Need a clarification regarding I2C bus frequency in FreeBSD

[Emmanuel Vadot]

On Sun, 19 Aug 2018 13:11:15 +0300
Daniel Braniss <danny at cs.huji.ac.il> wrote:

> 
> 
> > 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 :-)

 It is set at whatever frequency the dts set it and if there is no
frequency in the dts it fallback to 100Khz.
 twsi needs to support IICBUS_GET_FREQUENCY though, the main reason I
never really touched twsi is because the same driver is used on Marvell
SoC with hackish support for the clocks. Now that I have a Marvell
board with this i2c controller I'll probably rewrite this driver.

> I could change this, but is there some hidden reason that it?s so?
> 
> danny
> 
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-- 
Emmanuel Vadot <manu at bidouilliste.com> <manu at freebsd.org>