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

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Message-ID: <CAD=FV=XgLcOBOOp9kgShE4+T8g8UZcO_Ff3hhAbGTyLkdE7HNA@mail.gmail.com>
Date:   Tue, 14 Sep 2021 11:31:11 -0700
From:   Doug Anderson <dianders@...omium.org>
To:     Jani Nikula <jani.nikula@...ux.intel.com>
Cc:     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 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.

I _think_ you could make it work with four-character codes (only
specifying 3 characters), AKA single-quoting something like 'AUO' but
I think four-character codes are not dealt with in a standard enough
way between compilers so they're not allowed in Linux.

If you like it better, I could do something like this:

#define ACR_CODE 'A', 'C', 'R'
#define AUO_CODE 'A', 'U', 'O'
...
...

...then I could refer to the #defines...

> Just bikeshedding, really. ;)

I'll take this comment (without any formal tags) as:

* You've seen this patch (and the previous ones) and wouldn't object
to it merging.

* You're not planning on any deeper review / testing, so I shouldn't
wait for more stuff from you before merging. Please yell if this is
not the case. I'm happy to wait but I don't want to wait if no further
review is planned.

In general I'm still planning to give this series at least another
week for comments before merging. ${SUBJECT} patch also is the only
one lacking any type of Review / Ack tags so I'll see if I can find
someone to give it something before merging, too.

-Doug