[coreboot] i2c_writeb vs i2c_raw_write output

Julius Werner jwerner at chromium.org
Sat Oct 8 02:51:37 CEST 2016

I2C is a very simple bus format with some common conventions attached
to it that are followed by most (but not all) devices. You can find a
pretty good summary of how it works on Wikipedia:

For devices which follow this convention, the device exposes registers
that usually have a 1-byte register address and contain somewhere
between 1 to 4 bytes of data (the convention isn't super strict in
many aspects, so a lot of it is device dependent). In order to write
to a register you're supposed to make two consecutive transactions
(with what's called a "repeated start" in between): one transaction
that writes the 1-byte register address, and one transaction that
writes the 1-4 bytes of data you want to write into that register.
Similarly, for a read you first do a write transaction with the
register address, followed by a read transaction where the device will
return the register contents.

i2c_readb() and i2c_writeb() are convenience wrappers that encapsulate
this whole thing for registers containing 1-byte values for you. This
is what you want for most devices, but sometimes you have a device
that doesn't adhere to the conventions and needs special treatment
(e.g. maybe it doesn't really have a concept of registers at all and
just treats all data as a single stream or something). For those cases
you have i2c_read_raw() and i2c_write_raw(): these just directly read
or write any amount of raw bytes you want out on the bus. Therefore,
'val = i2c_readb(1, 2, 3, 4)' is roughly equivalent to 'reg = 3 &&
i2c_write_raw(1, 2, &reg, 1) && i2c_read_raw(1, 2, &val, 1)'.

Also, I think all of these functions only work on non-x86 devices
right now so they're probably not really helpful to you (I'd be
surprised if they even compile and link for your board). The I2C API
is unfortunately one of those areas where x86 and non-x86 coreboot
still lead very separate side-by-side lives right now. For x86 you
probably need the i2c_dev_xxx() functions, which should in theory work
similar but I have no experience with them. You might also need to use
the smbus_xxx() functions instead (SMBUS is some
kinda-the-same-but-not-quite thing related to I2C on x86 that I never
bothered to fully understand).

On Thu, Sep 29, 2016 at 3:10 PM, Haleigh Novak <haleigh at edt.com> wrote:
> Hello All,
> I am currently trying to send POST codes from Coreboot (downloaded
> approximately a month ago) to a Beagle I2C/SPI reader, specifically via i2c.
> I don't know the register needed for i2c_writeb() so I figured I would use
> i2c_raw_write(). In general since I have been unable to locate anything
> explaining the difference between the two functions I would really
> appriciate it if someone could explain it or direct me to where I can find
> documentation. I am hoping that the explaination/resource will either tell
> me where / or direct me to how I can figure out where the i2c_raw_write will
> write to (if it will just write directly to the Beagle or somewhere else)?
> Or would it be better to send it to a register with i2c_writeb()? I am
> running Coreboot on an ASROCK Mainboard, model IMB-A180.
> Thank you.
> HN
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