On 06.07.2017 21:03, Stephen Warren wrote:
> On 07/06/2017 11:16 AM, Dmitry Osipenko wrote:
>> On 06.07.2017 19:46, Stephen Warren wrote:
>>> On 07/05/2017 07:24 PM, Peter Chen wrote:
>>>> On Wed, Jul 05, 2017 at 08:19:53PM +0300, Dmitry Osipenko wrote:
>>>>> From: Thierry Reding <treding@xxxxxxxxxx>
>>>>>
>>>>> Override the compatible string of the first USB controller to enable
>>>>> device mode.
>>>>>
>>>>> Signed-off-by: Thierry Reding <treding@xxxxxxxxxx>
>>>>> ---
>>>>> arch/arm/boot/dts/tegra20-trimslice.dts | 2 ++
>>>>> 1 file changed, 2 insertions(+)
>>>>>
>>>>> diff --git a/arch/arm/boot/dts/tegra20-trimslice.dts
>>>>> b/arch/arm/boot/dts/tegra20-trimslice.dts
>>>>> index b902ab594afa..96b4f3f9827b 100644
>>>>> --- a/arch/arm/boot/dts/tegra20-trimslice.dts
>>>>> +++ b/arch/arm/boot/dts/tegra20-trimslice.dts
>>>>> @@ -336,7 +336,9 @@
>>>>> };
>>>>> usb@c5000000 {
>>>>> + compatible = "nvidia,tegra20-udc";
>>>>> status = "okay";
>>>>> + dr_mode = "otg";
>>>>
>>>> If this board supports peripheral-only, you need to
>>>> set dr_mode as "peripheral".
>>>
>>> I agree here. Even if the ports can theoretically do otg, and DT is supposed to
>>> represent HW not SW, given we have no SW support for actual OTG, it'd be best
>>> not to change the DT to OTG until it's actually useful to do so. This comment
>>> applies to the patches I ack'd already too.
>>
>> In case of absence of the "dr_mode" option or without a support of host mode
>> switching, UDC driver fallbacks to the peripheral mode. Given that DT is
>> supposed to be stable, I'm not sure what to do... should I specify the
>> peripheral mode for now (which doesn't represent the HW) or completely remove
>> the "dr_mode"?
>
> I think specifying dr_mode="peripheral" now is fine; we can change it later.
>
> DT ABI compatibility isn't about never changing the DT content, it's about
> making sure that any schema changes occur in a backwards-compatible fashion.
>
> Also in the case of a USB port that does support OTG, the dr_mode property
> represents both (a) the HW's capabilities, and (b) the user's desired (initial,
> if e.g. sysfs modification is possible) mode of operation if the HW has multiple
> capabilities. Given (b), I think it's fine to specify peripheral mode for now,
> and allow the user to change it back to host or OTG mode if they want (although
> OTG mode isn't useful/valid until SW support exists).
Thank you for the clarification.
--
Dmitry
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