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

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Message-ID: <CAHCN7xKZnHGkErwpT-zXR6P-nMxBbg4OVNSr1An2vBaTpOsuqA@mail.gmail.com>
Date:   Wed, 19 Apr 2023 21:24:02 -0500
From:   Adam Ford <aford173@...il.com>
To:     Marek Vasut <marex@...x.de>
Cc:     dri-devel@...ts.freedesktop.org, m.szyprowski@...sung.com,
        aford@...conembedded.com, Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Shawn Guo <shawnguo@...nel.org>,
        Sascha Hauer <s.hauer@...gutronix.de>,
        Pengutronix Kernel Team <kernel@...gutronix.de>,
        Fabio Estevam <festevam@...il.com>,
        NXP Linux Team <linux-imx@....com>,
        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>,
        Frieder Schrempf <frieder.schrempf@...tron.de>,
        devicetree@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org, Lucas Stach <l.stach@...gutronix.de>
Subject: Re: [PATCH 5/6] drm: bridge: samsung-dsim: Support non-burst mode

On Mon, Apr 17, 2023 at 6:23 PM Marek Vasut <marex@...x.de> wrote:
>
> On 4/18/23 00:24, Adam Ford wrote:
> > On Mon, Apr 17, 2023 at 3:08 PM Marek Vasut <marex@...x.de> wrote:
> >>
> >> On 4/17/23 13:57, Adam Ford wrote:
> >>> On Sun, Apr 16, 2023 at 5:13 PM Marek Vasut <marex@...x.de> wrote:
> >>>>
> >>>> On 4/15/23 12:41, 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.
> >>>>
> >>>> The link rate negotiation should happen internally between the nearest
> >>>> bridge and DSIM, so please add that to DRM core instead of hacking
> >>>> around it by tweaking the HS clock again.
> >>>
> >>> I thought you tried to add something like this before and had some resistance.
> >>
> >> Yes, all my attempts were rejected by a single reviewer. I suspended my
> >> efforts in that area for now.
> >>
> >>> The Pixel clock is set by the bridge already without any new code
> >>> added to the DRM core..  I am just reading that value that's there,
> >>> and setting the clock accordingly.  I don't see how this is a hack.
> >>
> >> Assume you have a DSI-to-HDMI bridge attached to your DSIM bridge, it
> >> operates in non-burst mode, like ADV7533 . How would you configure the
> >
> > I have an ADV7535
> >
> >> HS clock rate for such a bridge in DT ? (hint: you cannot, because the
> >> required clock comes from the EDID, which may not be available just yet)
> >
> > The whole idea is that you wouldn't want to or need to configure the
> > clock speed in the device tree because it comes from the
> > EDID->bridge->DSI.
> >
> > I've tested this configuration on imx8mm, imx8mn, and imx8mp and I can
> > change the resolution and refresh rate on the fly and the DSI will
> > automatically readjust accordingly.   If you fixed the clock in the
> > device tree, you wouldn't be able to do that, and that was the point
> > of this patch.
>
> Uh, I retract my comment, I was clearly confused here and we're talking
> about the same thing.

I'm working on a V2 for this series.  Are you OK with this if I update
the commit message a bit to make it more clear?

adam