Qualcomm External Bus Interface 2 (EBI2)

The EBI2 contains two peripheral blocks: XMEM and LCDC. The XMEM handles any
external memory (such as NAND or other memory-mapped peripherals) whereas
LCDC handles LCD displays.

As it says it connects devices to an external bus interface, meaning address
lines (up to 9 address lines so can only address 1KiB external memory space),
data lines (16 bits), OE (output enable), ADV (address valid, used on some
NOR flash memories), WE (write enable). This on top of 6 different chip selects
(CS0 thru CS5) so that in theory 6 different devices can be connected.

Apparently this bus is clocked at 64MHz. It has dedicated pins on the package
and the bus can only come out on these pins, however if some of the pins are
unused they can be left unconnected or remuxed to be used as GPIO or in some
cases other orthogonal functions as well.

Also CS1 and CS2 has -A and -B signals. Why they have that is unclear to me.

The chip selects have the following memory range assignments. This region of
memory is referred to as "Chip Peripheral SS FPB0" and is 168MB big.

Chip Select                     Physical address base
CS0 GPIO134                     0x1a800000-0x1b000000 (8MB)
CS1 GPIO39 (A) / GPIO123 (B)    0x1b000000-0x1b800000 (8MB)
CS2 GPIO40 (A) / GPIO124 (B)    0x1b800000-0x1c000000 (8MB)
CS3 GPIO133                     0x1d000000-0x25000000 (128 MB)
CS4 GPIO132                     0x1c800000-0x1d000000 (8MB)
CS5 GPIO131                     0x1c000000-0x1c800000 (8MB)

The APQ8060 Qualcomm Application Processor User Guide, 80-N7150-14 Rev. A,
August 6, 2012 contains some incomplete documentation of the EBI2.

FIXME: the manual mentions "write precharge cycles" and "precharge cycles".
We have not been able to figure out which bit fields these correspond to
in the hardware, or what valid values exist. The current hypothesis is that
this is something just used on the FAST chip selects and that the SLOW
chip selects are understood fully. There is also a "byte device enable"
flag somewhere for 8bit memories.

FIXME: The chipselects have SLOW and FAST configuration registers. It's a bit
unclear what this means, if they are mutually exclusive or can be used
together, or if some chip selects are hardwired to be FAST and others are SLOW
by design.

The XMEM registers are totally undocumented but could be partially decoded
because the Cypress AN49576 Antioch Westbridge apparently has suspiciously
similar register layout, see: http://www.cypress.com/file/105771/download

Required properties:
- compatible: should be one of:
  "qcom,msm8660-ebi2"
  "qcom,apq8060-ebi2"
- #address-cells: should be <2>: the first cell is the chipselect,
  the second cell is the offset inside the memory range
- #size-cells: should be <1>
- ranges: should be set to:
  ranges = <0 0x0 0x1a800000 0x00800000>,
           <1 0x0 0x1b000000 0x00800000>,
           <2 0x0 0x1b800000 0x00800000>,
           <3 0x0 0x1d000000 0x08000000>,
           <4 0x0 0x1c800000 0x00800000>,
           <5 0x0 0x1c000000 0x00800000>;
- reg: two ranges of registers: EBI2 config and XMEM config areas
- reg-names: should be "ebi2", "xmem"
- clocks: two clocks, EBI_2X and EBI
- clock-names: should be "ebi2x", "ebi2"

Optional subnodes:
- Nodes inside the EBI2 will be considered device nodes.

The following optional properties are properties that can be tagged onto
any device subnode. We are assuming that there can be only ONE device per
chipselect subnode, else the properties will become ambiguous.

Optional properties arrays for SLOW chip selects:
- qcom,xmem-recovery-cycles: recovery cycles is the time the memory continues to
  drive the data bus after OE is de-asserted, in order to avoid contention on
  the data bus. They are inserted when reading one CS and switching to another
  CS or read followed by write on the same CS. Valid values 0 thru 15. Minimum
  value is actually 1, so a value of 0 will still yield 1 recovery cycle.
- qcom,xmem-write-hold-cycles: write hold cycles, these are extra cycles
  inserted after every write minimum 1. The data out is driven from the time
  WE is asserted until CS is asserted. With a hold of 1 (value = 0), the CS
  stays active for 1 extra cycle etc. Valid values 0 thru 15.
- qcom,xmem-write-delta-cycles: initial latency for write cycles inserted for
  the first write to a page or burst memory. Valid values 0 thru 255.
- qcom,xmem-read-delta-cycles: initial latency for read cycles inserted for the
  first read to a page or burst memory. Valid values 0 thru 255.
- qcom,xmem-write-wait-cycles: number of wait cycles for every write access, 0=1
  cycle. Valid values 0 thru 15.
- qcom,xmem-read-wait-cycles: number of wait cycles for every read access, 0=1
  cycle. Valid values 0 thru 15.

Optional properties arrays for FAST chip selects:
- qcom,xmem-address-hold-enable: this is a boolean property stating that we
  shall hold the address for an extra cycle to meet hold time requirements
  with ADV assertion.
- qcom,xmem-adv-to-oe-recovery-cycles: the number of cycles elapsed before an OE
  assertion, with respect to the cycle where ADV (address valid) is asserted.
  2 means 2 cycles between ADV and OE. Valid values 0, 1, 2 or 3.
- qcom,xmem-read-hold-cycles: the length in cycles of the first segment of a
  read transfer. For a single read transfer this will be the time from CS
  assertion to OE assertion. Valid values 0 thru 15.


Example:

ebi2@1a100000 {
	compatible = "qcom,apq8060-ebi2";
	#address-cells = <2>;
	#size-cells = <1>;
	ranges = <0 0x0 0x1a800000 0x00800000>,
		 <1 0x0 0x1b000000 0x00800000>,
		 <2 0x0 0x1b800000 0x00800000>,
		 <3 0x0 0x1d000000 0x08000000>,
		 <4 0x0 0x1c800000 0x00800000>,
		 <5 0x0 0x1c000000 0x00800000>;
	reg = <0x1a100000 0x1000>, <0x1a110000 0x1000>;
	reg-names = "ebi2", "xmem";
	clocks = <&gcc EBI2_2X_CLK>, <&gcc EBI2_CLK>;
	clock-names = "ebi2x", "ebi2";
	/* Make sure to set up the pin control for the EBI2 */
	pinctrl-names = "default";
	pinctrl-0 = <&foo_ebi2_pins>;

	foo-ebi2@2,0 {
		compatible = "foo";
		reg = <2 0x0 0x100>;
		(...)
		qcom,xmem-recovery-cycles = <0>;
		qcom,xmem-write-hold-cycles = <3>;
		qcom,xmem-write-delta-cycles = <31>;
		qcom,xmem-read-delta-cycles = <28>;
		qcom,xmem-write-wait-cycles = <9>;
		qcom,xmem-read-wait-cycles = <9>;
	};
};