Re: [PATCH v4 1/2] watchdog: imx2_wdt: add external reset support via 'ext-reset-output' dt prop

[Tim Harvey <tharvey@gateworks.com>]

On Wed, Dec 2, 2015 at 11:11 AM, Akshay Bhat <akshay.bhat@timesys.com> wrote:
>
>
> On 11/06/2015 05:02 PM, Guenter Roeck wrote:
>>
>> On Fri, Nov 06, 2015 at 11:53:42AM -0800, Tim Harvey wrote:
>>>
>>> On Thu, Nov 5, 2015 at 2:23 PM, Guenter Roeck <linux@roeck-us.net> wrote:
>>>>
>>>> On Thu, Nov 05, 2015 at 04:19:21PM -0500, Akshay Bhat wrote:
>>>>>
>>>>> From: Tim Harvey <tharvey@gateworks.com>
>>>>>
>>>>> The IMX6 watchdog supports assertion of a signal (WDOG_B) which
>>>>> can be pinmux'd to an external pin. This is typically used for boards
>>>>> that
>>>>> have PMIC's in control of the IMX6 power rails. In fact, failure to use
>>>>> such an external reset on boards with external PMIC's can result in
>>>>> various
>>>>> hangs due to the IMX6 not being fully reset [1] as well as the board
>>>>> failing
>>>>> to reset because its PMIC has not been reset to provide adequate
>>>>> voltage for
>>>>> the CPU when coming out of reset at 800Mhz.
>>>>>
>>>>> This uses a new device-tree property 'ext-reset-output' to indicate the
>>>>> board has such a reset and to cause the watchdog to be configured to
>>>>> assert
>>>>> WDOG_B instead of an internal reset both on a watchdog timeout and in
>>>>> system_restart.
>>>>>
>>>>> [1]
>>>>> http://lists.infradead.org/pipermail/linux-arm-kernel/2015-March/333689.html
>>>>>
>>>>> Cc: Lucas Stach <l.stach@pengutronix.de>
>>>>> Signed-off-by: Tim Harvey <tharvey@gateworks.com>
>>>>> ---
>>>>>   .../devicetree/bindings/watchdog/fsl-imx-wdt.txt     |  2 ++
>>>>>   drivers/watchdog/imx2_wdt.c                          | 20
>>>>> ++++++++++++++++++--
>>>>>   2 files changed, 20 insertions(+), 2 deletions(-)
>>>>>
>>>>> diff --git a/Documentation/devicetree/bindings/watchdog/fsl-imx-wdt.txt
>>>>> b/Documentation/devicetree/bindings/watchdog/fsl-imx-wdt.txt
>>>>> index 8dab6fd..9b89b3a 100644
>>>>> --- a/Documentation/devicetree/bindings/watchdog/fsl-imx-wdt.txt
>>>>> +++ b/Documentation/devicetree/bindings/watchdog/fsl-imx-wdt.txt
>>>>> @@ -9,6 +9,8 @@ Optional property:
>>>>
>>>>
>>>> properties ?
>>>>
>>>>>   - big-endian: If present the watchdog device's registers are
>>>>> implemented
>>>>>     in big endian mode, otherwise in native mode(same with CPU), for
>>>>> more
>>>>>     detail please see:
>>>>> Documentation/devicetree/bindings/regmap/regmap.txt.
>>>>> +- ext-reset-output: If present the watchdog device is configured to
>>>>> assert its
>>>>
>>>>
>>>> Should that have a vendor prefix ? Also, not sure if "-output"
>>>> has any real value in the property name. "fsl,external-reset", maybe ?
>>>
>>>
>>> Hi Guenter,
>>>
>>> I don't see why a vendor prefix is necessary - its a feature of the
>>> IMX6 watchdog supported by this driver to be able to trigger an
>>> internal chip-level reset and/or an external signal that can be hooked
>>> to additional hardware.
>>>
>> Sounded like vendor specific to me, but then I am not a devicetree
>> maintainer,
>> so I am not an authority on the subject.
>
>
> Devicetree maintainers,
>
> Any thoughts?
>
> Tim,
>
> After looking at all the other watchdog drivers, it does not appear that
> there is any other processor which uses a similar feature. Since imx is the
> only processor that appears to support this feature, it might make sense in
> making this vendor specific. If in the future it is found more processors
> support a similar functionality, it can be revisited and moved out from
> being vendor specific?
>

I'm certainly no expert on device-tree policy. I understand your
point, but realize that the driver in question is imx2_wdt.c
(compatible = "fsl,imx21-wdt"). This is an IP block inside the silicon
of only Freescale chips, so its not like a future omap chip would be
using this driver - only fsl devices. So why would it need a 'vendor'
property any more than its other properties?

Regards,

Tim