Re: [PATCH v2] spi: orion.c: Add direct access mode

[Andrew Lunn <andrew-g2DYL2Zd6BY@public.gmane.org>]

> Please note that this direct access mode is really not suitable
> for "normal" SPI devices like SPI NOR chips. As the direct read
> mode (as described in chapter 22.5.1 of the Armada XP datasheet)
> always generates:
> 
> a) a write command to the SPI bus (opcode configurable via register)
> 
> and
> 
> b) writes 1-4 address bytes to the SPI bus
> 
> before the data is read from the SPI bus.
> 
> So its definitely nothing that should be enabled per default for
> all SPI devices.

Looking at the Kirkwood datasheet, it suggests it can be used with SPI
flash:

	If using SPI flash, set the <Attr> field in the CPU address
	decoding windows to match the SPI interface (see Section 2,
	Address Map, on page 34 for more details). Any CPU read to
	this address space is converted by the SPI controller to a SPI
	flash read transaction, composed of an address phase, followed
	by a data phase.

	The actual sequence that is driven on the SPI interface is:

	1. Assert SPI_CSn. Write command to the SPI. The command is
	either Read or Fast_Read, based on the configuration of the
	<DirectRdCommand> field.

	2. Write the address. The Address is driven in 1 to 4 phases
	based on the configuration of the <DirectAddrLen>> field. The order of
	the bytes is MSB to LSB.

	3. In Fast_Read mode, add a one-byte dummy write.

	4. Read the data, according to the length given by the request.

	5. De-assert the SPI_CSn signal.

There is also something interesting in the Direct Write to SPI
section:

	4. Address Phase.

	This is a 1-4 byte field that is taken from the address of the
	request.  The size of the address is configured via the
	<DirectAddrLen> field in the Serial Memory Interface
	Configuration Register (Table 629 p. 686).  The entire Address
	phase is omitted when <Direct Wr Addr Enable> field in the
	Serial Memory Direct Write Configuration Register (Table 634
	p. 688) is cleared.

Turning off the address phase may mean it is possible to do normal SPI
writes using the direct mode.

Quite a few Kirkwood boards have SPI flash, so if this brings better
performance, it would be nice to be able to use it.

	     Andrew
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