Re: [PATCH v3 1/4] dt-bindings: power: reset: add document for reboot-mode driver

[John Stultz]

On Thu, Feb 4, 2016 at 8:35 PM, Rob Herring <robh@xxxxxxxxxx> wrote:
> On Thu, Feb 04, 2016 at 03:46:15PM -0800, John Stultz wrote:
>> On Thu, Feb 4, 2016 at 3:08 PM, Rob Herring <robh@xxxxxxxxxx> wrote:
>> > On Tue, Feb 02, 2016 at 05:59:11PM +0800, Andy Yan wrote:
>> >> +Example:
>> >> +     reboot-mode {
>> >> +             mode-normal = <BOOT_NORMAL>;
>> >> +             mode-recovery = <BOOT_RECOVERY>;
>> >> +             mode-fastboot = <BOOT_FASTBOOT>;
>> >
>> > I tend to agree with John on calling this mode-bootloader.
>> >
>> > OTOH, fastboot is more specific about what the mode is. The name in DT
>> > and the userspace name don't necessarily have to be the same.
>>
>> Wait. This is a bit confusing. The utility of adding a property name
>> and using that name be the reboot command parsed for made sense
>> (compared to earlier versions which had command strings) as it made
>> the dts more terse.   But it sounds here like you're suggesting we
>> should have some logic in the driver that translates "reboot fastboot"
>> to mode-bootloader or vice versa.
>
> I said early on the DT names and kernel-userspace names should not
> necessarily be linked. They can be, but we shouldn't require that.

Sigh. Ok. It seemed it was due to earlier comments (maybe from others,
but I thought it was you), that we moved from specifying a command
string, to using the label. But if you think the label name and the
commands shouldn't be linked, it seems like we should re-introduce
that. No?

Unless your thinking we need some sort of static in-kernel mapping of
commands to label names? But that just seems painfully indirect for
little gain ("Its obvious! For that mode, you use this term here, and
that different term over there!").

> My concern with mode-bootloader is what if you can boot into multiple
> bootloader modes. Say USB mass storage is one option. "bootloader" is
> not real specific.

True. But as I think we agreed below, "bootloader" and "recovery" are
basically defacto standards, and I think it would be a bad idea to try
to declare all the existing android tooling and docs wrong just
because the command is vague, technically.


>> >> +             mode-loader = <BOOT_LOADER>;
>> >
>> > This one needs a better name. Maybe it should be 'rockchip,mode-loader'
>> > as it is vendor specific. Either way, loader is vague. Perhaps
>> > rockchip,mode-bl-download?
>>
>> Hrm. So how what reboot command do you expect to trigger that?
>
> Whatever your OS has defined to map to that.
>
> We could just decide the kernel will strip <vendor> and 'mode-' and
> match commands against what remains.

That part sounds sane, although I do think having vendor prefixes are
reasonable for actual commands as well.


>> I think one of the difficult things here is that there's no real
>> standards for all bios/bootloader modes. So they are somewhat
>> firmware/bootloader/device specific, and thus we need something that
>> is flexible enough to allow lots of different modes to be easily
>> specified.  That said, this does expose a userspace interface (though
>> one could argue kernel ABI doesn't cross reboots :) so we should try
>> to have some consistency so the same userspace can work on various
>> devices.
>
> There is: UEFI. Boot mode efivars are standard. But then they are pretty
> much PC oriented though. It is more which device to boot off of, but
> there is network boot or boot to bios setup.

Well, there's a partial standard there.  I'm told for android on x86,
there is no UEFI standard way to communicate rebooting to fastboot or
recovery. Every device does its own device specific driver.


>> I do think the "bootloader" and "recovery" arguments are somewhat
>> defacto standards, well established on most android devices.
>
> Yes, otherwise I'd be completely against "mode-bootloader" as the
> property.

Ok.

>> I think here the concern is rockchip probably has some userspace that
>> is already using "reboot maskrom" or "reboot loader" for their own
>> uses. And its a bit of a pain to ask that userspace to be reworked to
>
> Perhaps those are only for development and change would not be so
> painful.

Andy: Can you comment here? How critical are the specific commands
you've used here for your userspace?


>> use "reboot rom-download" or "reboot rockchip,rom-download" depending
>> on how we try to deal with these.  (Granted, non-upstream interfaces
>> aren't official, so that is their risk somewhat, but we avoid being
>> smug about that :)
>
> Or vendor specific modes require vendor specific translation in the
> kernel.

True.


> To some extent we need to design what is right and worry about future
> devices rather than cater to past devices. There's always some
> compromise. What would you design ignoring the existing conditions.
> Start there and then figure out how to make it work with current
> designs.

Fair enough. I just want to make sure we're not getting too caught up
with design purity and are willing the meet the world where it is.

>> Another part of the issue is there isn't really a way to probe for
>> reboot cmd capability here. As much as I'd rather not complicate
>> things, one couldn't easily extend existing userspace to work with
>> current kernels as well as future kernels, since the reboot with an
>> invalid command won't fail. The machine still resets. So you can't try
>> one and fallback to the other.
>
> Well, that's nice. Maybe we should change that? Or we're stuck with
> that ABI?

Maybe? I'm not sure what might have trouble with reboot failing if it
sticks any extra noise in the reboot command.
I'd probably lean towards sticking with the existing behavior.

>> Maybe there needs to be a sysfs entry with the list of the supported commands?
>
> You can just read the DT. Although, the problem then is what happens
> when we move to the next firmware interface. We see that some with
> devices having userspace dependencies on ATAGS.

Heh. I guess. Thought I suspect "Just read the DT to sort out the
available reboot modes" is probably not what most userspace wants to
hear. :)

thanks
-john