Hi Pierre,
Before the begining: I don’t want to disrupt your idea - sorry. My intention is simply to inspire you
So, the main thesis is: bitbanging is slow. Within your design, it is even slower. Writing a 128MB
flash this way will put you to sleep - I’ve already done it)
You are placing the controlling logic too high. Just imagine: your software will take a long journey
with every level change - Soft ↔ libgpio ↔ kernel IPC ↔ GPIO controller ↔ GPIO pin. And you
have to do this because you need to track error codes.
GPIO is slow, especially when controlling multiple pins. With this approach, you must provide
an interface that allows the user to configure any GPIO in the system. Otherwise, it will be both
impractical and confusing. This means you will need to manage multiple GPIO controllers and
synchronize them as well - but it won’t necessarily speed things up.
But even bitbang is useful when you do not have another way to do the job. And you already
mentioned the solution. Linux has already bitbang drivers for i2c
Moreover flashrom has interfaces to work with standard linux buses (though I'm not sure about
the status of i2c). As you said you just need to configure them. And I agree that it could be boring.
I’m suggesting that it would be great to have a script or a top-level application that configures the
system and Flashrom to work with bitbanging through just a few clicks or a simple configuration.
For example, the application could detect the system it is running on, check dependencies,
generate a device tree (DT) overlay, install it for the next boot, and either provide the necessary
command-line instructions or launch Flashrom as needed.
This approach might only save 2–3 steps, but those are the slowest ones.
But maybe you are talking about not linux devices, so hope you have direct access to gpios there.
Answering your question: pony_spi.c looks like a good example if you’re interested.
But you will have to study bitbang_spi.c as well and its register_spi_bitbang_master
you need to implement minimum set of function: set_cs, set_sck, set_mosi, get_miso
So copy pony, register its as a new programmer in programmers_table.c, cut everything
around pony's serial and implement 4 bitbanging functions
Good luck
Dmitry