linux-kernel - Re: [PATCH V2 5/6] drm: bridge: samsung-dsim: Support non-burst mode

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Message-ID: <CAHCN7xKcHELG3n8+f4pjj05NCFUhBfz0_q_huN9bP65Fv6Ombg@mail.gmail.com>
Date:   Fri, 28 Apr 2023 09:09:44 -0500
From:   Adam Ford <aford173@...il.com>
To:     Marek Szyprowski <m.szyprowski@...sung.com>
Cc:     dri-devel@...ts.freedesktop.org, aford@...conembedded.com,
        dario.binacchi@...rulasolutions.com, l.stach@...gutronix.de,
        Inki Dae <inki.dae@...sung.com>,
        Jagan Teki <jagan@...rulasolutions.com>,
        Andrzej Hajda <andrzej.hajda@...el.com>,
        Neil Armstrong <neil.armstrong@...aro.org>,
        Robert Foss <rfoss@...nel.org>,
        Laurent Pinchart <Laurent.pinchart@...asonboard.com>,
        Jonas Karlman <jonas@...boo.se>,
        Jernej Skrabec <jernej.skrabec@...il.com>,
        David Airlie <airlied@...il.com>,
        Daniel Vetter <daniel@...ll.ch>, Marek Vasut <marex@...x.de>,
        Frieder Schrempf <frieder.schrempf@...tron.de>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH V2 5/6] drm: bridge: samsung-dsim: Support non-burst mode

On Fri, Apr 28, 2023 at 9:04 AM Marek Szyprowski
<m.szyprowski@...sung.com> wrote:
>
> On 28.04.2023 15:35, Adam Ford wrote:
> > On Fri, Apr 28, 2023 at 7:31 AM Marek Szyprowski
> > <m.szyprowski@...sung.com> wrote:
> >> On 24.04.2023 12:00, Adam Ford wrote:
> >>> On Mon, Apr 24, 2023 at 3:25 AM Marek Szyprowski
> >>> <m.szyprowski@...sung.com> wrote:
> >>>> On 23.04.2023 14:12, Adam Ford wrote:
> >>>>> The high-speed clock is hard-coded to the burst-clock
> >>>>> frequency specified in the device tree.  However, when
> >>>>> using devices like certain bridge chips without burst mode
> >>>>> and varying resolutions and refresh rates, it may be
> >>>>> necessary to set the high-speed clock dynamically based
> >>>>> on the desired pixel clock for the connected device.
> >>>>>
> >>>>> This also removes the need to set a clock speed from
> >>>>> the device tree for non-burst mode operation, since the
> >>>>> pixel clock rate is the rate requested from the attached
> >>>>> device like an HDMI bridge chip.  This should have no
> >>>>> impact for people using burst-mode and setting the burst
> >>>>> clock rate is still required for those users.
> >>>>>
> >>>>> Signed-off-by: Adam Ford <aford173@...il.com>
> >>>> This one breaks Exynos-5433 based TM2e board with a DSI panel.
> >>> Marek S,
> >>>
> >>> Thank you for testing!  I knoiw there are several of us who appreciate
> >>> your testing this since it's hard to know if something broke without
> >>> hardware.  Is there any way you can tell me if the flag is set to
> >>> enable MIPI_DSI_MODE_VIDEO_BURST?
> >> TM2e board uses the DSI panel operated in command mode and handled by
> >> panel-samsung-s6e3ha2.c driver. The MIPI_DSI_MODE_VIDEO_BURST flag is
> >> not set by the driver. However, the MIPI_DSI_CLOCK_NON_CONTINUOUS flags
> >> is set there. I really have no idea if setting VIDEO_BURST would make
> >> sense together with CLOCK_NON_CONTINUOUS or not. Maybe the driver lacks
> >> setting it?
> >>
> >>
> >>> I was trying to be diligent about not breaking your boards, but
> >>> without your boards, it's difficult.  The theory was that if
> >>> MIPI_DSI_MODE_VIDEO_BURST is set and there is a burst clock set in the
> >>> device tree, it would use the burst clock.
> >>>
> >>> As a fall-back I could just simply check for the presence of the
> >>> burst_clock_rate instead of both MIPI_DSI_MODE_VIDEO_BURST and
> >>> burst_clock_rate.
> >> Maybe you should extend your check also for the
> >> MIPI_DSI_CLOCK_NON_CONTINUOUS flag? Does it make sense?
> > Looking at some of the devices that might attach in the future, It
> > appears that ti-sn65dsi86.c sets this flag.  It's a display port
> > bridge, so I would expect it to need a variable clock rate similar to
> > how the HDMI bridge that I need works.  I am concerned that I make the
> > burst clock dependent on MIPI_DSI_CLOCK_NON_CONTINUOUS, it might break
> > the Display Port bridge.
> >
> > I think it's better to just check if the samsung,burst-clock-frequency
> > is present in the device tree and use it when present.  If it's not
> > present, then fall back to the pixel clock of the connected device.
>
> Right, this sounds rational.
>
> > I looked at a bunch of Exynos parts, and it looks like they all use
> > the samsung,burst-clock-frequency device tree setting.  Is that true,
> > or did I miss one?
>
> That true. All Exynos based boards with DSI panels use constant DSI
> burst frequency defined in the device tree.

Thanks for the feedback.  I'll try to get another rev of this series
pushed later today or Monday.

adam
>
>
> Best regards
> --
> Marek Szyprowski, PhD
> Samsung R&D Institute Poland
>