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

Frank Oltmanns
2023-12-22T09:11:13Z

On 2023-12-20 at 19:57:06 +0100, Frank Oltmanns <frank@oltmanns.dev> wrote:
> Ok, I've done more detailed testing, and it seems this patch results in
> lots of dropped frames. I'm sorry for not being more thorough earlier.
> I'll do some more testing without this patch and might have to either
> remove it from V2 of this series.
>
> I need to see if the same stability can be achieved when running
> PLL-MIPI outside its specied range.

I've done some more (load) testing and observing the panel for dropped
frames.

The conclusion I draw from those results is that this patch isn't
necessary for the pinephone. It would be enough to use the correct clock
rate based on the existing values [*]:
- .clock = 69000,
+ .clock = (720 + 40 + 40 + 40) * (1440 + 18 + 10 + 17) * 60 / 1000,

I've asked in the postmarketOS community for a bit more testing. They
already have a merge request that contains these changes [2].

This means that we would continue to drive PLL-MIPI outside it's
specified range. I have, so far, not experienced any downside of doing
so. It seems enough to fix the ratios that are part of the first four
patches in this series without introducing a min and max rate.

In conclusion, I'll soon (after some more feedback from the fine folks
at postmarketOS) submit a V2 that addresses the fixes requested in the
first four patches of this series. I'll drop the existing PATCH 5 and
replace it with the one I sent in February [1] instead.

After that, just for fun, I'll probably look into min_rate and max_rate
for nkm clocks and which consequences it has on the pinephone. I might
or might not send a follow up series for that. However, if the pinephone
runs stable without it, it's not a high priority for me.

Best regards,
Frank

[*] I've already submitted a patch in February '23 [1]. It was of little
use back then because the A64's PLL-MIPI clock was not able to run
close to that rate. But since kernel 6.6 PLL-MIPI is able to set
it's parent rate, so that it can come quite close to the required
rate:
+ Panel requires 74.844 MHz with the current timings.
+-> tcon-data-clock rate should be 112.266 MHz (panel*24/4/4).
+-> PLL-MIPI rate should be 449.064 MHz (TCON0 * 4)

The 6.6 kernel the following rates are possible:
+ PLL-MIPI: ~448.984615 MHz
+-> tcon-data-clock: ~112.246153
+-> panel: ~74.830768 MHz

Which leaves us with a vertical refresh rate of ~59.989 Hz,
deviating less then 0.2% from the ideal 60Hz. That's probably closer
than the accumulated accuracy of all involved components can
reliably achieve. I'd say, let's leave it at that.

[1]: https://lore.kernel.org/lkml/20230219114553.288057-2-frank@oltmanns.dev/
[2]: https://gitlab.com/postmarketOS/pmaports/-/merge_requests/4645
>
> Best regards,
> Frank
>
> On 2023-12-20 at 16:18:49 +0100, Jernej Škrabec <jernej.skrabec@gmail.com> wrote:
>> 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,
>>> >>
>>> >>
>>>