Need a clarification regarding I2C bus frequency in FreeBSD

[Daniel Braniss]

> On 17 Aug 2018, at 19:26, Ian Lepore <ian at freebsd.org> wrote:
> 
> On Fri, 2018-08-17 at 11:48 +0530, Rajesh Kumar wrote:
>> Hi,
>> 
>> I am trying to use the I2C designware controller driver available in
>> FreeBSD (ig4_iic.c) in our boards.
>> 
>> Is there a clean way, I can set the I2C bus frequency from the controller
>> driver itself, rather than using device hints, FDT, tunables etc.,
>> Something like, if the driver is loaded for our boards (identified using
>> the PCI or ACPI ID's), then the frequency of the I2C bus needs to be
>> hardcoded from driver itself. This is to avoid additional configs from the
>> config file.
>> 
>> I tried adding a new interface "iicbus_set_frequency" (in line with
>> iicbus_get_frequency) and tried calling that from the ig4 driver after the
>> "iicbus" child is added.  But, iicbus instance is created only after ig4
>> driver is loaded. So, calling iicbus_set_frequency after child addition
>> leads to system panic (as there is no iicbus softc at this point).
>> 
>> Let me know if you need any details.
>> 
>> Thanks,
>> Rajesh.
> 
> I don't really understand what you're asking for. The ig4_iic
> controller driver doesn't appear to support bus frequency settings at
> all, it just loads hard-coded values into the clock high/low registers
> and never changes them. If you want to locally modify the driver to run
> at a different hard-coded speed for your application, just change lines
> 589-592 in ig4_iic.c and continue to ignore the speed set by the bus
> driver when handling iicbus_reset.
> 

on the arm/allwinner it’s set at 100Khz, and though there are sysctls to change it,
it’s ignored :-)

I could change this, but is there some hidden reason that it’s so?

danny