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

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Message-ID: <5af6cd50-0ed3-1a70-d7d0-66ca18a83d65@samsung.com>
Date:   Tue, 14 Sep 2021 21:36:01 +0200
From:   Andrzej Hajda <a.hajda@...sung.com>
To:     Jani Nikula <jani.nikula@...ux.intel.com>,
        Doug Anderson <dianders@...omium.org>
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


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))


Regards

Andrzej


>
>> 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...
> Nah.
>
>>
>>> 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.
> I have now reviewed the ones where my review is relevant, and certainly
> don't expect me to comment on the rest. ;)
>
> BR,
> Jani.
>
>>
>> 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