[PATCH 13/20] drm/bridge: Add RGB to VGA bridge support

Laurent Pinchart
2016-05-16T13:24:15Z

Hi Maxime,

Thank you for the patch.

On Monday 16 May 2016 14:47:13 Maxime Ripard wrote:
> Some boards have an entirely passive RGB to VGA bridge, based on either
> DACs or resistor ladders.
>
> Those might or might not have an i2c bus routed to the VGA connector in
> order to access the screen EDIDs.
>
> Add a bridge that doesn't do anything but expose the modes available on the
> screen, either based on the EDIDs if available, or based on the XGA
> standards.
>
> Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
> ---
> .../bindings/display/bridge/dumb-vga.txt | 40 +++++
> drivers/gpu/drm/bridge/Kconfig | 6 +
> drivers/gpu/drm/bridge/Makefile | 1 +
> drivers/gpu/drm/bridge/dumb-vga.c | 186 ++++++++++++++++++
> 4 files changed, 233 insertions(+)
> create mode 100644
> Documentation/devicetree/bindings/display/bridge/dumb-vga.txt create mode
> 100644 drivers/gpu/drm/bridge/dumb-vga.c
>
> diff --git a/Documentation/devicetree/bindings/display/bridge/dumb-vga.txt
> b/Documentation/devicetree/bindings/display/bridge/dumb-vga.txt new file
> mode 100644
> index 000000000000..757f04de97f3
> --- /dev/null
> +++ b/Documentation/devicetree/bindings/display/bridge/dumb-vga.txt
> @@ -0,0 +1,40 @@
> +Passive RGB to VGA bridge
> +-------------------------
> +
> +This binding is aimed for entirely passive RGB to VGA bridges that do not
> +require any configuration.
> +
> +Required properties:
> +
> +- compatible: Should be "dumb-vga-bridge"
> +
> +Optional properties:
> +
> +- ddc-i2c-bus: phandle to the i2c bus used to communicate with the monitor
> +
> +Required nodes:
> +
> +This device has one video port. Its connection is modelled using the OF
> +graph bindings specified in Documentation/devicetree/bindings/graph.txt.
> This +connection is meant to be the RGB input bus.

Should the bridge have two ports, one input port connected to a display engine
and one output port connected to a VGA connector ?

