linux-kernel - Re: [PATCH 2/2] drm/panel: panel-edp: Match with panel hash for overridden modes

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Date: Mon, 26 Feb 2024 14:29:17 -0800
From: Doug Anderson <dianders@...omium.org>
To: Hsin-Yi Wang <hsinyi@...omium.org>
Cc: Neil Armstrong <neil.armstrong@...aro.org>, Jessica Zhang <quic_jesszhan@...cinc.com>, 
	Sam Ravnborg <sam@...nborg.org>, Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>, 
	Maxime Ripard <mripard@...nel.org>, Thomas Zimmermann <tzimmermann@...e.de>, 
	David Airlie <airlied@...il.com>, Daniel Vetter <daniel@...ll.ch>, dri-devel@...ts.freedesktop.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] drm/panel: panel-edp: Match with panel hash for
 overridden modes

Hi,

On Fri, Feb 23, 2024 at 2:40 PM Hsin-Yi Wang <hsinyi@...omium.org> wrote:
>
> It's found that some panels have variants that they share the same panel id
> although their EDID and names are different. One of the variants requires
> using overridden modes to resolve glitching issue as described in commit
> 70e0d5550f5c ("drm/panel-edp: Add auo_b116xa3_mode"). Other variants should
> use the modes parsed from EDID.
>
> For example, AUO 0x405c B116XAK01.0, it has at least 2 different variants
> that EDID and panel name are different, but using the same panel id. One of
> the variants require using overridden mode. Same case for AUO 0x615c
> B116XAN06.1.
>
> Add such entries and use the hash of the EDID to match the panel needs the
> overridden modes.

As pointed out in an offline discussion, it's possible that we might
want to "ignore" some of these bytes for the purpose of the CRC.
Specifically, we might want to ignore:
* byte 16 - Week of manufacture
* byte 17 - Year of manufacture
* byte 127 - Checksum

That way if a manufacturer actually is updating those numbers in
production we can still have one hash that captures all the panels. I
have no idea if manufacturers actually are, but IMO the hash of the
rest of the base block should be sufficient to differentiate between
different panels anyway. It would be easy to just zero out those 3
bytes before computing the CRC.

What do you think?


> @@ -758,13 +762,13 @@ static void panel_edp_parse_panel_timing_node(struct device *dev,
>                 dev_err(dev, "Reject override mode: No display_timing found\n");
>  }
>
> -static const struct edp_panel_entry *find_edp_panel(u32 panel_id);
> +static const struct edp_panel_entry *find_edp_panel(u32 panel_id, u32 panel_hash);
>
>  static int generic_edp_panel_probe(struct device *dev, struct panel_edp *panel)
>  {
>         struct panel_desc *desc;
>         void *base_block;
> -       u32 panel_id;
> +       u32 panel_id, panel_hash;
>         char vend[4];
>         u16 product_id;
>         u32 reliable_ms = 0;
> @@ -796,15 +800,17 @@ static int generic_edp_panel_probe(struct device *dev, struct panel_edp *panel)
>         base_block = drm_edid_get_base_block(panel->ddc);
>         if (base_block) {
>                 panel_id = drm_edid_get_panel_id(base_block);
> +               panel_hash = crc32_le(~0, base_block, EDID_LENGTH) ^ 0xffffffff;

Any reason you need to XOR with 0xffffffff?


> @@ -2077,13 +2098,32 @@ static const struct edp_panel_entry edp_panels[] = {
>         { /* sentinal */ }
>  };
>
> -static const struct edp_panel_entry *find_edp_panel(u32 panel_id)
> +/*
> + * Similar to edp_panels, this table lists panel variants that require using
> + * overridden modes but have the same panel id as one of the entries in edp_panels.
> + *
> + * Sort first by vendor, then by product ID.

Add ", then by hash" just in case we need it.


> +static const struct edp_panel_entry *find_edp_panel(u32 panel_id, u32 panel_hash)
>  {
>         const struct edp_panel_entry *panel;
>
> -       if (!panel_id)
> +       if (!panel_id || !panel_hash)
>                 return NULL;

IMO just remove the check above. Not sure why it was there in the
first place. Maybe I had it from some older version of the code?
Callers shouldn't be calling us with a panel ID / hash of 0 anyway,
and if they do they'll go through the loop and return NULL anyway.



-Doug