Documentation: spi-nor: rewrite some portions

                          SPI NOR framework

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Part I - why we need this framework?

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Part I - Why do we need this framework?

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The SPI bus controller only deals with the byte stream.

Some controller does not works like a SPI bus controller, it works

like a SPI NOR controller instead, such as the Freescale's QuadSPI controller.

SPI bus controllers (drivers/spi/) only deal with streams of bytes; the bus

controller operates agnostic of the specific device attached. However, some

controllers (such as Freescale's QuadSPI controller) cannot easily handle

arbitrary streams of bytes, but rather are designed specifically for SPI NOR.

The Freescale's QuadSPI controller should know the NOR commands to

find the right LUT sequence. Unfortunately, the old code can not meet

this requirement.

In particular, Freescale's QuadSPI controller must know the NOR commands to

find the right LUT sequence. Unfortunately, the SPI subsystem has no notion of

opcodes, addresses, or data payloads; a SPI controller simply knows to send or

receive bytes (Tx and Rx). Therefore, we must define a new layering scheme under

which the controller driver is aware of the opcodes, addressing, and other

details of the SPI NOR protocol.

Part II - How does the framework work?

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This framework just adds a new layer between the MTD and the SPI bus driver.

With this new layer, the SPI NOR controller driver does not depend on the

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	       SPI NOR chip

Part III - How can the drivers use the framework

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Part III - How can drivers use the framework?

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The main API is the spi_nor_scan(). Before you call the hook, you should

initialize the necessary fields for spi_nor{}.

Please see the drivers/mtd/spi-nor/spi-nor.c for detail.

Please also reference to the fsl-quadspi.c when you want to write a new driver

for a SPI NOR controller.

The main API is spi_nor_scan(). Before you call the hook, a driver should

initialize the necessary fields for spi_nor{}. Please see

drivers/mtd/spi-nor/spi-nor.c for detail. Please also refer to fsl-quadspi.c

when you want to write a new driver for a SPI NOR controller.