Re: SPI driver

[Swift Griggs <swiftgriggs%gmail.com@localhost>]

On Thu, 7 Feb 2019, Michael van Elst wrote:
> I've written a userland interface to our spi(4) driver.

Neat. I'm interested in SPI, but pardon me if I am a bit ignorant. If you 
don't have time to answer it's no problem.

> crw-------  1 root  wheel  346, 0 Jan  5 16:02 /dev/spi0

I assume there is no concept of a block-mode for SPI, directly. It's only 
a transport like I2C or JTAG right? Don't SD cards use it for IO or 
something related ?

> SPI_IOCTL_CONFIGURE calls spi_configure() to chose operational mode and 
> clock.

I see the struct sets a clock in Hz. Is SPI always synchronous ? Can one 
vary the clock frequency ? Is there a way to sense what the other side 
will max out at ? How can one know the width of data-bits in a cycle or 
does it not matter and you just set sendlen & recvlen and not worry about 
it ?

> The interface is different from spigen(4) used by FreeBSD and also 
> different from an early example of an SPI driver by Jonathan Kollasch.

Why didn't you just steal it? I'm kidding, but still a bit curious.

> As there is no kind of session handling,

Is SPI meant to be stateful but the driver can't do it yet or is that just 
how SPI works intrinsically ? Isn't it kinda low level bit-banging stuff 
where you control which wire is doing what and who's on edge vs level 
etc... ? So, you're saying "sessions" are just the responsibility of the 
user/implementer, right ?

Thanks for your hard work, Michael.

-Swift