[RFC RESEND 1/4] arm/dts: OMAP: Add timer nodes

Rob Herring robherring2 at gmail.com
Thu Sep 6 09:45:26 EDT 2012

On 07/23/2012 10:24 AM, Jon Hunter wrote:
> Hi Rob,
> On 07/16/2012 10:56 AM, Jon Hunter wrote:
>> Hi Rob,
>> On 07/13/2012 09:15 PM, Rob Herring wrote:
>>> On 07/13/2012 05:26 PM, Jon Hunter wrote:
>>>> Add the 12 GP timers nodes present in OMAP3.
>>>> Add the 11 GP timers nodes present in OMAP4.
>>>> Add documentation for timer properties specific to OMAP.
>>>> For each timer an alias is being added. The purpose for doing this is because
>>>> the OMAP dmtimer driver uses an ID to distinguish between the different timer
>>>> instances. For example, a timer can be requested by its ID. By adding an alias
>>>> for each timer we can then use the function of_alias_get_id() to extract the
>>>> ID for each timer from the alias name. The same method is used for the TTY
>>>> serial devices. If it is preferred that such an alias is not added and there
>>>> is a better way to pass an ID from device-tree let me know.
>>> I'm not sure this is really a good use of aliases. UARTs use aliases
>>> because it is important that the UART number to tty number is known and
>>> fixed. IIUC, as an example you are picking timer1 because it has
>>> properties X, Y and Z. If so, then you should describe those h/w
>>> properties within the timer nodes so you can pick which timer to use
>>> based on it's h/w properties.
>> Thanks for the feedback. What you suggest could definitely work for most
>> timers. The only item that I would need to resolve here is the handling
>> of system timers (ie. those used for clockevents and clocksource). These
>> system timers (for OMAP) are reserved during early boot based upon the
>> timer ID today and so this is before the actual main timer driver has
>> been probed and all the attributes of the timers has been read for
>> device-tree.
>> One thought would be to move the reservation of the system timers out of
>> the kernel and into device-tree itself. Then we query device tree on
>> start-up to see which we should use. I am wondering if this could be a
>> better use of alias? For example, say I want to use timer1 as my
>> clockevent timer and so I could have an alias of ...
>> alias {
>> 	clockevent_timer = &timer1;
>> }
>> However, I am not sure if this is even correct, because there does not
>> appear to be an API to search the aliases by name and return the
>> phandle, just of_alias_get_id(). Alternatively, I could add another
>> property called "ti,timer-clockevent" that is populated for the timer
>> used as the clockevent timer.
> Do you have any inputs on the above? Does it make sense to reserve timer
> resources for kernel system timers in device-tree?

This issue is not unique to omap. So if we do specify clockevent and
clocksource in dts, then it should be in a common way. I think using
"linux,clockevent" either as an alias name or property within the timer
node would work. I don't have a strong preference, but I tend to lean
toward an alias. Primarily this is because we are already using aliases
for similar purposes.

However, I still think this could be done by looking at properties.
There's not really any reason you can't check properties at timer init
stage. The FDT has already been un-flattened. What are the features or
lack of features you care about to determine which timer to use?


More information about the linux-arm-kernel mailing list