Re: [PATCH v18 0/4] Introduce usb charger framework to deal with the usb gadget power negotation

[Baolin Wang <baolin.wang@linaro.org>]

On 14 November 2016 at 12:21, NeilBrown <neilb@suse.com> wrote:
> On Thu, Nov 10 2016, Baolin Wang wrote:
>
>> Hi
>>
>> On 8 November 2016 at 04:36, NeilBrown <neilb@suse.com> wrote:
>>> On Mon, Nov 07 2016, Baolin Wang wrote:
>>>
>>>> On 3 November 2016 at 09:25, NeilBrown <neilb@suse.com> wrote:
>>>>> On Tue, Nov 01 2016, Baolin Wang wrote:
>>>>
>>>> I agree with your most opinions, but these are optimization.
>>>
>>> I see them as bug fixes, not optimizations.
>>>
>>>>                                                              Firstly I
>>>> think we should upstream the USB charger driver.
>>>
>>> I think you missed the point.  The point is that we don't *need* your
>>> "USB charger driver" because all the infrastructure we need is *already*
>>> present in the kernel.  It is buggy and not used uniformly, and could
>>> usefully be polished and improved.  But the structure is already
>>> present.
>>>
>>> If everyone just added new infrastructure when they didn't like, or
>>> didn't understand, what was already present, the kernel would become
>>> like the Mad Hatter's tea party, full of dirty dishes.
>>>
>>>>                                                  What I want to ask is
>>>> how can we notify power driver if we don't set the
>>>> usb_register_notifier(), then I think you give the answer is: power
>>>> driver can register by 'struct usb_phy->notifier'. But why usb phy
>>>> should notify the power driver how much current should be drawn, and I
>>>> still think we should notify the current in usb charger driver which
>>>> is better, and do not need to notify current for power driver in usb
>>>> phy driver.
>>>
>>> I accept that it isn't clear that the phy *should* be involved in
>>> communicating the negotiated power availability, but nor is it clear
>>> that it shouldn't.  The power does travel through the physical
>>> interface, so physically it plays a role.
>>>
>>> But more importantly, it already *does* get told (in some cases).
>>> There is an interface "usb_phy_set_power()" which exists explicitly to
>>> tell the phy what power has been negotiated.  Given that infrastructure
>>> exists and works, it make sense to use it.
>>>
>>> If you think it is a broken design and should be removed, then fine:
>>> make a case for that.  Examine the history.  Make sure you know why it
>>> is there (or make sure that information cannot be found), and then
>>> present a case as to why it should be removed and replaced with
>>> something else.  But don't just leave it there and pretend it doesn't
>>> exist and create something similar-but-different and hope people will
>>> know why yours is better.  That way lies madness.
>>
>> Like Peter said, it is not only PHY can detect the USB charger type,
>> which means there are other places can detect the charger type.
>
> If I understand Peter's example correctly, it shows a configuration
> where the USB PHysical interface is partly implemented in the SOC and
> partly in the PMIC.  I appreciate that would make it more challenging to
> implement a PHY driver, but there is no reason it should impact anything
> outside of the PHY.

Like Peter's example, it need to use controller register to pull up dp
to begin the secondary detection, which is not belonged to phy driver
and I don't think it is suitable we implemented these in phy driver.

>
>> Second, some controller need to detect the charger type manually which
>> USB phy did not support.
>
> "manually" is an odd term to use in this context.

Sorry for the confusing.

> I think you mean that to detect the charger type you need to issue some
> command to the hardware and wait for it to respond, then assess the
> response.

Yes.

> That isn't at all surprising.  The charger type is detected by measuring
> resistance between ID and GND, which may require setting up a potential
> and activating ADCs to measure the voltage.  This can all be done
> internally to the phy driver.
> Sometimes it is easy (I did https://lkml.org/lkml/2015/2/23/746 for
> twl4030, though it never got upstream).
> The code for the imx7d does look more complex, but not intrinsically
> different.

But you should implement these in every phy driver, why not one
standard framework?

>
>> Third, it did not handle what current should
>> be drawn in USB phy.
>
> The standards define that.  The extcon just reports the cable type.
> Certainly it would be sensible to provide a library function to
> translate from cable type to current range.  You don't need a new
> subsystem to do that, just a library function.

I don't think the extcon should handle current things. For example,
the extcon can not know the gadget speed, which is used to change the
default current values for super speed gadget.

>
>> Fourth, we need integrate all charger plugin/out
>> event in one framework, not from extcon, maybe type-c in future.
>
> Why not extcon?  Given that a charger is connected by an external
> connector, extcon seems like exactly the right thing to use.

My mistake, what I mean is not only from extcon, maybe from other
places in future.

>
> Obviously extcon doesn't report the current that was negotiated, but
> that is best kept separate.  The battery charger can be advised of the
> available current either via extcon or separately via the usb
> subsystem.  Don't conflate the two.
>
>
>>  In a
>> word, we need one standard integration of this feature we need, though
>> like you said we should do some clean up or fix to make it better.
>
> But really, I'm not the person you need to convince.  I'm just a vaguely
> interested bystander who is rapidly losing interest.  You need to
> convince a maintainer, but they have so far shown remarkably little
> interest.  I don't know why, but I'd guess that reviewing a complex new
> subsystem isn't much fun.  Reviewing and applying clear bugfixes and
> incremental improvements is much easier and more enjoyable.  But that is
> just a guess.

Maybe you missed previous comments, and we had a lot of discussion
about this patchset. Also Felipe had reviewed this patchset with some
suggestion.

-- 
Baolin.wang
Best Regards