[flashrom] [PATCH][RFC] uisp_spi programmer support

[Andrew]

Hello, all.

I've been playing around with buspirate and my uISP ( 
https://github.com/uISP/ ) dongle.
Even with newer firmware buspirate is really slow, and on average, takes 
3m 49s to read out
a 2MiB SPI flash.

So I decided to play around and see if I can make a faster programmer 
with my uISP
dongle. Basically it's an atmega8 with 12M crystal running vusb stack 
(no hardware usb). Any
other similar hardware would do. This hardware is really cheap (BOM is 
less than 5$ including
the pcb).

My first attempt was creating a serprog-compatible firmware for uISP. It 
can be found here:

https://github.com/uISP/uisp-app-serprog

The results weren't very exciting. It took 4m 30s to read out a 2MiB SPI 
flash (EN25QH16) on
12M crystal. Same for 20M crystal. It seems like it was a limitation of 
bulk out transfers via
vusb.

The second attempt required patching flashrom itself, but resulted in a 
MUCH simpler firmware
that fits roughly 115 lines of code (not counting vusb). It just uses 
control transfers for both
reads and writes. With vusb long transfers enabled, 20Mhz crystal, 10Mhz 
SPI speed and
max_data_read/max_data_write set to 4096 bytes reading a 2MiB SPI flash 
took only 2m 13s which I
consider a WIN.

4096 is not a vusb limit. USB spec limits maximum transfers to control 
endpoint to 4096 bytes anyway.

Using 254 byte max_data_read/max_data_write and long transfers disabled 
would be slower, but it is
possible to fit everything into 2K flash and use some attiny2313, which 
will make it even cheaper.
The protocol's simple as hell, so anyone with a better hardware around 
(STM32, atmega8u2, pic32, whatever)
can implement it with very little effort and get lighting-fast speeds.
My WIP patch to flashrom's attached, although I have a few silly 
questions left:

* Do I need to implement SPI frequency changing via a dedicated command 
or is it okay to hardcode it
   to 10Mhz in firmware?
* Right now I'm using the same spi_read as serprog does. Is okay, or 
should I add an option to disable
   it?
* firmware version checking, etc. (Is it a good idea to implement?)
* Is there a better way to benchmark read/write speed in flashrom, since 
delay loop calibration interferes
   with speed measurements (currently I just used "time flashrom ...")


-- 
Regards,
Andrew
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