linux-kernel - Re: [PATCH v4 04/15] drm/edid: Use new encoded panel id style for quirks matching

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Date:   Tue, 14 Sep 2021 13:02:20 -0700
From:   Doug Anderson <dianders@...omium.org>
To:     Andrzej Hajda <a.hajda@...sung.com>
Cc:     Jani Nikula <jani.nikula@...ux.intel.com>,
        Thierry Reding <thierry.reding@...il.com>,
        Rob Herring <robh+dt@...nel.org>,
        Sam Ravnborg <sam@...nborg.org>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>, Maxime Ripard <mripard@...nel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        Linus W <linus.walleij@...aro.org>,
        Daniel Vetter <daniel@...ll.ch>,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>,
        Steev Klimaszewski <steev@...i.org>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        David Airlie <airlied@...ux.ie>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4 04/15] drm/edid: Use new encoded panel id style for
 quirks matching

Hi,

On Tue, Sep 14, 2021 at 12:36 PM Andrzej Hajda <a.hajda@...sung.com> wrote:
>
>
> W dniu 14.09.2021 o 20:59, Jani Nikula pisze:
> > On Tue, 14 Sep 2021, Doug Anderson <dianders@...omium.org> wrote:
> >> Hi,
> >>
> >> On Tue, Sep 14, 2021 at 11:16 AM Jani Nikula
> >> <jani.nikula@...ux.intel.com> wrote:
> >>> On Thu, 09 Sep 2021, Douglas Anderson <dianders@...omium.org> wrote:
> >>>> In the patch ("drm/edid: Allow the querying/working with the panel ID
> >>>> from the EDID") we introduced a different way of working with the
> >>>> panel ID stored in the EDID. Let's use this new way for the quirks
> >>>> code.
> >>>>
> >>>> Advantages of the new style:
> >>>> * Smaller data structure size. Saves 4 bytes per panel.
> >>>> * Iterate through quirks structure with just "==" instead of strncmp()
> >>>> * In-kernel storage is more similar to what's stored in the EDID
> >>>>    itself making it easier to grok that they are referring to the same
> >>>>    value.
> >>>>
> >>>> The quirk table itself is arguably a bit less readable in the new
> >>>> style but not a ton less and it feels like the above advantages make
> >>>> up for it.
> >>> I suppose you could pass vendor as a string to EDID_QUIRK() to retain
> >>> better readability?
> >> I would love to, but I couldn't figure out how to do this and have it
> >> compile! Notably I need the compiler to be able to do math at compile
> >> time to compute the final u32 to store in the init data. I don't think
> >> the compiler can dereference strings (even constant strings) and do
> >> math on the result at compile time.
> > Ah, right.
>
>
> What about:
>
> +#define drm_edid_encode_panel_id(vend, product_id) \
> +       ((((u32)((vend)[0]) - '@') & 0x1f) << 26 | \
> +        (((u32)((vend)[1]) - '@') & 0x1f) << 21 | \
> +        (((u32)((vend)[2]) - '@') & 0x1f) << 16 | \
> +        ((product_id) & 0xffff))

Wow, I _swear_ I tried exactly that syntax, but clearly I didn't. It
works great and it looks _sooo_ much nicer now. Thanks! I'll send out
a v5 shortly with this.

-Doug