Hi Linus,
On 07/01/20 2:52 PM, Harish Jenny K N wrote:
> On 06/01/20 1:42 PM, Geert Uytterhoeven wrote:
>> Hi Rob,
>>
>> On Fri, Dec 6, 2019 at 4:04 PM Rob Herring <robh@xxxxxxxxxx> wrote:
>>> On Fri, Dec 6, 2019 at 3:17 AM Geert Uytterhoeven <geert@xxxxxxxxxxxxxx> wrote:
>>>> On Thu, Dec 5, 2019 at 10:06 PM Rob Herring <robh@xxxxxxxxxx> wrote:
>>>>> On Wed, Nov 27, 2019 at 09:42:50AM +0100, Geert Uytterhoeven wrote:
>>>>>> Add Device Tree bindings for a GPIO repeater, with optional translation
>>>>>> of physical signal properties. This is useful for describing explicitly
>>>>>> the presence of e.g. an inverter on a GPIO line, and was inspired by the
>>>>>> non-YAML gpio-inverter bindings by Harish Jenny K N
>>>>>> <harish_kandiga@xxxxxxxxxx>[1].
>>>>>>
>>>>>> Note that this is different from a GPIO Nexus Node[2], which cannot do
>>>>>> physical signal property translation.
>>>>> It can't? Why not? The point of the passthru mask is to not do
>>>>> translation of flags, but without it you are always doing translation of
>>>>> cells.
>>>> Thanks for pushing me deeper into nexuses!
>>>> You're right, you can map from one type to another.
>>>> However, you cannot handle the "double inversion" of an ACTIVE_LOW
>>>> signal with a physical inverter added:
>>>>
>>>> nexus: led-nexus {
>>>> #gpio-cells = <2>;
>>>> gpio-map = <0 0 &gpio2 19 GPIO_ACTIVE_LOW>, // inverted
>>>> <1 0 &gpio2 20 GPIO_ACTIVE_HIGH>, // noninverted
>>>> <2 0 &gpio2 21 GPIO_ACTIVE_LOW>; // inverted
>>>> gpio-map-mask = <3 0>;
>>>> // default gpio-map-pass-thru = <0 0>;
>>>> };
>>>>
>>>> leds {
>>>> compatible = "gpio-leds";
>>>> led6-inverted {
>>>> gpios = <&nexus 0 GPIO_ACTIVE_HIGH>;
>>>> };
>>>> led7-noninverted {
>>>> gpios = <&nexus 1 GPIO_ACTIVE_HIGH>;
>>>> };
>>>> led8-double-inverted { // FAILS: still inverted
>>>> gpios = <&nexus 2 GPIO_ACTIVE_LOW>;
>>>> };
>>>> };
>>>>
>>>> It "works" if the last entry in gpio-map is changed to GPIO_ACTIVE_HIGH.
>>>> Still, the consumer would see the final translated polarity, and not the
>>>> actual one it needs to program the consumer for.
>>> I'm not really following. Why isn't a double inversion just the same
>>> as no inversion?
>> Because the nexus can only mask and/or substitute bits.
>> It cannot do a XOR operation on the GPIO flags.
>>
>>>>>> While an inverter can be described implicitly by exchanging the
>>>>>> GPIO_ACTIVE_HIGH and GPIO_ACTIVE_LOW flags, this has its limitations.
>>>>>> Each GPIO line has only a single GPIO_ACTIVE_* flag, but applies to both
>>>>>> th provider and consumer sides:
>>>>>> 1. The GPIO provider (controller) looks at the flags to know the
>>>>>> polarity, so it can translate between logical (active/not active)
>>>>>> and physical (high/low) signal levels.
>>>>>> 2. While the signal polarity is usually fixed on the GPIO consumer
>>>>>> side (e.g. an LED is tied to either the supply voltage or GND),
>>>>>> it may be configurable on some devices, and both sides need to
>>>>>> agree. Hence the GPIO_ACTIVE_* flag as seen by the consumer must
>>>>>> match the actual polarity.
