proposing to change is defined as this:
> 
>     static const struct st7703_panel_desc xbd599_desc = {
>     	.mode = &xbd599_mode,
>     	.lanes = 4,
>     	.mode_flags = MIPI_DSI_MODE_VIDEO | MIPI_DSI_MODE_VIDEO_SYNC_PULSE,
>     	.format = MIPI_DSI_FMT_RGB888,
>     	.init_sequence = xbd599_init_sequence,
>     };
> 
> So, we have 24 bpp and 4 lanes. Therefore, the resulting requested
> tcon-data-clock rate is
>     crtc_clock * 1000 * (24 / 4) / 4
> 
> tcon-data-clock therefore requests a parent rate of
>     4 * (crtc_clock * 1000 * (24 / 4) / 4)
> 
> The initial 4 is the fixed divisor between tcon0 and tcon-data-clock.
> Since tcon0 is a ccu_mux, the rate of tcon0 equals the rate of pll-mipi.
> 
> Since PLL-MIPI has to run at at least at 500MHz this forces us to have a
> crtc_clock >= 83.333 MHz. The mode I'm prorposing results in a rate of
> 83.502 MHz.

This is much better explanation why this change is needed. Still, I think
adding min and max rate to PLL_MIPI would make sense, so proper rates
are guaranteed.

Anyway, do you know where are all those old values come from? And how did
you come up with new ones? I guess you can't just simply change timings,
there are probably some HW limitations? Do you know if BSP kernel support
this panel and how this situation is solved there?

> 
> If we only changed the constraints on the PLL_MIPI without changing the
> panel mode, we end up with a mismatch. This, in turn, would result in
> dropped frames, right?

From what I read, I think frame rate would be higher than 60 fps. What
exactly would happen depends on the panel.

Best regards,
Jernej

> 
> Best regards,
>   Frank
> 
> [1] Source:
> https://elixir.bootlin.com/linux/v6.6.7/source/drivers/gpu/drm/sun4i/sun4i_tcon.c#L346
> 
> >
> > Best regards,
> > Jernej
> >
> >> ---
> >>  drivers/gpu/drm/panel/panel-sitronix-st7703.c | 14 +++++++-------
> >>  1 file changed, 7 insertions(+), 7 deletions(-)
> >>
> >> diff --git a/drivers/gpu/drm/panel/panel-sitronix-st7703.c b/drivers/gpu/drm/panel/panel-sitronix-st7703.c
> >> index b55bafd1a8be..6886fd7f765e 100644
> >> --- a/drivers/gpu/drm/panel/panel-sitronix-st7703.c
> >> +++ b/drivers/gpu/drm/panel/panel-sitronix-st7703.c
> >> @@ -320,14 +320,14 @@ static int xbd599_init_sequence(struct st7703 *ctx)
> >>
> >>  static const struct drm_display_mode xbd599_mode = {
> >>  	.hdisplay    = 720,
> >> -	.hsync_start = 720 + 40,
> >> -	.hsync_end   = 720 + 40 + 40,
> >> -	.htotal	     = 720 + 40 + 40 + 40,
> >> +	.hsync_start = 720 + 65,
> >> +	.hsync_end   = 720 + 65 + 65,
> >> +	.htotal      = 720 + 65 + 65 + 65,
> >>  	.vdisplay    = 1440,
> >> -	.vsync_start = 1440 + 18,
> >> -	.vsync_end   = 1440 + 18 + 10,
> >> -	.vtotal	     = 1440 + 18 + 10 + 17,
> >> -	.clock	     = 69000,
> >> +	.vsync_start = 1440 + 30,
> >> +	.vsync_end   = 1440 + 30 + 22,
> >> +	.vtotal	     = 1440 + 30 + 22 + 29,
> >> +	.clock	     = (720 + 65 + 65 + 65) * (1440 + 30 + 22 + 29) * 60 / 1000,
> >>  	.flags	     = DRM_MODE_FLAG_NHSYNC | DRM_MODE_FLAG_NVSYNC,
> >>  	.width_mm    = 68,
> >>  	.height_mm   = 136,
> >>
> >>
>