[RFC v2 5/6] drivers: boot_constraint: Add initial DT bindings

[Chen-Yu Tsai]

On Thu, Jul 13, 2017 at 5:28 AM, Rob Herring <robh+dt at kernel.org> wrote:
> On Wed, Jul 12, 2017 at 1:34 AM, Viresh Kumar <viresh.kumar at linaro.org> wrote:
>> This adds device tree bindings for boot constraints. Only power supply
>> constraint types are supported currently.
>>
>> Signed-off-by: Viresh Kumar <viresh.kumar at linaro.org>
>> ---
>>  .../devicetree/bindings/boot-constraints.txt       | 68 ++++++++++++++++++++++
>>  1 file changed, 68 insertions(+)
>>  create mode 100644 Documentation/devicetree/bindings/boot-constraints.txt
>>
>> diff --git a/Documentation/devicetree/bindings/boot-constraints.txt b/Documentation/devicetree/bindings/boot-constraints.txt
>> new file mode 100644
>> index 000000000000..9a01ea1e6e72
>> --- /dev/null
>> +++ b/Documentation/devicetree/bindings/boot-constraints.txt
>> @@ -0,0 +1,68 @@
>> +BOOT CONSTRAINTS
>> +================
>> +
>> +Some devices are powered ON by the bootloader before the bootloader handovers
>> +control to the Operating System (OS). It maybe important for those devices to
>> +keep working until the time the OS takes over and starts configuring the devices
>> +again.
>> +
>> +A typical example of that can be the LCD controller, which is used by the
>> +bootloaders to show image(s) while the platform is booting into the Operating
>> +System. The LCD controller can be using some resources, like clk, supplies, etc,
>> +that are shared between several devices. These shared resources should be
>> +configured to satisfy need of all the users. If another device's (X) driver gets
>> +probed before the LCD controller driver in this case, then it may end up
>> +reconfiguring these resources to ranges satisfying the current users (only
>> +device X) and that can make the LCD screen unstable.
>
> Display is a pretty well known use case here. Do you have other
> examples in mind? Other cases I've seen are automotive with keeping
> the backup camera going and CAN bus handling. Though my new car has a
> flicker shortly after coming on, so I guess the handoff doesn't have
> to be completely seemless. :)
>
> [...]
>
>> +       mmc: mmc at 0x0 {
>> +               ...
>> +               ...
>> +               vmmc-supply = <&twl_reg1>;
>> +               vmmcaux-supply = <&twl_reg2>;
>> +               boot-constraint-supplies = "vmmc", "vmmcaux";
>> +               boot-constraint-uV = <1800000 2000000>, /* vmmc */
>> +                                    <2000000 2000000>; /* vmmcaux */
>
> No. I don't like how this is going to extend to all the other bindings
> people are going to want constraints for. We don't need a parallel set
> of properties for each type of binding.
>
> I'm not convinced that we need a general solution for what's probably
> a handful of things that need a handoff versus just re-initialize.

I'm afraid the regulator case still doesn't make sense. The voltage
constraints should be set within each supplies device node. This was
explained in the discussion in v1 [1].

ChenYu

[1] https://www.spinics.net/lists/arm-kernel/msg591692.html