On Thu, Jul 21, 2016 at 11:52:36AM +0530, Sekhar Nori wrote: > Nishanth, > > On Wednesday 20 July 2016 09:03 PM, Nishanth Menon wrote: > > On 07/20/2016 09:56 AM, Mugunthan V N wrote: > >> Add documention of ti,impedance-control which can be used to > >> correct MAC impedance mismatch using phy extended registers. > >> > >> Signed-off-by: Mugunthan V N <mugunthanvnm@xxxxxx> > >> --- > >> Documentation/devicetree/bindings/net/ti,dp83867.txt | 7 +++++++ > >> 1 file changed, 7 insertions(+) > >> > >> diff --git a/Documentation/devicetree/bindings/net/ti,dp83867.txt > >> b/Documentation/devicetree/bindings/net/ti,dp83867.txt > >> index 5d21141..531ea3c5 100644 > >> --- a/Documentation/devicetree/bindings/net/ti,dp83867.txt > >> +++ b/Documentation/devicetree/bindings/net/ti,dp83867.txt > >> @@ -9,6 +9,13 @@ Required properties: > >> - ti,fifo-depth - Transmitt FIFO depth- see > >> dt-bindings/net/ti-dp83867.h > >> for applicable values > >> > >> +Optional property: > >> + - ti,impedance-control - MAC Interface Impedance control to vary the > >> + output impedance with an approximate range > >> + from 35-70 ohms in 32 steps. Value range can > >> + be 0x0 to 0x1f. Lowest impedance value is > >> + 0x1f and highest impedance being 0x0. > >> + > > > > Should'nt you be using the impedance values of 35 to 70 as the valid > > values here? that would be the hardware description, and the values to > > program corresponding to those are 0x00 to 0x1f. Right? > > > > Rob: is'nt that the right way to do it? Normally, yes that is preferred. I don't have an issue with it here if that is what makes sense. > Agree that that is usually the right way to do it. I believe this case > is bit peculiar though. Here is the extract from the datasheet[1] about > how the register in question works. > > " > Output impedance approximate range from 35-70 ohms in 32 steps. > Lowest being 11110 and highest being 00000. Range and Step size > will vary with process. > > Default is set to 50 ohms by trim. But the default register value can > vary by process. Non default values of MAC I/O impedance can be > used based on trace impedance. Mismatch between device and > trace impedance can cause voltage overshoot and undershoot. > " > > So clearly, there is no easy correspondence that the hardware guarantees > between output impedance ohmage and the register value programmed. Only > couple of things are guaranteed. > > 1) Programming a value of 0 gives approximately 35 ohms > 2) Programming a value of 0x1F gives approximately 70 ohms > 3) Default value of the register gives 50 ohms (the default value could > vary by device). > 4) Programming a value in between will give some ohmage in the > approximate range 35-70 (curve is unknown). > > Given this, I am not sure how one can convert a given user supplied ohms > values to a reasonable register value. Clearly, the register is supposed > to be programmed by experimentation, not calculation. So your are going to change from a known value tuned for the individual Si (50ohm) to the same value for all chips (at least for a given board). Or does this get set by firmware at boot? If not, seems like you would want to specify a delta from the default register value in this case. > That said, we could take another approach. At least for the current > issue we are trying to address, we only need to configure the register > to max value. And given the nature of the register, I am pretty sure > that is what most people will end up doing. How does the max value get determined? Possibly it should just be set in the kernel. > So, may be we have two properties: > > ti,max-output-impedance > ti,min-output-imepdance Just define max if that's all you currently need. > which configure the impedance to maximum and minimum values > respectively. If, in future, someone needs to configure a value in > between, he can introduce a more granular property which when then > override whatever else is specified. > > Regards, > Sekhar > > [1] http://www.ti.com/lit/ds/symlink/dp83867ir.pdf -- To unsubscribe from this list: send the line "unsubscribe devicetree" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
