This is a driver for the DMMC-STAMP "mailbox" acting as a data interface between the MMC and the payload FPGA/SoC.
The "mailbox" is a 2 KByte dual port memory inside the STAMP's CPLD acting as a at24-like EEPROM. This driver is based on at24.c from the mainline Linux kernel.
To avoid race conditions, the uppermost byte has a "lock" flag preventing the STAMP from updating the memory. This driver implements a critical section, setting the lock flag when more than one byte is read or written.
To notify the STAMP that the Linux system on FPGA/SoC is ready to be powered off, this driver uses the Linux kernel's pm_power_off callback to set a "shutdown finished" flag in the mailbox.
This requires that
- No other driver is using
pm_power_off - The I2C driver providing access to the I2C bus towards the DMMC-STAMP mailbox supports the
master_xfer_atomic()method (see also i2c-xiic-atomic)