Description

The BCM283X GPU provides information about and control over various aspects of the system. A hardware mailbox mechanism is used to exchange messages with the GPU to query or control these aspects. The BCM283X HAL provides some functions to access this interface. The default functions are limited to just those exchanges that are of direct relevance to eCos, although there is scope for expanding these, or for performing a property exchange directly.

hal_bcm283x_board_model() and hal_bcm283x_board_revision() fetch the board model and revision numbers. The model number is usually zero, but the revision number encodes the board properties as described Raspberry Pi Revision codes page. Normally the eCos HAL reads the revision code at startup and decodes it into a set of variables.

hal_bcm283x_board_serial() returns the board's serial number; this is a 64 bit identifier unique to this board. The function hal_bcm283x_mac_address() returns a MAC address for this board; this consists of the lower 24 bits of the serial number combined with a vendor ID assigned to the Raspberry Pi Foundation.

hal_bcm283x_arm_memory() returns the base and size of the RAM available to the ARM processor. The base will be zero and the size will indicate the top of memory after the GPU memory specified in the config.txt file has been allocated.

hal_bcm283x_clock_rate() returns the current rate of the given clock in Hz. Clock identifiers can be found in bcm238x.h. hal_bcm283x_clock_rate_max() and hal_bcm283x_clock_rate_min() return the maximum and minimum rates for the clock. hal_bcm283x_clock_rate_set() sets the given clock to the supplied rate.

hal_bcm283x_dma_channels() returns a mask of the available DMA channels.

hal_bcm283x_set_gpu_gpio() sets the value (0 or 1) of a GPIO pin controlled by the GPU through the external GPIO extender. This is mainly used to control the ACT LED on Pi3B and Pi3A boards.

hal_bcm283x_temperature() reads the temperature of the SoC in thousandths of a degree C. hal_bcm283x_temperature_max() returns the maximum SoC temperature, above which overclocking will be disabled. In both cases the id should be zero.

hal_bcm283x_property_exchange() performs a property exchange with the GPU and may be used to implement requests that are not covered above. The protocol used is described on the Raspberry Pi Mailbox properties GitHub page. This function copies the array of property tags pointed to by prop into a properly aligned buffer after the standard buffer header and terminates it an end tag. It then sends a mailbox message to the GPU and waits for a response. Finally, the updated property buffer is copied back out to prop. In SMP systems a spinlock is used to control concurrent access from different CPUs.