Re: [PATCH V8 0/7] drm: bridge: samsung-dsim: Support variable clocking

From: Adam Ford <aford173@gmail.com>
To: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
Cc: dri-devel@lists.freedesktop.org, aford@beaconembedded.com,
	Inki Dae <inki.dae@samsung.com>,
	Jagan Teki <jagan@amarulasolutions.com>,
	Marek Szyprowski <m.szyprowski@samsung.com>,
	Andrzej Hajda <andrzej.hajda@intel.com>,
	Neil Armstrong <neil.armstrong@linaro.org>,
	Robert Foss <rfoss@kernel.org>,
	Laurent Pinchart <Laurent.pinchart@ideasonboard.com>,
	Jonas Karlman <jonas@kwiboo.se>,
	Jernej Skrabec <jernej.skrabec@gmail.com>,
	David Airlie <airlied@gmail.com>, Daniel Vetter <daniel@ffwll.ch>,
	Rob Herring <robh+dt@kernel.org>,
	Krzysztof Kozlowski <krzysztof.kozlowski+dt@linaro.org>,
	Conor Dooley <conor+dt@kernel.org>, Marek Vasut <marex@denx.de>,
	Frieder Schrempf <frieder.schrempf@kontron.de>,
	devicetree@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH V8 0/7] drm: bridge: samsung-dsim: Support variable clocking
Date: Thu, 8 Jun 2023 07:30:56 -0500	[thread overview]
Message-ID: <CAHCN7xKdKGA02=ZDNQkVVVDV0AZTqd7QpHA2Nq9LNnbmK=hKxA@mail.gmail.com> (raw)
In-Reply-To: <bfd050f2-b39e-c091-614e-0c77fe324435@prevas.dk>

On Thu, Jun 8, 2023 at 6:40 AM Rasmus Villemoes
<rasmus.villemoes@prevas.dk> wrote:
>
> On 07/06/2023 15.27, Adam Ford wrote:
> > On Wed, Jun 7, 2023 at 8:15 AM Rasmus Villemoes
> > <rasmus.villemoes@prevas.dk> wrote:
> >>
> >> On 26/05/2023 05.05, Adam Ford wrote:
> >>> This series fixes the blanking pack size and the PMS calculation.  It then
> >>> adds support to allows the DSIM to dynamically DPHY clocks, and support
> >>> non-burst mode while allowing the removal of the hard-coded clock values
> >>> for the PLL for imx8m mini/nano/plus, and it allows the removal of the
> >>> burst-clock device tree entry when burst-mode isn't supported by connected
> >>> devices like an HDMI brige.  In that event, the HS clock is set to the
> >>> value requested by the bridge chip.
> >>>
> >>> This has been tested on both an i.MX8M Nano and i.MX8M Plus, and should
> >>> work on i.MX8M Mini as well. Marek Szyprowski has tested it on various
> >>> Exynos boards.
> >>
> >> Hi all
> >>
> >> We're testing this on top of v6.4-rc4 on our imx8mp board, which has a
> >> ti-sn65dsi86 DSI -> DisplayPort bridge. We do get an image at
> >> 1920x1200, but the monitor says it's only at 58Hz, and measuring on the
> >> DSI signals does seem to confirm that the update frequency is about 57.7
> >> or 57.8Hz (it's pretty hard to get a good measurement). It looks like
> >> it's the lines that are too long, by a time that corresponds to about 80
> >> pixels. But all the frontporch/backporch/hsync values look sane and
> >> completely standard for that resolution.
> >>
> >> Setting samsung,burst-clock-frequency explicitly to something large
> >> enough or letting it be derived from the 154MHz pixel clock makes no
> >> difference.
> >>
> >> Any ideas?
> >
> > What refresh rate are you trying to achieve?  It seems like 57.7 or
> > 57.8 is really close to the 58 the Monitor states.
>
> Oh, sorry, I thought that was clear, but it should be/we're aiming
> for/expecting 60Hz, or (154MHz / (2080 * 1235)) which is about 59.95Hz.
> We've tried with a variety of monitors that all have 1920x1200@60Hz as
> max resolution, and parse-edid always gives the same hfp/hbp/...
> numbers, namely
>
>        Modeline        "Mode 0" 154.00 1920 1968 2000 2080 1200 1203
> 1209 1235 +hsync -vsync
>
> > I would expect the
> > refresh to be driven by whatever the monitor states it can handle.
>
> Well, it states that it can handle 60Hz, and the pixel clock is also
> computed to be the 154MHz, but still, the actual signals on the wire,
> and hence also what the monitor ends up reporting, do not end up with 60
> full frames per second.
>
> > Have you tried using modetest to see what refresh rates are available?
>
> Hm. My userspace may be a little weird. When I run modetest I just get
>
> trying to open device 'i915'...failed
> trying to open device 'amdgpu'...failed
> ...
> trying to open device 'imx-dcss'...failed
> trying to open device 'mxsfb-drm'...failed
> no device found
>

One the 8MP, I think you need to append "-M imx-lcdif" to the modetest
command  to specify the driver being used.
I don't have my 8MP with me right now, but I think that's the right name.

> > The 8MP shares the video-pll clock with both disp1 and disp2 clocks,
> > and the imx-lcdif driver, which sends the display signals to the DSI,
> > uses the disp clock, so the video-pll needs to be an exact multiple of
> > the pixel clock or the output won't sink.
>
> Bingo! I enabled the
>
>   DRM_DEV_DEBUG_DRIVER(drm->dev, "Pixel clock: %dkHz (actual: %dkHz)\n",
>
> in drivers/gpu/drm/mxsfb/lcdif_kms.c, and indeed it got me
>
>   Pixel clock: 154000kHz (actual: 148500kHz)
>
> Modifying the 1039500000 in imx8mp.dtsi to 1078000000 (i.e. 7 times the
> desired pixel clock) gave me "actual" matching the desired pixel clock,
> and the monitor now reports 60Hz.

I am glad that worked!

>
> This product also has an LVDS display on lcdif2, so I'll have to
> investigate how changing the video_pll1 rate affects that. And also what
> to do about the case where somebody plugs in, say, a 1080p monitor that
> would indeed require 148.5MHz pixel clock.

That's the down-side to the 8MP with the shared clock.  According to
the processor reference manual, It looks like the MEDIA_LDB_CLK can be
a child of Audio_PLL2.  i don't know if you need both AUDIO_PLL1 and
Audio_PLL2, but the Audio_PLL2 clock is fairly flexible, so if you can
use Audio_pll1 for all your audio needs, and configure the audio_pll2
for your LVDS, you might be able to get both LDB and DSI to sync at
the nominal values.

adam
>
> Thanks,
> Rasmus
>