Re: [PATCH 29/29] arm64: dts: qcom: Harmonize DWC USB3 DT nodes name

Serge Semin
2021-07-21T11:39:34Z

On Wed, Jul 21, 2021 at 01:10:19PM +0200, Krzysztof Kozlowski wrote:
> On 21/07/2021 13:02, Greg Kroah-Hartman wrote:
> > On Wed, Jul 21, 2021 at 12:45:32PM +0200, Krzysztof Kozlowski wrote:
> >> On 21/07/2021 12:29, Greg Kroah-Hartman wrote:
> >>> On Wed, Jul 21, 2021 at 01:02:20PM +0300, Serge Semin wrote:
> >>>> Hi Greg,
> >>>> @Krzysztof, @Rob, please join the discussion so to finally get done
> >>>> with the concerned issue.
> >>>>
> >>>> On Wed, Jul 21, 2021 at 09:38:54AM +0200, Greg Kroah-Hartman wrote:
> >>>>> On Wed, Jul 14, 2021 at 03:48:07PM +0300, Serge Semin wrote:
> >>>>>> Hello John,
> >>>>>>
> >>>>>> On Tue, Jul 13, 2021 at 05:07:00PM -0700, John Stultz wrote:
> >>>>>>> On Tue, Oct 20, 2020 at 5:10 AM Serge Semin
> >>>>>>> <Sergey.Semin@baikalelectronics.ru> wrote:
> >>>>>>>>
> >>>>>>>> In accordance with the DWC USB3 bindings the corresponding node
> >>>>>>>> name is suppose to comply with the Generic USB HCD DT schema, which
> >>>>>>>> requires the USB nodes to have the name acceptable by the regexp:
> >>>>>>>> "^usb(@.*)?" . Make sure the "snps,dwc3"-compatible nodes are correctly
> >>>>>>>> named.
> >>>>>>>>
> >>>>>>>> Signed-off-by: Serge Semin <Sergey.Semin@baikalelectronics.ru>
> >>>>>>>
> >>>>>>
> >>>>>>> I know folks like to ignore this, but this patch breaks AOSP on db845c. :(
> >>>>>>
> >>>>>> Sorry to hear that. Alas there is no much can be done about it.
> >>>>>
> >>>>> Yes there is, we can revert the change. We do not break existing
> >>>>> configurations, sorry.
> >>>>
> >>>> By reverting this patch we'll get back to the broken dt-bindings
> >>>> since it won't comply to the current USB DT-nodes requirements
> >>>> which at this state well describe the latest DT spec:
> >>>> https://github.com/devicetree-org/devicetree-specification/releases/tag/v0.3
> >>>> Thus the dtbs_check will fail for these nodes.
> >>>>
> >>>> Originally this whole patchset was connected with finally getting the
> >>>> DT-node names in order to comply with the standard requirement and it
> >>>> was successful mostly except a few patches which still haven't been
> >>>> merged in.
> >>>>
> >>>> Anyway @Krzysztof has already responded to the complain regarding this
> >>>> issue here:
> >>>> https://lore.kernel.org/lkml/20201221210423.GA2504@kozik-lap/
> >>>> but noone cared to respond on his reasonable questions in order to
> >>>> get to a suitable solution for everyone. Instead we are
> >>>> getting another email with the same request to revert the changes.
> >>>> Here is the quote from the Krzysztof email so we could continue the
> >>>> discussion:
> >>>>
> >>>> On Mon, 21 Dec 2020 13:04:27 -0800 (PST), Krzysztof Kozlowski <krzk@kernel.org> wrote:
> >>>>> On Mon, Dec 21, 2020 at 12:24:11PM -0800, John Stultz wrote:
> >>>>>> On Sat, Dec 19, 2020 at 3:06 AM Krzysztof Kozlowski <krzk@kernel.org> wrote:
> >>>>>>> ...
> >>>>>>>
> >>>>>>> The node names are not part of an ABI, are they? I expect only
> >>>>>>> compatibles and properties to be stable. If user-space looks for
> >>>>>>> something by name, it's a user-space's mistake. Not mentioning that you
> >>>>>>> also look for specific address... Imagine remapping of addresses with
> >>>>>>> ranges (for whatever reason) - AOSP also would be broken? Addresses are
> >>>>>>> definitely not an ABI.
> >>>>>>
> >>>>>> Though that is how it's exported through sysfs.
> >>>>>
> >>>>> The ABI is the format of sysfs file for example in /sys/devices. However
> >>>>> the ABI is not the exact address or node name of each device.
> >>>>>
> >>>>>> In AOSP it is then used to setup the configfs gadget by writing that
> >>>>>> value into /config/usb_gadget/g1/UDC.
> >>>>>>
> >>>>>> Given there may be multiple controllers on a device, or even if its
> >>>>>> just one and the dummy hcd driver is enabled, I'm not sure how folks
> >>>>>> reference the "right" one without the node name?
> >>>>>
> >>>>> I think it is the same type of problem as for all other subsystems, e.g.
> >>>>> mmc, hwmon/iio. They usually solve it either with aliases or with
> >>>>> special property with the name/label.
> >>>>>
> >>>>>> I understand the fuzziness with sysfs ABI, and I get that having
> >>>>>> consistent naming is important, but like the eth0 -> enp3s0 changes,
> >>>>>> it seems like this is going to break things.
> >>>>>
> >>>>> One could argue whether interface name is or is not ABI. But please tell
> >>>>> me how the address of a device in one's representation (for example DT)
> >>>>> is a part of a stable interface?
> >>>>>
> >>>>>> Greg? Is there some better way AOSP should be doing this?
> >>>>>
> >>>>> If you need to find specific device, maybe go through the given bus and
> >>>>> check compatibles?
> >>>>>
> >>>>> Best regards,
> >>>>> Krzysztof
> >>>>
> >>>> So the main question is how is the DT-node really connected with ABI
> >>>> and is supposed to be stable in that concern?
> >>>>
> >>>> As I see it even if it affects the configfs node name, then we may
> >>>> either need to break that connection and somehow deliver DT-node-name
> >>>> independent interface to the user-space or we have no choice but to
> >>>> export the node with an updated name and ask of user-space to deal
> >>>> with it. In both suggested cases the DT-node name will still conform
> >>>> to the USB-node name DT spec. Currently we are at the second one.
> >>>
> >>> I really do not care what you all decide on, but you CAN NOT break
> >>> existing working systems, sorry. That is why I have reverted this
> >>> change in my tree and will send it to Linus soon.
> >>
> >> I had impression that kernel defines interfaces which should be used and
> >> are stable (e.g. syscalls, sysfs and so on). This case is example of
> >> user-space relying on something not being marked as part of ABI. Instead
> >> they found something working for them and now it is being used in "we
> >> cannot break existing systems". Basically, AOSP unilaterally created a
> >> stable ABI and now kernel has to stick to it.
> >
> > Since when are configfs names NOT a user-visable api?
> >
> > Why would you not depend on them?
>

