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MIPI host driver and panel driver

Introduction

The relationship of host driver and panel driver is introduced here. The design is a very good example of a design pattern: aggregation or facade. Declaimer: this document is not a complete reference document for the drivers, but just a notes to understand from one perspective.

Concepts

MIPI DSI Host Driver

What is the host? The host basically means the SOC itself. The MIPI Host driver is in e.g.:

drivers/gpu/drm/rockchip/dw-mipi-dsi.c

It controls the registers in SOC in order to connect to a panel.

MIPI Panel Driver

What is the panel? It basically means the Panel, which could be controlled BY SoC via communication channels, for example in case of MIPI DSI, the host could control the panel by DSI commands via D0.

Connector

A connector is an aggregate concept. The separation of Host driver and Panel driver is for decoupling the two logic parts. However it might be confusing for the client of host driver and panel driver, as the client just how to show or display. A connector is an aggregation of two logical parts and expose only one interface to the client.

How these two drivers are bound together

So how these two driver interact with each other? This is no magic here, it is done via callbacks/registrations. Specifically, it is done via component.

Refer to link for the details of concept of component:

In short, component allows to aggregate drivers into an aggregated driver. In our case, it helps to aggregate the MIPI DSI host driver and sub-dev panel driver.

In a bit more details, the host driver register its self as a component, once it is done, the bind callback will be called by the component system. In this bind callback, the host driver will try to do the binding with its panel driver.

More details

The design of component is quite simple but powerful. It allows the host driver and panel driver to become more static, but move the dynamic part under the cover of component helper system.

probing of host driver

During probe of MIPI DSI Host driver, following component (ops) is registered:

ret = component_add(dev, &dw_mipi_dsi_ops);

The host DSI ops has two callback functions: bind and unbind.

static const struct component_ops dw_mipi_dsi_ops = {
    .bind   = dw_mipi_dsi_bind,
    .unbind = dw_mipi_dsi_unbind,
};

Component calling bind callback

In this binding function it will check if the panel driver is available. If not, it will return EPROBE_DEFER to indicate the component system that ‘please call me later’. The component system will call it again a bit later.

Following function is called to check if the panel is available:

dsi->panel = of_drm_find_panel(dsi->client);

of_drm_find_panel is a helper function to check if the panel is already added by function drm_panel_add, which is called by panel driver during probing.

Refer to:

drivers/gpu/drm/drm_panel.c

probing of panel driver

Refer to following source code for for the calling of drm_panel_add by the panel driver:

drivers/gpu/drm/panel/panel-simple.c

continuing of probing of Host driver

Once the host driver (in binding function) find the correct panel, it will continue its logic, it will other factors and it may tell to be called later again by return -EPROBE_DEFER.

Refer to a specific host driver:

drivers/gpu/drm/rockchip/dw-mipi-dsi.c

connector

get_modes

As we understand earlier, connector will expose some functions. One of them is get_modes function, as shown in below:

static const struct drm_connector_helper_funcs
dw_mipi_dsi_connector_helper_funcs = {
    .get_modes = dw_mipi_dsi_connector_get_modes,
    .best_encoder = dw_mipi_dsi_connector_best_encoder,
};

It will call the panel’s driver’s function eventually:

int panel_simple_get_modes(struct drm_panel *panel)
encoder

Another part of a connector is the encoder, the encoder has several callbacks to be used by the client:

static const struct drm_encoder_helper_funcs
dw_mipi_dsi_encoder_helper_funcs = {
    .loader_protect = dw_mipi_dsi_encoder_loader_protect,
    .mode_set = dw_mipi_dsi_encoder_mode_set,
    .enable = dw_mipi_dsi_encoder_enable,
    .disable = dw_mipi_dsi_encoder_disable,
    .atomic_check = dw_mipi_dsi_encoder_atomic_check,
};

Initialization Steps

  • DSI Encoder Enabled (Set the Lane/Clock)
  • Set the VoP routing to DSI
  • Set the DSI to command mode
  • Toggle Panel’s Reset pin and Enable pin to activate panel (for receiving commands)
  • After some delay, send init-sequence commands
  • Set the DSI to video mode (so DSI could output video packets)
  • Enable Panel’s backlight

Summary

The aggregation design patten allows logic separation of host driver and panel driver. While facade design patten allows an unified interface to the client.