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

[Alexander Stein <alexander.stein@ew.tq-group.com>]

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@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.

> The DSI84 EN pin already has a built-in pullup per DSI84 datasheet (see
> Table 5-1. Pin Functions), so that should make the signal rise fast,
> certainly not for seconds.

Here it is >100ms, so the current waiting time is far too less. This results 
in errors regarding PLL lock failure.

Best regards,
Alexander