>>>>>> There exists a similar issue with interrupt flags, where both the
>>>>>> interrupt controller and the device generating the interrupt need
>>>>>> to agree, which breaks in the presence of a physical inverter not
>>>>>> described in DT (see e.g. [3]).
>>>>> Adding an inverted flag as I've suggested would also solve this issue.
>>>> As per your suggestion in "Re: [PATCH V4 2/2] gpio: inverter: document
>>>> the inverter bindings"?
>>>> https://lore.kernel.org/linux-devicetree/CAL_JsqLp___2O-naU+2PPQy0QmJX6+aN3hByz-OB9+qFvWgN9Q@xxxxxxxxxxxxxx/
>>>>
>>>> Oh, now I understand. I was misguided by Harish' interpretation
>>>> https://lore.kernel.org/linux-devicetree/dde73334-a26d-b53f-6b97-4101c1cdc185@xxxxxxxxxx/
>>>> which assumed an "inverted" property, e.g.
>>>>
>>>> inverted = /bits/ 8 <0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0 0>;
>>>>
>>>> But you actually meant a new GPIO_INVERTED flag, to be ORed into the 2nd
>>>> cell of a GPIO specifier? I.e. add to include/dt-bindings/gpio/gpio.h"
>>>>
>>>> /* Bit 6 expresses the presence of a physical inverter */
>>>> #define GPIO_INVERTED 64
>>> Exactly.
>> OK, makes sense.
>
> The reason I went for "inverted" property is because, we can specify this for gpios at provider side.
>
> The usecase needed to define the polarity which did not have kernel space consumer driver.
>
>
> I am not sure how do we achieve this using GPIO_INVERTED flag. We need some sort of node/gpio-hog to specify these
>
> type of properties? Otherwise gpio-pin will be held by kernel or the module using the hog property and the user space application will not be able to access pin.
>
>
> or please let me know if I am missing something.
>
>
>>>> We need to be very careful in defining to which side the GPIO_ACTIVE_*
>>>> applies to (consumer?), and which side the GPIO_INVERTED flag (provider?).
>>>> Still, this doesn't help if e.g. a FET is used instead of a push-pull
>>>> inverter, as the former needs translation of other flags (which the
>>>> nexus can do, the caveats above still applies, though).
>>> Yes. Historically the cells values are meaningful to the provider and
>>> opaque to the consumer. Standardized cell values changes that
>>> somewhat. I think we want the active flag to be from the provider's
>>> prospective because the provider always needs to know. The consumer
>>> often doesn't need to know. That also means things work without the
>>> GPIO_INVERTED flag if the consumer doesn't care which is what we have
>>> today already and we can't go back in time.
>>>
> Things will work without GPIO_INVERTED flag for consumers which can specify GPIO_ACTIVE_* flags.
>
>
>
>>>> Same for adding IRQ_TYPE_INVERTED.
>>> I suppose so, yes.
>>>
>>>> Related issue: how to handle physical inverters on SPI chip select lines,
>>>> if the SPI slave can be configured for both polarities?
>>> Good question. Perhaps in a different way because we have to handle
>>> both h/w controlled and gpio chip selects.
>>>
>>> However, how would one configure the polarity in the device in the
>>> first place? You have to assert the CS first to give a command to
>>> reprogram it.
>> That's indeed true for a simple SPI slave.
>> But if it is a smarter device (e.g. a generic micro controller), it may use the
>> system's DTB to configure itself.
>>
>> Gr{oetje,eeting}s,
>>
>> Geert
>>
Can you please let me know your inputs on this ?
Now that Geert has sent v4 patch of GPIO Aggregator by "Dropping controversial GPIO repeater", I do not see the above mentioned inverter usecase can be handled anymore.
Is the observation/patch submitted in https://lore.kernel.org/linux-devicetree/dde73334-a26d-b53f-6b97-4101c1cdc185@xxxxxxxxxx/ still not acceptable?
Thanks,
Harish
