DSI command and video mode
MIPI DSI names a link, not a place where the picture lives. In video mode the host streams pixels without stopping. In command mode a panel with its own memory stores what it is sent and refreshes itself. The interface name does not tell you which.
Lesson two asked who keeps the picture. DSI does not answer it, because one interface supports two very different arrangements.
In video mode the host sends pixels continuously, with display timing, like the RGB stream of lesson nine carried on DSI lanes. The panel has no frame memory to fall back on. If the stream stops, the picture goes with it.
In command mode the panel has frame memory. The host writes pixels into it with commands, and the panel refreshes itself from that memory on its own timing. Updating a small clock rectangle writes only that rectangle. The rest of the stored frame is untouched, and the host can stop sending entirely while the stored picture stays lit.
What stopping costs
Stopping the host is a test that separates the two. A video-mode panel goes empty when the stream disappears. A command-mode panel keeps the last thing it was given. A screen that stays lit tells you the panel has memory, nothing more.
The price of command mode is what it asks of the panel: memory, and a controller that refreshes from it. The price of video mode is what it asks of the host, which has to hold the frame and keep the link busy.
On the ESP32
In ESP-IDF the ESP32-P4's DSI DPI path carries the video pixel stream, and its DBI path carries commands and parameters to the display controller. The ESP-IDF documentation describes the panel as having no GRAM and the host as keeping the frame buffer. A display that depends only on command mode and panel GRAM is not documented in the stock driver. That is a statement about the driver, and not about what the chip can do. The ESP32-S3 has no DSI host. More in MIPI displays on the ESP32.
eDP has a related idea, Panel Self Refresh, added in eDP 1.3. The display keeps a static image while the main link sleeps.
Common mistakes
- Assuming the picture vanishes when pixels stop. That is true of a video-mode panel and false of a command-mode one. The interface name alone does not say which you have.
- Reading a lit screen as proof of the interface. It shows the panel has memory, or that the host is still streaming. Only a test on the bench separates them.
- Assuming the driver is the hardware. Command-only panels are not documented in the stock P4 driver. Do not read that as a chip limit.
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