66 lines
2.5 KiB
Plaintext
66 lines
2.5 KiB
Plaintext
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SPI NOR framework
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============================================
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Part I - Why do we need this framework?
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---------------------------------------
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SPI bus controllers (drivers/spi/) only deal with streams of bytes; the bus
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controller operates agnostic of the specific device attached. However, some
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controllers (such as Freescale's QuadSPI controller) cannot easily handle
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arbitrary streams of bytes, but rather are designed specifically for SPI NOR.
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In particular, Freescale's QuadSPI controller must know the NOR commands to
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find the right LUT sequence. Unfortunately, the SPI subsystem has no notion of
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opcodes, addresses, or data payloads; a SPI controller simply knows to send or
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receive bytes (Tx and Rx). Therefore, we must define a new layering scheme under
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which the controller driver is aware of the opcodes, addressing, and other
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details of the SPI NOR protocol.
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Part II - How does the framework work?
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--------------------------------------
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This framework just adds a new layer between the MTD and the SPI bus driver.
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With this new layer, the SPI NOR controller driver does not depend on the
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m25p80 code anymore.
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Before this framework, the layer is like:
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MTD
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------------------------
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m25p80
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------------------------
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SPI bus driver
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------------------------
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SPI NOR chip
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After this framework, the layer is like:
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MTD
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------------------------
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SPI NOR framework
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------------------------
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m25p80
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------------------------
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SPI bus driver
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------------------------
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SPI NOR chip
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With the SPI NOR controller driver (Freescale QuadSPI), it looks like:
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MTD
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------------------------
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SPI NOR framework
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------------------------
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fsl-quadSPI
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------------------------
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SPI NOR chip
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Part III - How can drivers use the framework?
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---------------------------------------------
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The main API is spi_nor_scan(). Before you call the hook, a driver should
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initialize the necessary fields for spi_nor{}. Please see
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drivers/mtd/spi-nor/spi-nor.c for detail. Please also refer to fsl-quadspi.c
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when you want to write a new driver for a SPI NOR controller.
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Another API is spi_nor_restore(), this is used to restore the status of SPI
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flash chip such as addressing mode. Call it whenever detach the driver from
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device or reboot the system.
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