> +
> +
> +Example
> +-------
> +
> +bridge {
> + compatible = "dumb-vga-bridge";
> + #address-cells = <1>;
> + #size-cells = <0>;
> +
> + port at 0 {
> + reg = <0>;
> + #address-cells = <1>;
> + #size-cells = <0>;
> +
> + vga_input: endpoint at 0 {
> + reg = <0>;
> + remote-endpoint = <&tcon0_out_vga>;
> + };
> + };
> +};
> diff --git a/drivers/gpu/drm/bridge/Kconfig b/drivers/gpu/drm/bridge/Kconfig
> index 8f7423f18da5..cf279956cbec 100644
> --- a/drivers/gpu/drm/bridge/Kconfig
> +++ b/drivers/gpu/drm/bridge/Kconfig
> @@ -17,6 +17,12 @@ config DRM_ANALOGIX_ANX78XX
> the HDMI output of an application processor to MyDP
> or DisplayPort.
>
> +config DRM_DUMB_VGA
> + tristate "Dumb RGB to VGA Bridge support"
> + select DRM_KMS_HELPER
> + help
> + Support for passive RGB to VGA bridges
> +
> config DRM_DW_HDMI
> tristate
> select DRM_KMS_HELPER
> diff --git a/drivers/gpu/drm/bridge/Makefile
> b/drivers/gpu/drm/bridge/Makefile index 96b13b30e6ab..63f21120e8f8 100644
> --- a/drivers/gpu/drm/bridge/Makefile
> +++ b/drivers/gpu/drm/bridge/Makefile
> @@ -1,6 +1,7 @@
> ccflags-y := -Iinclude/drm
>
> obj-$(CONFIG_DRM_ANALOGIX_ANX78XX) += analogix-anx78xx.o
> +obj-$(CONFIG_DRM_DUMB_VGA) += dumb-vga.o
> obj-$(CONFIG_DRM_DW_HDMI) += dw-hdmi.o
> obj-$(CONFIG_DRM_DW_HDMI_AHB_AUDIO) += dw-hdmi-ahb-audio.o
> obj-$(CONFIG_DRM_NXP_PTN3460) += nxp-ptn3460.o
> diff --git a/drivers/gpu/drm/bridge/dumb-vga.c
> b/drivers/gpu/drm/bridge/dumb-vga.c new file mode 100644
> index 000000000000..2864fa535bf8
> --- /dev/null
> +++ b/drivers/gpu/drm/bridge/dumb-vga.c
> @@ -0,0 +1,186 @@
> +
> +#include <linux/module.h>
> +
> +#include <drm/drmP.h>
> +#include <drm/drm_atomic_helper.h>
> +#include <drm/drm_crtc.h>
> +#include <drm/drm_crtc_helper.h>
> +
> +struct dumb_vga {
> + struct drm_bridge bridge;
> + struct drm_connector connector;
> +
> + struct i2c_adapter *ddc;
> +};
> +
> +static inline struct dumb_vga *
> +drm_bridge_to_dumb_vga(struct drm_bridge *bridge)
> +{
> + return container_of(bridge, struct dumb_vga, bridge);
> +}
> +
> +static inline struct dumb_vga *
> +drm_connector_to_dumb_vga(struct drm_connector *connector)
> +{
> + return container_of(connector, struct dumb_vga, connector);
> +}
> +
> +static int dumb_vga_get_modes(struct drm_connector *connector)
> +{
> + struct dumb_vga *vga = drm_connector_to_dumb_vga(connector);
> + struct edid *edid;
> + int ret;
> +
> + if (!vga->ddc)
> + goto fallback;
> +
> + edid = drm_get_edid(connector, vga->ddc);
> + if (!edid) {
> + DRM_INFO("EDID readout failed, falling back to standard modes\n");
> + goto fallback;
> + }
> +
> + drm_mode_connector_update_edid_property(connector, edid);
> + return drm_add_edid_modes(connector, edid);
> +
> +fallback:
> + /*
> + * In case we cannot retrieve the EDIDs (broken or missing i2c
> + * bus), fallback on the XGA standards
> + */
> + ret = drm_add_modes_noedid(connector, 1920, 1200);

The DRM core adds modes up to 1024x768 in
drm_helper_probe_single_connector_modes(). I wonder if it really makes sense
to override that in drivers, compared to increasing the maximum resolution in
the core. What we can reasonable expect from a VGA monitor doesn't really
depend on which display engine it is connected to.

> + /* And prefer a mode pretty much anyone can handle */
> + drm_set_preferred_mode(connector, 1024, 768);
> +
> + return ret;
> +}
> +
> +static struct drm_encoder *
> +dumb_vga_best_encoder(struct drm_connector *connector)
> +{
> + struct dumb_vga *vga = drm_connector_to_dumb_vga(connector);
> +
> + return vga->bridge.encoder;
> +}
> +
> +static struct drm_connector_helper_funcs dumb_vga_con_helper_funcs = {
> + .get_modes = dumb_vga_get_modes,
> + .best_encoder = dumb_vga_best_encoder,
> +};
> +
> +static enum drm_connector_status
> +dumb_vga_connector_detect(struct drm_connector *connector, bool force)
> +{
> + return connector_status_connected;

Shouldn't that be connector_status_unknown if you have no information about
the connection status ?

Would it make sense to add an optional GPIO-based connection detection to this
driver ?

