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

From: Doug Anderson <dianders@chromium.org>
To: Andrzej Hajda <a.hajda@samsung.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>,
	Thierry Reding <thierry.reding@gmail.com>,
	Rob Herring <robh+dt@kernel.org>, Sam Ravnborg <sam@ravnborg.org>,
	"open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
	<devicetree@vger.kernel.org>, Maxime Ripard <mripard@kernel.org>,
	Thomas Zimmermann <tzimmermann@suse.de>,
	Linus W <linus.walleij@linaro.org>,
	Daniel Vetter <daniel@ffwll.ch>,
	linux-arm-msm <linux-arm-msm@vger.kernel.org>,
	Steev Klimaszewski <steev@kali.org>,
	Maarten Lankhorst <maarten.lankhorst@linux.intel.com>,
	Bjorn Andersson <bjorn.andersson@linaro.org>,
	David Airlie <airlied@linux.ie>,
	dri-devel <dri-devel@lists.freedesktop.org>,
	LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v4 04/15] drm/edid: Use new encoded panel id style for quirks matching
Date: Tue, 14 Sep 2021 13:02:20 -0700	[thread overview]
Message-ID: <CAD=FV=XHvFq5+Rtax7WNq2-BieQr-BM4UnmOcma_eTzkX2ZtNA@mail.gmail.com> (raw)
In-Reply-To: <5af6cd50-0ed3-1a70-d7d0-66ca18a83d65@samsung.com>

Hi,

On Tue, Sep 14, 2021 at 12:36 PM Andrzej Hajda <a.hajda@samsung.com> wrote:
>
>
> W dniu 14.09.2021 o 20:59, Jani Nikula pisze:
> > On Tue, 14 Sep 2021, Doug Anderson <dianders@chromium.org> wrote:
> >> Hi,
> >>
> >> On Tue, Sep 14, 2021 at 11:16 AM Jani Nikula
> >> <jani.nikula@linux.intel.com> wrote:
> >>> On Thu, 09 Sep 2021, Douglas Anderson <dianders@chromium.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