Re: [PATCH 1/4] dt-bindings: display: vga-connector: Allow hardcoding EDID

[Laurent Pinchart <laurent.pinchart@ideasonboard.com>]

Hi Maxime,

On Tue, Jul 07, 2026 at 11:01:43AM +0300, Laurent Pinchart wrote:
> On Tue, Jul 07, 2026 at 09:46:52AM +0200, Maxime Ripard wrote:
> > On Mon, Jul 06, 2026 at 07:02:43PM +0300, Laurent Pinchart wrote:
> > > On Mon, Jul 06, 2026 at 05:40:30PM +0200, Maxime Ripard wrote:
> > > > On Mon, Jul 06, 2026 at 12:45:22PM +0300, Laurent Pinchart wrote:
> > > > > On Mon, Jul 06, 2026 at 10:52:32AM +0200, Maxime Ripard wrote:
> > > > > > On Mon, Jul 06, 2026 at 12:35:39AM +0300, Laurent Pinchart wrote:
> > > > > > > Since DDC version 2, introduced in 1996, VGA monitors have exposed EDID
> > > > > > > data over an I2C bus. The bus is also used to detect the presence of a
> > > > > > > connected monitor by trying to read the EDID data.
> > > > > > > 
> > > > > > > Some devices where the VGA display is integrated in the device and
> > > > > > > always connected do not connect the DDC pins. Some development boards,
> > > > > > > such as the Renesas M3N Salvator-XS, also do not connect the DDC pins.
> > > > > > > 
> > > > > > > To support those, add the ability to provide hardcoded EDID data in the
> > > > > > > device tree. This is mutually exclusive with specifying a DDC bus, and
> > > > > > > can only be done when the VGA display is guaranteed to be always
> > > > > > > connected.
> > > > > > > 
> > > > > > > Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
> > > > > > 
> > > > > > This feels redundant with the EDID firmware loading mechanism we have in
> > > > > > KMS already. It should at least be mentioned why we would need to set it
> > > > > > in the device tree at all.
> > > > > 
> > > > > Very good question.
> > > > > 
> > > > > I assume you're talking about CONFIG_DRM_LOAD_EDID_FIRMWARE, as the
> > > > > debugfs override_edid feature isn't meant for production. If there are
> > > > > other relevant mechanisms I'm not aware of, please let me know.
> > > > 
> > > > No, that's indeed what I meant.
> > > > 
> > > > > In the use case at hand, the VGA display is an integral part of the
> > > > > device, the same way an LVDS or DSI panel would be. Using
> > > > > CONFIG_DRM_LOAD_EDID_FIRMWARE, the manufacturer would need to set the
> > > > > drm.edid_firmware command line parameter, and provide EDID as a file in
> > > > > /lib/firmware/ (possibly in an initramfs). Beside the complexity, and
> > > > > the fact it won't be very friendly to people who run a different
> > > > > userspace on the device, I think EDID counts in this case as system
> > > > > description, the same way we support specifying panel timings in device
> > > > > tree.
> > > > 
> > > > It's *some* hardware description, but you have no idea whether it's
> > > > actually the hardware you're running from. What would be in that EDID
> > > > anyway?
> > > 
> > > It would be the EDID corresponding to the connected display :-) I see it
> > > as identical to how a user of the simple panel driver is responsible for
> > > listing the timings corresponding to the hardware in the device tree.
> > 
> > What's the expected user-experience for someone with that board
> > unplugging a display and plugging another then? Update the device tree,
> > rebuild it, rebuild the firmware and / or resign it and reboot?
> 
> The whole point is that they can't easily unplug the display. This patch
> series is meant to support devices where the display is an integral part
> of the device. The user experience is expected to be the same as for a
> user who will take a phone, remove the display panel, and plug a
> different one. It's certainly possible, and they would have to update
> the DT indeed.
> 
> > That doesn't sound very practical, and even impossible in some cases.
> > 
> > > > There's another alternative we've used several times already, in
> > > > simple-bridge for example: just register any VESA mode up to a given
> > > > resolution:
> > > > https://elixir.bootlin.com/linux/v7.1.2/source/drivers/gpu/drm/bridge/simple-bridge.c#L66
> > > 
> > > I don't see how that would help. The goal is to provide the kernel with
> > > the data corresponding to the display integrated in the device.
> > 
> > How is that not helping? Your problem is that you have a display you
> > can't get the supported resolutions from. What I'm suggesting allows you
> > to create all the possible resolutions for that display, allowing the
> > user to use the display, which is what we did for multiple drivers with
> > the same problem.
> 
> How would the possible resolutions be provided to the kernel ? Isn't it
> the whole point of EDID ?
> 
> > You seem to be set on providing the EDID to the kernel, but it's a
> > terrible idea to begin with, so I'd rather discuss alternatives.
> 
> Could you elaborate on why it is a terrible idea ?

I'd be happy to continue this discussion when you'll have time.

-- 
Regards,

Laurent Pinchart