[RFC 0/2] spi: aspeed: SPI1 mode control driver enabling sysfs control

[Kun Yi]

Hello,

I'm looking for feedback on a simple Aspeed SPI1 mode control driver that does two things mainly:
1. Expose a sysfs interface for userland control of SPI1 interface.
2. Optionally override the SPI1 mode at probe if "start_mode" is configured in 
   device tree.

Aspeed 24XX/25XX support three interface modes for their SPI1 interface,
which can be configured via HW strap register. In the typical case of connecting
a host flash chip SPI1 interface:
- SPI master mode makes BMC the master to the host SPI flash
- SPI pass-through mode make whichever external SPI interface connected to SYSSPI 
  be the master
- There are also "disable" and "debug" modes. Aspeed 25XX specified the debug
  mode is reserved though.

In particular, SPI pass-through mode would enable an external SPI master to
directly program host SPI chip and possibly serve as an alternative way to LPC
for host CPU accessing SPI flash. I think by providing a sysfs handle, userspace
applications can have a flexible way to mux host SPI flash ownership between 
BMC/external on the fly.

Overriding the SPI mode at probe time can be useful to make sure 2500-smc flash
controller always identifies the flash chip and create /dev/mtd files correctly.

I wrote the driver as a standalone misc driver for the ease of initial implementation.
There are other options such as merging the functionality into the SMC driver
itself, possibly enabled by optional device tree arguments/nodes, but I just want to
send the patches out first to gather comments on perceived usefulness of
the feature and how you think the driver should be constructed.

Feedback greatly appreciated!

Regards,
Kun