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

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Date:   Tue, 18 Apr 2023 10:30:59 +0200
From:   Marek Vasut <marex@...x.de>
To:     Adam Ford <aford173@...il.com>
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>, devicetree@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/6] drm: bridge: samsung-dsim: Fetch pll-clock-frequency
 automatically

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 ?