[PATCH v6] spi: orion.c: Add direct access mode

[Arnd Bergmann]

On Monday 02 May 2016 08:47:15 Stefan Roese wrote:
> On 20.04.2016 12:11, Stefan Roese wrote:
> > This patch adds support for the direct write mode to the Orion SPI
> > driver which is used on the Marvell Armada based SoCs. In this direct
> > mode, all data written to (or read from) a specifically mapped MBus
> > window (linked to one SPI chip-select on one of the SPI controllers)
> > will be transferred directly to the SPI bus. Without the need to control
> > the SPI registers in between. This can improve the SPI transfer rate in
> > such cases.
> > 
> > Currently only the direct write mode is supported. This mode especially
> > benefits from the SPI direct mode, as the data bytes are written
> > head-to-head to the SPI bus, without any additional addresses, that
> > are also written in the direct read mode.
> > 
> > One use-case for this direct write mode is, programming a FPGA bitstream
> > image into the FPGA connected to the SPI bus at maximum speed.
> > 
> > This mode is described in chapter "22.5.2 Direct Write to SPI" in the
> > Marvell Armada XP Functional Spec Datasheet.
> > 
> > It should be possible to support SPI-NOR and SPI-NAND devices via
> > this direct access mode as well. But this needs further work, e.g.:
> > - The mapping of the MBus window needs to get extended to span
> >    the complete flash device size
> > - The address / control data needs to get inserted into the SPI
> >    controller registers
> > 
> > Signed-off-by: Stefan Roese <sr at denx.de>
> > Cc: Thomas Petazzoni <thomas.petazzoni at free-electrons.com>
> > Cc: Gregory CLEMENT <gregory.clement at free-electrons.com>
> > Cc: Andrew Lunn <andrew at lunn.ch>
> > Cc: Arnd Bergmann <arnd at arndb.de>
> > Cc: Mark Brown <broonie at kernel.org>
> 
> Its been a while since v6 of this direct write access mode has been
> posted. This is a gentle ping on its status.
> 
> Arnd / Mark, do you have any additional change requests or are you okay
> with the current version?

No objections, please add

Acked-by: Arnd Bergmann <arnd at arndb.de>

Two questions though:

- you now always send a multiple of four byte in each transfer, are
  there any downsides in doing this, e.g. some SPI devices that might
  get confused by receiving additional uninitialized data?

- How does the performance compare to the normal mode, is it basically
  unchanged, or does this patch make things faster?

	Arnd