[PATCH 1/2] dt-bindings: drm/bridge: ti-sn65dsi83: Add enable delay property

[Marek Vasut]

On 12/9/22 13:21, Alexander Stein wrote:
> Hi Marek,
> 
> Am Freitag, 9. Dezember 2022, 13:02:10 CET schrieb Marek Vasut:
>> On 12/9/22 10:36, Alexander Stein wrote:
>>> Hello Krzysztof,
>>
>> Hi,
>>
>>> Am Freitag, 9. Dezember 2022, 10:07:45 CET schrieb Krzysztof Kozlowski:
>>>> On 09/12/2022 09:54, Alexander Stein wrote:
>>>>> Hello Krzysztof,
>>>>>
>>>>> thanks for the fast feedback.
>>>>>
>>>>> Am Freitag, 9. Dezember 2022, 09:39:49 CET schrieb Krzysztof Kozlowski:
>>>>>> On 09/12/2022 09:33, Alexander Stein wrote:
>>>>>>> It takes some time until the enable GPIO has settled when turning on.
>>>>>>> This delay is platform specific and may be caused by e.g. voltage
>>>>>>> shifts, capacitors etc.
>>>>>>>
>>>>>>> Signed-off-by: Alexander Stein <alexander.stein at ew.tq-group.com>
>>>>>>> ---
>>>>>>>
>>>>>>>    .../devicetree/bindings/display/bridge/ti,sn65dsi83.yaml      | 4
>>>>>>>    ++++
>>>>>>>    1 file changed, 4 insertions(+)
>>>>>>>
>>>>>>> diff --git
>>>>>>> a/Documentation/devicetree/bindings/display/bridge/ti,sn65dsi83.yaml
>>>>>>> b/Documentation/devicetree/bindings/display/bridge/ti,sn65dsi83.yaml
>>>>>>> index 48a97bb3e2e0d..3f50d497cf8ac 100644
>>>>>>> ---
>>>>>>> a/Documentation/devicetree/bindings/display/bridge/ti,sn65dsi83.yaml
>>>>>>> +++
>>>>>>> b/Documentation/devicetree/bindings/display/bridge/ti,sn65dsi83.yaml
>>>>>>>
>>>>>>> @@ -32,6 +32,10 @@ properties:
>>>>>>>        maxItems: 1
>>>>>>>        description: GPIO specifier for bridge_en pin (active high).
>>>>>>>
>>>>>>> +  ti,enable-delay-us:
>>>>>>> +    default: 10000
>>>>>>> +    description: Enable time delay for enable-gpios
>>>>>>
>>>>>> Aren't you now mixing two separate delays? One for entire block on (I
>>>>>> would assume mostly fixed delay) and one depending on regulators
>>>>>> (regulator-ramp-delay, regulator-enable-ramp-delay). Maybe you miss the
>>>>>> second delays in your power supply? If so, the first one might be fixed
>>>>>> and hard-coded in the driver?
>>>>>
>>>>> Apparently there are two different delays: reset time (t_reset) of 10ms
>>>>> as
>>>>> specified by datasheet. This is already ensured by a following delay
>>>>> after
>>>>> requesting enable_gpio as low and switching the GPIO to low in disable
>>>>> path.
>>>>>
>>>>> When enabling this GPIO it takes some time until it is valid on the
>>>>> chip,
>>>>> this is what this series is about. It's highly platform specific.
>>>>>
>>>>> Unfortunately this is completely unrelated to the vcc-supply regulator.
>>>>> This one has to be enabled before the enable GPIO can be enabled. So
>>>>> there is no regulator-ramp-delay.
>>>>
>>>> Your driver does one after another - regulator followed immediately by
>>>> gpio - so this as well can be a delay from regulator (maybe not ramp but
>>>> enable delay).
>>
>> The chip has two separate input pins:
>>
>> VCC -- power supply that's regulator
>> EN -- reset line, that's GPIO
>>
>> Alexander is talking about EN line here.
>>
>>> But this will introduce a section which must not be interrupted or
>>> delayed.
>>> This is impossible as the enable gpio is attached to an i2c expander in my
>>> case.
>>>
>>> Given the following time chart:
>>>    vcc                  set             EN
>>>
>>> enable               GPIO             PAD
>>>
>>>     |                    |<-- t_raise -->|
>>>     |
>>>     | <-- t_vcc_gpio --> |               |
>>>     | <--        t_enable_delay      --> |
>>>
>>> t_raise is the time from changing the GPIO output at the expander until
>>> voltage on the EN (input) pad from the bridge has reached high voltage
>>> level. This is an electrical characteristic I can not change and have to
>>> take into account.
>>> t_vcc_gpio is the time from enabling supply voltage to enabling the bridge
>>> (removing from reset). Minimum t_vcc_gpio is something which can be
>>> addressed by the regulator and is no problem so far. But there is no
>>> upper bound to it.
>> What exactly is your EN signal rise time (should be ns or so)? Can you
>> look at that with a scope , maybe even with relation to the VCC regulator ?
> 
> I checked EN rise time using a scope, it's ~110ms. I not an expert in hardware
> but on the mainboard there is some capacitor attached to this line, which
> increased the time, independent from the internal pull-up.

This does seem like a hardware bug right there, can you double-check 
this with the hardware engineer ?

I would expect the capacitor to charge quickly when you flip the I2C 
expander output HIGH, unless the I2C expander output is open drain, at 
which point the transistor in the output is closed when the output is 
set to HIGH and the capacitor is charging over the DSI83 EN pullup , 
which might be slow.