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

[Adam Ford <aford173@gmail.com>]

On Wed, Jun 7, 2023 at 8:27 AM Adam Ford <aford173@gmail.com> 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.  I would expect the
> refresh to be driven by whatever the monitor states it can handle.
>
> Have you tried using modetest to see what refresh rates are available?
>  When I was doing this driver work, I would use modetest to determine
> the connector ID, then use modetest -s
> <connector-id>:<resolution>-<refresh> to display various resolutions
> and refresh rates.
>
> 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.  Modetest should also show
> you the desired pixel clock for a given resolution and refresh.
> My displays didn't show 19200x1200 as an option, so I wasn't able to
> test that configuration.

Another thing you could try would be this rounding patch that I'm
experimenting with [1].

From what I can see, some resolutions end up with math that end up
rounding down, and this patch corrects the timings a bit to attempt to
compensate.  I haven't tested this extensively yet, but you can try it
to see if it helps.

adam
[1] - https://github.com/aford173/linux/commit/183cf6d154afeb9b0300500b09d7b8ec53047a12


>
> adam
> >
> > Thanks,
> > Rasmus
> >