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+SPI/QSPI Dual flash connection modes:
+=====================================
+
+This describes how SPI/QSPI flash memories are connected to a given
+controller in a single chip select line.
+
+Current spi_flash framework supports, single flash memory connected
+to a given controller with single chip select line, but there are some
+hw logics(ex: xilinx zynq qspi) that describes two/dual memories are
+connected with a single chip select line from a controller.
+
+"dual_flash" from include/spi.h describes these types of connection mode
+
+Possible connections:
+--------------------
+SF_SINGLE_FLASH:
+ - single spi flash memory connected with single chip select line.
+
+ +------------+ CS +---------------+
+ | |----------------------->| |
+ | Controller | I0[3:0] | Flash memory |
+ | SPI/QSPI |<======================>| (SPI/QSPI) |
+ | | CLK | |
+ | |----------------------->| |
+ +------------+ +---------------+
+
+SF_DUAL_STACKED_FLASH:
+ - dual spi/qspi flash memories are connected with a single chipselect
+ line and these two memories are operating stacked fasion with shared buses.
+ - xilinx zynq qspi controller has implemented this feature [1]
+
+ +------------+ CS +---------------+
+ | |---------------------->| |
+ | | I0[3:0] | Upper Flash |
+ | | +=========>| memory |
+ | | | CLK | (SPI/QSPI) |
+ | | | +---->| |
+ | Controller | CS | | +---------------+
+ | SPI/QSPI |------------|----|---->| |
+ | | I0[3:0] | | | Lower Flash |
+ | |<===========+====|====>| memory |
+ | | CLK | | (SPI/QSPI) |
+ | |-----------------+---->| |
+ +------------+ +---------------+
+
+ - two memory flash devices should has same hw part attributes (like size,
+ vendor..etc)
+ - Configurations:
+ on LQSPI_CFG register, Enable TWO_MEM[BIT:30] on LQSPI_CFG
+ Enable U_PAGE[BIT:28] if U_PAGE flag set - upper memory
+ Disable U_PAGE[BIT:28] if U_PAGE flag unset - lower memory
+ - Operation:
+ accessing memories serially like one after another.
+ by default, if U_PAGE is unset lower memory should accessible,
+ once user wants to access upper memory need to set U_PAGE.
+
+Note: Technically there is only one CS line from the controller, but
+zynq qspi controller has an internal hw logic to enable additional CS
+when controller is configured for dual memories.
+
+[1] http://www.xilinx.com/support/documentation/user_guides/ug585-Zynq-7000-TRM.pdf
+
+--
+Jagannadha Sutradharudu Teki <jaganna@xilinx.com>
+05-01-2014.