> It's not good example. The configfs entries (file names) are
> user-visible however the USB gadget exposes specific value for specific
> one device. It encodes device specific DT node name and HW address and
> gives it to user-space. It is valid only on this one HW, all other
> devices will have different values.
>
> User-space has hard-coded this value (DT node name and hardware
> address). This value was never part of configfs ABI, maybe except of its
> format "[a-z]+\.[0-9a-f]+". Format is not broken. Just the value changes
> for a specific device/hardware.
>
> It's like you depend that lsusb will always report:
> Bus 003 Device 008: ID 046d:c52b Logitech, Inc. Unifying Receiver
> and then probing order changed and this Logitech ends as Device 009.
> Then AOSP guys come, wait, we hard-coded that Logitech on our device
> will be always Device 008, not 009. Please revert it, we depend on
> specific value of Device number. It must be always 009...
>
> For the record - the change discussed here it's nothing like USB VID/PID. :)

Right I was wrong referring to the configfs names in this context.
That must have mislead Greg.

Getting back to the topic AFAICS from what John said in here
https://lore.kernel.org/lkml/CALAqxLWGujgR7p8Vb5S_RimRVYxwm5XF-c4NkKgMH-43wEBaWg@mail.gmail.com/

AOSP developers somehow hardcoded a USB-controller UDC name in the
internal property called "sys.usb.controller" with a value
"ff100000.dwc3". That value is generated by the kernel based on the
corresponding DT-node name. The property is then used to
pre-initialize the system like it's done here:
https://android.googlesource.com/platform/system/core/+/master/rootdir/init.usb.configfs.rc

Since we changed the DT-node name in the recent kernel, we thus changed the
UDC controller name so AOSP init procedure now fails to bring up the Linux
USB-gadget using on the older UDC name. UDC is supposed to be ff100000.usb now
(after this patch has been merged in).

What problems I see here:
1) the AOSP developers shouldn't have hard-coded the value but read
from the /sys/class/udc/* directory and then decided which controller
to use. As it's described for instance here:
https://www.kernel.org/doc/Documentation/usb/gadget_configfs.txt
2) even if they hard-coded the value, then they should have used an
older dts file for their platform, since DTS is more
platform-specific, but not the kernel one. Even if a dts-file is
supplied in the kernel it isn't supposed to have the node names
unchanged from release to release.

Regards,
-Sergey

>
> Best regards,
> Krzysztof