> +}
> +
> +static void
> +dumb_vga_connector_destroy(struct drm_connector *connector)
> +{
> + drm_connector_cleanup(connector);
> +}
> +
> +static struct drm_connector_funcs dumb_vga_con_funcs = {
> + .dpms = drm_atomic_helper_connector_dpms,
> + .detect = dumb_vga_connector_detect,
> + .fill_modes = drm_helper_probe_single_connector_modes,
> + .destroy = dumb_vga_connector_destroy,
> + .reset = drm_atomic_helper_connector_reset,
> + .atomic_duplicate_state = drm_atomic_helper_connector_duplicate_state,
> + .atomic_destroy_state = drm_atomic_helper_connector_destroy_state,
> +};
> +
> +static int dumb_vga_attach(struct drm_bridge *bridge)
> +{
> + struct dumb_vga *vga = drm_bridge_to_dumb_vga(bridge);
> + int ret;
> +
> + if (!bridge->encoder) {
> + DRM_ERROR("Missing encoder\n");
> + return -ENODEV;
> + }
> +
> + drm_connector_helper_add(&vga->connector,
> + &dumb_vga_con_helper_funcs);
> + ret = drm_connector_init(bridge->dev, &vga->connector,
> + &dumb_vga_con_funcs, DRM_MODE_CONNECTOR_VGA);
> + if (ret) {
> + DRM_ERROR("Failed to initialize connector\n");
> + return ret;
> + }
> +
> + drm_mode_connector_attach_encoder(&vga->connector,
> + bridge->encoder);
> +
> + return 0;
> +}
> +
> +static void dumb_vga_nop(struct drm_bridge *bridge) {};
> +
> +static struct drm_bridge_funcs dumb_vga_bridge_funcs = {
> + .attach = dumb_vga_attach,
> + .enable = dumb_vga_nop,
> + .disable = dumb_vga_nop,
> + .pre_enable = dumb_vga_nop,
> + .post_disable = dumb_vga_nop,
> +};
> +
> +static int dumb_vga_probe(struct platform_device *pdev)
> +{
> + struct dumb_vga *vga;
> + struct device_node *ddc;
> +
> + vga = devm_kzalloc(&pdev->dev, sizeof(*vga), GFP_KERNEL);
> + if (!vga)
> + return -ENOMEM;
> + platform_set_drvdata(pdev, vga);
> +
> + ddc = of_parse_phandle(pdev->dev.of_node, "ddc-i2c-bus", 0);
> + if (ddc) {
> + vga->ddc = of_find_i2c_adapter_by_node(ddc);

You're leaking the reference to the I2C adapter.

Also, shouldn't you use of_get_i2c_adapter_by_node() ? Otherwise you won't
take a reference to the adapter module.

> + of_node_put(ddc);
> +
> + if (!vga->ddc) {
> + dev_err(&pdev->dev, "Couldn't retrieve i2c bus\n");
> + return -EPROBE_DEFER;
> + }
> + } else {
> + dev_info(&pdev->dev,
> + "No i2c bus specified... Disabling EDID readout\n");
> + }
> +
> + vga->bridge.funcs = &dumb_vga_bridge_funcs;
> + vga->bridge.of_node = pdev->dev.of_node;
> +
> + return drm_bridge_add(&vga->bridge);
> +}
> +
> +static int dumb_vga_remove(struct platform_device *pdev)
> +{
> + struct dumb_vga *vga = platform_get_drvdata(pdev);
> +
> + drm_bridge_remove(&vga->bridge);
> +
> + return 0;
> +}
> +
> +static const struct of_device_id dumb_vga_match[] = {
> + { .compatible = "dumb-vga-bridge" },
> + {},
> +};
> +MODULE_DEVICE_TABLE(of, dumb_vga_match);
> +
> +struct platform_driver dumb_vga_driver = {
> + .probe = dumb_vga_probe,
> + .remove = dumb_vga_remove,
> + .driver = {
> + .name = "dumb-vga-bridge",
> + .of_match_table = dumb_vga_match,
> + },
> +};
> +module_platform_driver(dumb_vga_driver);
> +
> +MODULE_AUTHOR("Maxime Ripard <maxime.ripard@free-electrons.com>");
> +MODULE_DESCRIPTION("Dumb RGB to VGA bridge driver");
> +MODULE_LICENSE("GPL");

--
Regards,

Laurent Pinchart