linux-kernel - Re: [PATCH v1] drm: panel: Add Waveshare MIPI-DSI panel driver

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Message-ID: <yp3khykumcomvfsgcwpeh7uhnpz26jv3wnpjdmsh5pzpsq4wzr@6e5he2witixd>
Date: Mon, 17 Nov 2025 17:21:06 -0600
From: Bjorn Andersson <andersson@...nel.org>
To: Sudarshan Shetty <tessolveupstream@...il.com>
Cc: neil.armstrong@...aro.org, jessica.zhang@....qualcomm.com, 
	maarten.lankhorst@...ux.intel.com, mripard@...nel.org, tzimmermann@...e.de, airlied@...il.com, 
	simona@...ll.ch, dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1] drm: panel: Add Waveshare MIPI-DSI panel driver

On Tue, Nov 11, 2025 at 04:13:09PM +0530, Sudarshan Shetty wrote:
> This patch adds a DRM panel driver for Waveshare MIPI-DSI panels
> of various sizes (5.0", 5.5", 7.0", 8.0", and 10.1") with integrated
> touch functionality.
> 
> The driver provides:
>  - Initialization and reset sequencing
>  - MIPI-DSI command transfer for panel setup
>  - Backlight integration through the backlight class device
>  - Support for multiple panel variants via compatible strings
> 
> These panels are commonly used with Waveshare development kits
> and require proper power sequencing and regulator support.
> 

In order to facilitate any form of discussion about the DeviceTree
binding and how this relates to the Waveshare MCU/regulator module,
the patches for these separate parts should be sent together as one
series.


That said, I was able to puzzle the pieces together and get the
Waveshare 12.3 DSI panel working, with touchscreen, on my Particle
Tachyon.

> Signed-off-by: Sudarshan Shetty <tessolveupstream@...il.com>
> ---
[..]
> diff --git a/drivers/gpu/drm/panel/panel-waveshare-dsi-v2.c b/drivers/gpu/drm/panel/panel-waveshare-dsi-v2.c
[..]
> +static int ws_panel_dsi_probe(struct mipi_dsi_device *dsi)
> +{
> +	struct ws_panel *ctx;
> +	int ret;
> +
> +	dev_info(&dsi->dev, "dsi panel: %s\n",
> +		 (char *)of_get_property(
> +			 dsi->dev.of_node, "compatible", NULL));
> +
> +	ctx = devm_kzalloc(&dsi->dev, sizeof(*ctx), GFP_KERNEL);
> +	if (!ctx)
> +		return -ENOMEM;
> +	mipi_dsi_set_drvdata(dsi, ctx);
> +	ctx->dsi = dsi;
> +	ctx->desc = of_device_get_match_data(&dsi->dev);
> +
> +	ctx->panel.prepare_prev_first = true;
> +	drm_panel_init(&ctx->panel, &dsi->dev, &ws_panel_funcs,
> +		       DRM_MODE_CONNECTOR_DSI);
> +
> +	ctx->reset = devm_gpiod_get_optional(
> +			&dsi->dev, "reset", GPIOD_OUT_LOW);

Your reset signal is active-low, but you describe it in DeviceTree as
GPIO_ACTIVE_HIGH and then you invert the logic here (i.e. the think you
call "reset" is "not_reset").

Invert the high/low to mean high == reset, low == not-reset, here in the
driver and then say GPIO_ACTIVE_LOW in the DeviceTree.

> +	if (IS_ERR(ctx->reset))
> +		return dev_err_probe(&dsi->dev, PTR_ERR(ctx->reset),
> +				     "Couldn't get our reset GPIO\n");
> +
> +	ctx->iovcc = devm_gpiod_get_optional(
> +			&dsi->dev, "iovcc", GPIOD_OUT_LOW);

This really sounds like a regulator, not a GPIO.

> +	if (IS_ERR(ctx->iovcc))
> +		return dev_err_probe(&dsi->dev, PTR_ERR(ctx->iovcc),
> +					"Couldn't get our iovcc GPIO\n");
> +
> +	ctx->avdd = devm_gpiod_get_optional(&dsi->dev, "avdd", GPIOD_OUT_LOW);

This too sounds like a regulator, not a GPIO.

> +	if (IS_ERR(ctx->avdd))
> +		return dev_err_probe(&dsi->dev, PTR_ERR(ctx->avdd),
> +					"Couldn't get our avdd GPIO\n");
> +
> +	ret = of_drm_get_panel_orientation(
> +			dsi->dev.of_node, &ctx->orientation);

Do you know why this doesn't work? (Not sure it relates to your code...)

> +	if (ret) {
> +		dev_err(&dsi->dev, "%pOF: failed to get orientation: %d\n",
> +			dsi->dev.of_node, ret);
> +		return ret;
> +	}
> +
> +	ret = drm_panel_of_backlight(&ctx->panel);
> +	if (ret)
> +		return ret;
> +
> +	drm_panel_add(&ctx->panel);
> +
> +	dsi->mode_flags = ctx->desc->mode_flags;
> +	dsi->format = ctx->desc->format;
> +	dsi->lanes = ctx->desc->lanes;
> +	dev_info(&dsi->dev, "lanes: %d\n", dsi->lanes);
> +
> +	ret = mipi_dsi_attach(dsi);
> +	if (ret)
> +		drm_panel_remove(&ctx->panel);
> +
> +	return ret;
> +}

Regards,
Bjorn