linux-kernel - Re: [PATCH 3/6] drm: bridge: samsung-dsim: Fetch pll-clock-frequency automatically

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Date:   Wed, 19 Apr 2023 05:41:49 -0500
From:   Adam Ford <aford173@...il.com>
To:     Lucas Stach <l.stach@...gutronix.de>
Cc:     Marek Vasut <marex@...x.de>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        aford@...conembedded.com, dri-devel@...ts.freedesktop.org,
        Laurent Pinchart <Laurent.pinchart@...asonboard.com>,
        Andrzej Hajda <andrzej.hajda@...el.com>,
        m.szyprowski@...sung.com, Robert Foss <rfoss@...nel.org>,
        Jernej Skrabec <jernej.skrabec@...il.com>,
        Jagan Teki <jagan@...rulasolutions.com>,
        NXP Linux Team <linux-imx@....com>,
        devicetree@...r.kernel.org, Jonas Karlman <jonas@...boo.se>,
        Sascha Hauer <s.hauer@...gutronix.de>,
        Rob Herring <robh+dt@...nel.org>,
        linux-arm-kernel@...ts.infradead.org,
        Neil Armstrong <neil.armstrong@...aro.org>,
        linux-kernel@...r.kernel.org,
        Pengutronix Kernel Team <kernel@...gutronix.de>,
        Shawn Guo <shawnguo@...nel.org>
Subject: Re: [PATCH 3/6] drm: bridge: samsung-dsim: Fetch pll-clock-frequency automatically

On Tue, Apr 18, 2023 at 3:47 AM Lucas Stach <l.stach@...gutronix.de> wrote:
>
> Am Dienstag, dem 18.04.2023 um 10:30 +0200 schrieb Marek Vasut:
> > On 4/18/23 04:29, Adam Ford wrote:
> > > On Sun, Apr 16, 2023 at 5:08 PM Marek Vasut <marex@...x.de> wrote:
> > > >
> > > > On 4/15/23 12:41, Adam Ford wrote:
> > > > > Fetch the clock rate of "sclk_mipi" (or "pll_clk") instead of
> > > > > having an entry in the device tree for samsung,pll-clock-frequency.
> > > > >
> > > > > Signed-off-by: Adam Ford <aford173@...il.com>
> > > > > ---
> > > > >    drivers/gpu/drm/bridge/samsung-dsim.c | 12 ++++++------
> > > > >    1 file changed, 6 insertions(+), 6 deletions(-)
> > > > >
> > > > > diff --git a/drivers/gpu/drm/bridge/samsung-dsim.c b/drivers/gpu/drm/bridge/samsung-dsim.c
> > > > > index 9fec32b44e05..73f0c3fbbdf5 100644
> > > > > --- a/drivers/gpu/drm/bridge/samsung-dsim.c
> > > > > +++ b/drivers/gpu/drm/bridge/samsung-dsim.c
> > > > > @@ -1744,11 +1744,6 @@ static int samsung_dsim_parse_dt(struct samsung_dsim *dsi)
> > > > >        struct device_node *node = dev->of_node;
> > > > >        int ret;
> > > > >
> > > > > -     ret = samsung_dsim_of_read_u32(node, "samsung,pll-clock-frequency",
> > > > > -                                    &dsi->pll_clk_rate);
> > > > > -     if (ret < 0)
> > > > > -             return ret;
> > > > > -
> > > > >        ret = samsung_dsim_of_read_u32(node, "samsung,burst-clock-frequency",
> > > > >                                       &dsi->burst_clk_rate);
> > > > >        if (ret < 0)
> > > >
> > > > Does this break compatibility with old samsung DTs ?
> > >
> > > My goal here was to declutter the device tree stuff and fetch data
> > > automatically if possible. What if I changed this to make them
> > > optional?  If they exist, we can use them, if they don't exist, we
> > > could read the clock rate.  Would that be acceptable?
> >
> > If you do not see any potential problem with ignoring the DT property
> > altogether, that would be better of course, but I think you cannot do
> > that with old DTs, so you should retain backward compatibility fallback,
> > yes. What do you think ?
>
> I'm very much in favor of this patch, but I also think we shouldn't
> risk breaking Samsung devices, where we don't now 100% that the input
> clock rate provided by the clock driver is correct.
>
> So I think the right approach is to use "samsung,pll-clock-frequency"
> when present in DT and get it from the clock provider otherwise. Then
> just remove the property from the DTs where we can validate that the
> input clock rate is correct, i.e. all i.MX8M*.

I'll update this accordingly when I do a V2 of this series.

adam
>
> Regards,
> Lucas