How to use the secure display feature

Applicable for

STM32MP13x lines, STM32MP25x lines

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1. Article purpose

This article aims to explain how to use the secure display hardware feature thanks to:

• the STM32 LTDC internal peripheral
• the following secure hardware internal peripherals:
  • on STM32MP13x lines , the STM32 ETZPC internal peripheral
  • on STM32MP25x lines , the STM32 RIFSC internal peripheral
• the STM32 OP-TEE framework and its related libutee library<ref name="libutee>OP-TEE libraries (libutee) man pages that offers services to Trusted Applications (TA)" including the Trusted User Interface (TUI) API

This article also explains how to configure and use a Trusted User Interface / Trusted Application example for demonstrating the secure display hardware feature.

2. Prerequisites

To run the Trusted User Interface / Trusted Application example, required hardware boards and software stacks are listed below:

• On STM32MP13x lines : The STM32MP135x-DK Discovery kit
• On STM32MP25x lines : The STM32MP257x-EV1 Evaluation board or the STM32MP257x-DK Discovery kit
• A display panel or the HDMI interface
• An OpenSTLinux distribution

3. LTDC Secure layer description

On STM32MP13x lines , the LTDC layer2 can be set as secure (under ETZPC control), whereas the layer1 is always non-secure.

On STM32MP25x lines , the LTDC layer3 can be set as secure (under RIFSC internal peripheral control), whereas layer1 and layer2 are always non-secure, with grouped regs and additional interrupt set:

• The RISUP differentiates the access right of accesses performed toward the following RIF protected peripheral ID:
  • "LTDC common": LTDC common registers, about panel info, synchronization, interface
  • "LTDC_L1L2" (layer 1 and 2): for the window of any two default applications
  • "LTDC_L3": LTDC layer 3, for the window of a potentially secure application, or any default application if there is no secure layer
  • "LTDC_ROT": LTDC rotation, with information about the rotation buffers
• The RIMU differentiates the bus transactions emitted by the following AXI masters:
  • "RIMU_L1L2": read access only for layer 1 and 2, always non-protected
  • "RIMU_L3": read access only for layer 3, potentially protected
  • "RIMU_ROT": write of blended pixels, and read of to-be-rotated pixels, potentially protected, because containing blended pixels of the protected layer 3

Information

Please refer to the STM32 MPU reference manuals for more details on the LTDC secure display hardware feature.

4. Configuration

header file of LTDC HAL module

Trusted application

Application programming interface

LCD TFT display controller (STM32 specific)

Resource Isolation Slave Unit for Peripherals - NEW

Resource Isolation Framework

Resource Isolation Master Unit - NEW

Microprocessor unit