[RFC,0/6] drm/bridge: Add mux input selection bridge

This bridge driver allows to select the input to a downstream bridge (or panel)
via device tree.

It can be useful to separate the pixel source selection from the actual bridge
processing the pixel data. E.g. NXP's imx8mq has two display controllers. Both
can feed the pixel data to the NWL DSI IP core. The input selection is done via
a separate mux register elsewhere on the chip, so separating this out avoids SoC
specific code in such drivers.

The current implementation allows to select the input source via device tree.
The long term goal is to allow to switch the input source at run time. This
can be useful to e.g. use the less power hungry display controller for normal
operation but switch to a the other display controller when running full screen
games (since it can detile textures more efficiently).

This was initially suggested by Laurent Pinchart¹. It is similar in spirit to
the video-mux in the media subsystem but for DRM bridges.

Besides the actual driver this series includes the necessary bits to demo the
usage for the Librem5 devkit.

The series is based on linux-next as of next-20200512.

¹ https://lore.kernel.org/dri-devel/20200415021908.GH19819@pendragon.ideasonboard.com/

Guido Günther (6):
  dt-bindings: display/bridge: Add binding for input mux bridge
  drm/bridge: Add mux-input bridge
  dt-bindings: display/bridge/nwl-dsi: Drop mux handling
  drm/bridge/nwl-dsi: Drop mux handling
  arm64: dts: imx8mq: Add NWL dsi controller
  arm64: dts: imx8mq-librem5-devkit: Enable MIPI DSI panel

 .../display/bridge/mux-input-bridge.yaml      | 123 +++++++++
 .../bindings/display/bridge/nwl-dsi.yaml      |   6 -
 .../dts/freescale/imx8mq-librem5-devkit.dts   |  81 ++++++
 arch/arm64/boot/dts/freescale/imx8mq.dtsi     |  31 +++
 drivers/gpu/drm/bridge/Kconfig                |  10 +-
 drivers/gpu/drm/bridge/Makefile               |   1 +
 drivers/gpu/drm/bridge/mux-input.c            | 238 ++++++++++++++++++
 drivers/gpu/drm/bridge/nwl-dsi.c              |  61 -----
 8 files changed, 483 insertions(+), 68 deletions(-)
 create mode 100644 Documentation/devicetree/bindings/display/bridge/mux-input-bridge.yaml
 create mode 100644 drivers/gpu/drm/bridge/mux-input.c

On Fri, May 15, 2020 at 03:12:13PM +0200, Guido Günther wrote:
> This will be handled via the mux-input-bridge.

You can't do this. What happens booting a kernel with this change and an 
un-modified dtb? You just broke it.

> 
> Signed-off-by: Guido Günther <agx@sigxcpu.org>
> ---
>  drivers/gpu/drm/bridge/Kconfig   |  1 -
>  drivers/gpu/drm/bridge/nwl-dsi.c | 61 --------------------------------
>  2 files changed, 62 deletions(-)
> 
> diff --git a/drivers/gpu/drm/bridge/Kconfig b/drivers/gpu/drm/bridge/Kconfig
> index 3886c0f41bdd..11444f841e35 100644
> --- a/drivers/gpu/drm/bridge/Kconfig
> +++ b/drivers/gpu/drm/bridge/Kconfig
> @@ -78,7 +78,6 @@ config DRM_NWL_MIPI_DSI
>  	select DRM_PANEL_BRIDGE
>  	select GENERIC_PHY_MIPI_DPHY
>  	select MFD_SYSCON
> -	select MULTIPLEXER
>  	select REGMAP_MMIO
>  	help
>  	  This enables the Northwest Logic MIPI DSI Host controller as
> diff --git a/drivers/gpu/drm/bridge/nwl-dsi.c b/drivers/gpu/drm/bridge/nwl-dsi.c
> index b14d725bf609..8839f333f39c 100644
> --- a/drivers/gpu/drm/bridge/nwl-dsi.c
> +++ b/drivers/gpu/drm/bridge/nwl-dsi.c
> @@ -12,7 +12,6 @@
>  #include <linux/math64.h>
>  #include <linux/mfd/syscon.h>
>  #include <linux/module.h>
> -#include <linux/mux/consumer.h>
>  #include <linux/of.h>
>  #include <linux/of_platform.h>
>  #include <linux/phy/phy.h>
> @@ -44,9 +43,6 @@ enum transfer_direction {
>  	DSI_PACKET_RECEIVE,
>  };
>  
> -#define NWL_DSI_ENDPOINT_LCDIF 0
> -#define NWL_DSI_ENDPOINT_DCSS 1
> -
>  struct nwl_dsi_plat_clk_config {
>  	const char *id;
>  	struct clk *clk;
> @@ -94,7 +90,6 @@ struct nwl_dsi {
>  	struct reset_control *rst_esc;
>  	struct reset_control *rst_dpi;
>  	struct reset_control *rst_pclk;
> -	struct mux_control *mux;
>  
>  	/* DSI clocks */
>  	struct clk *phy_ref_clk;
> @@ -1018,14 +1013,6 @@ static int nwl_dsi_parse_dt(struct nwl_dsi *dsi)
>  	}
>  	dsi->tx_esc_clk = clk;
>  
> -	dsi->mux = devm_mux_control_get(dsi->dev, NULL);
> -	if (IS_ERR(dsi->mux)) {
> -		ret = PTR_ERR(dsi->mux);
> -		if (ret != -EPROBE_DEFER)
> -			DRM_DEV_ERROR(dsi->dev, "Failed to get mux: %d\n", ret);
> -		return ret;
> -	}
> -
>  	base = devm_platform_ioremap_resource(pdev, 0);
>  	if (IS_ERR(base))
>  		return PTR_ERR(base);
> @@ -1073,47 +1060,6 @@ static int nwl_dsi_parse_dt(struct nwl_dsi *dsi)
>  	return 0;
>  }
>  
> -static int nwl_dsi_select_input(struct nwl_dsi *dsi)
> -{
> -	struct device_node *remote;
> -	u32 use_dcss = 1;
> -	int ret;
> -
> -	remote = of_graph_get_remote_node(dsi->dev->of_node, 0,
> -					  NWL_DSI_ENDPOINT_LCDIF);
> -	if (remote) {
> -		use_dcss = 0;
> -	} else {
> -		remote = of_graph_get_remote_node(dsi->dev->of_node, 0,
> -						  NWL_DSI_ENDPOINT_DCSS);
> -		if (!remote) {
> -			DRM_DEV_ERROR(dsi->dev,
> -				      "No valid input endpoint found\n");
> -			return -EINVAL;
> -		}
> -	}
> -
> -	DRM_DEV_INFO(dsi->dev, "Using %s as input source\n",
> -		     (use_dcss) ? "DCSS" : "LCDIF");
> -	ret = mux_control_try_select(dsi->mux, use_dcss);
> -	if (ret < 0)
> -		DRM_DEV_ERROR(dsi->dev, "Failed to select input: %d\n", ret);
> -
> -	of_node_put(remote);
> -	return ret;
> -}

You could however make these functions generic for any bridge to use. 
Define a function that checks for mux-control property and if found sets 
up the mux (IIRC, there's already a concept of a default state). That 
should be callable from somewhere generic too.

Rob