Subject: Re: [RFC 5/5] [powerpc] Implement a p1010rdb clock source.

From: Wolfgang Grandegger <[email-masked]>
Date: Mon, 08 Aug 2011 16:19:13 +0200

On 08/08/2011 03:44 PM, Marc Kleine-Budde wrote:
> On 08/08/2011 03:08 PM, Wolfgang Grandegger wrote:
>> On 08/08/2011 01:31 PM, Robin Holt wrote:
>>> On Mon, Aug 08, 2011 at 10:37:58AM +0200, Wolfgang Grandegger wrote:
>>>> On 08/06/2011 04:34 PM, Robin Holt wrote:
>>>>> flexcan driver needs the clk_get, clk_get_rate, etc functions
>>>>> to work.  This patch provides the minimum functionality.
>>>>
>>>> This needs some more general thoughts... apart from the question where
>>>> the code should go.
>>>>
>>>> Like for the MSCAN on the MPC5200, the user should be *able* to select
>>>> an appropriate clock source and divider via DTS node properties.
>>>> Currently it seems, that the DTS properties must match some
>>>> pre-configured values, most likely set by the boot loader. Please
>>>> correct me if I'm wrong. For me this is generic and should go into the
>>>> Flexcan driver. From there, a platform specific function, e.g.
>>>> flexcan_set_clock() might be called.
>>>
>>> OK.  Dug a bit more.  The p1010 built-in clocksource seems to be the
>>> periphereal clock frequency which is system bus frequency divided
>>> by 2.  The clock source can not be changed, but the clock divider can
>>> by freezing the interface and setting the CTRL register.  This appears
>>> to only be done by the boot loader.  I do not see why we can not leave
>>
>> And likely Freescale's bootloader does also fixup the DTS Flexcan node.
>> Ah, oh, there's already someting in the mainline U-BOOT:
>>
>> commit 65bb8b060a873fa4f5188f2951081f6011259614
>> Author: Bhaskar Upadhaya <[email-masked]>
>> Date:   Fri Mar 4 20:27:58 2011 +0530
>>
>>     powerpc/85xx: Fix up clock_freq property in CAN node of dts
>>
>>     Fix up the device tree property associated with the Flexcan clock
>>     frequency. This property is used to calculate the bit timing parameters
>>     for Flexcan.
>>
>>     Signed-off-by: Bhaskar Upadhaya <[email-masked]>
>>     Signed-off-by: Kumar Gala <[email-masked]>
>>
>>
>>> that functionality in the boot loader and then go back to a variation
>>> on my earlier flexcan_clk_* patch.  Is that close to the direction you
>>> think we should go or have I completely misunderstood your wishes?
>>
>> The boot loader might not chose the optimum clock source and frequency,
>> which might even be application dependent. Therefore it would be nice to
>> allow the user to change it if necessary. Some CAN interfaces do even
>> allow to use an external clock source. The main question is where we add
>> that functionality. As more as I think of it, the clock interface would
>> not be that bad, especially if it's available.
>>
>> Furthermore, if the bootloader sets the clock source and divider, we do
>> not need device tree properties for it. A simply register lookup would
>> reveal what values are used. We may just need the input clock source.
> 
> If the bootloader touches the divider _in_ the flexcan core, that would
> make absolutely no sense. The clock divider in the flexcan core (in the
> CTRL register) is the bitrate pre-scaler calculated by the bit-timing
> algorithm.

Right, as I realized in the meantime. I'm still looking for some special
p1010 registers for the divider. Unfortunately, the manual is only
available under NDA :-(.

> What we need in the device tree is, from my point of view.
> a) the used clock source (bus clock or xtal clock)
> b) the frequency of that clock

Yes, and maybe an additional divider, like available for the MPC512x:

http://lxr.linux.no/#linux+v3.0.1/Documentation/devicetree/bindings/net/can/mpc5xxx-mscan.txt
http://lxr.linux.no/#linux+v3.0.1/drivers/net/can/mscan/mpc5xxx_can.c#L132

Here is documented what you can expect from the PowerPC SOCs:

http://lxr.linux.no/#linux+v3.0.1/Documentation/devicetree/bindings/net/can/fsl-flexcan.txt

And there they also speak an *additional" clock divider. Maybe that
forseen for the next generations. U Bhaska, could you clarify that? Thanks?

> These problems are solved on arm via:
> a) bus clock is hard coded [1]
> b) get that clock frequency via clk_get_rate().

OK. The clk interface is fine and it should derive the frequency from
the relevant register settings and the bus clock frequency.

> Marc
> 
> [1] I just talked to Sascha (the i.mx maintainer), there's no support
> for the xtal clock, which is the OSC_AUDIO on mx35, in the i.mx clock
> framework so far.

OK. We may want to provide an interface to select taht sometimes later,
also because the P1010 does only support *one* clock source.

Wolfgang.