Proper Linux drivers for ZeroPhone hardware features

[CRImier]

ZeroPhone hardware would benefit from having corresponding Linux drivers. Why?

• It makes it easier to interface to ZeroPhone hardware - both to users and to existing software
• It also makes ZeroPhone compatible with a range of existing software
• You can implement more critical features (like undervoltage shutdown) in a way that is less likely to glitch
• You can also implement features inside the driver itself - they're going to be faster, more reliable and won't depend on external binaries (like Bash and Python scripts do)
• It would also allow for un-tying ZeroPhone from its software - allowing the user to easily avoid using the existing APIs and software (like ZPUI)

Current situation

• The MCP23017 chip uses the mcp23x08 Linux driver (working, but it only takes care of GPIOs, not the actual functions attached to them)
• The 5V step-up (that controls the USB) is not exposed to the kernel, so it has no idea when the USB port power is turned on or off (usually, a kernel driver is used, for example, Raspberry Pi has one AFAIK)
• RGB LED is only available through sysfs GPIO or Python API - not so good (though there's not yet a good API for RGB LEDs, we could use the individual LED API)
  • However, what will happen when we switch to APA102, which is digitally-controlled and has proper PWM support?
• Power management FET is switched on by a script It's working, but not fully, and it's a hack (which is undesirable)
  • In addition, it doesn't support turning the power off on shutdown
• There's charger detection (using a resistor divider). However, it only really is accessible through the Python API so far.
• There's a thermistor for sensing overheating of the phone, which is interfaced to ZP through a comparator (Delta version).
• GSM modem RST, DTR and RING lines are exposed, but not used
  • How can we use them with ofono? Do we even need a Linux driver for those?
• Charger input has undervoltage detection (Delta version). It could be somehow signalled to the kernel, but as of now there's no software support.
• 5V output has undervoltage (and, as a result, overcurrent) detection (Delta version). Again, it could be somehow signalled to the kernel (and maybe the USB output could be shut off), but there's no software support yet.
• GSM UART can be disabled by the 74HC125 IC on the board. However, there's no software control of that whatsoever yet.
• The ATMega328P has the keypad connected to it and it's controlled through I2C. However, there's no Linux driver for it yet - which means that it can't be used from, say, X.
• It also has a vibromotor (not yet supported in ATMega firmware). I don't know if Linux has a defined interface for using those, but it would be cool.
• There's an ADC, measuring VSYS, it would be great to expose that to the kernel (not yet supported in ATMega firmware)
• There's also an user-exposed ADC that should be easily accessible to users - making it accessible through a driver would achieve that goal (not yet supported in ATMega firmware)
• There are two user-accessible GPIOs that are accessible through I2C. They could easily be exposed to sysfs. (not yet supported in ATMega firmware)
  • Those GPIOs both have PWM support (not yet supported in ATMega firmware); I'm wondering if this could be easily interfaced as well.

Problems

• How do we implement cross-revision compatibility? Delta revision will have an EEPROM on I2C1, but I'm not sure that's enough.
  • Specifically, when you compare Delta rev. to Gamma rev., new features are added and pin mapping is changed

Tracking the status

• MCP23017-connected devices:
  • RGB LED
  • Power enable FET
  • GSM modem RST, DTR and RING lines
  • 5V DC-DC control FET
  • Charger sensing
  • Charger undervoltage warning (Delta revision)
  • 5V output undervoltage warning (Delta revision)
  • Charger board overheating warning (Delta revision)
  • GSM UART enable switch (Delta revision)
• ATMega-controlled devices
  • Keypad input
  • Vibromotor
  • User-accessible GPIO(s)
  • UART
  • User-accessible ADC
  • VSYS-measuring ADC

Links

• gpio-regulator Linux driver
• leds-gpio Linux driver