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

[Serge Semin]

On Thu, Jul 22, 2021 at 01:09:05PM -0700, John Stultz wrote:
> On Thu, Jul 22, 2021 at 12:17 PM Bjorn Andersson
> <bjorn.andersson@xxxxxxxxxx> wrote:
> > > On Jul 21, 2021, 1:45 PM +0200, Krzysztof Kozlowski wrote:
> > > > 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.
> > > >
> > > > Really, all normal systems depend on aliases or names and here we have
> > > > dependency on device address. I proposed way how AOSP should be fixed.
> > > > Anything happened? Nope.
> > >
> > > First time he sent a possible solution for the problem:
> > > https://lore.kernel.org/lkml/20201221210423.GA2504@kozik-lap/
> > >
> > > To sum up you could have used one of the more portable approaches
> > > 1. add an udc alias to the controller and use it then to refer to the
> > > corresponding USB controller
> >
> > Is there such a thing as "UDC alias"? Or are you suggesting that we
> > should add such feature?
> >
> > I think it would be wonderful if we could identify the UDCs on our
> > boards as "USB1" and "USB2", or "the one and only USB-C connector". But
> > unless that will fall back to the existing naming it would break John's
> > _existing_ userspace.
> 

> Well, I'd not hold up the existing userspace I'm using as sacrosanct
> (AOSP devices still usually don't have to work cross-kernel versions -
> devboards being the main exception). I'm fine if we can rework
> userland as proposed, so that the issues can be avoided, but I
> honestly don't have enough context to really understand what that
> looks like (no idea what udc aliases are).
> 
> And whatever we do, the main constraint is that userland has to be
> able to work with existing LTS kernels and newer kernels.

As I said in my response to Bjorn even if it is added to the kernel it
won't get to the official LTSes as it would be a new kernel feature.
New features aren't normally backported to the older kernels.

> 
> > > 2. search through DT-nodes looking for a specific compatible/reg
> > > DT-properties.
> > >
> >
> > We could define that this is the way, but again we didn't yesterday so
> > your proposal is not backwards compatible.
> 

> It may be backwards compatible, but I'm still not clear exactly how it
> would work.
> 
> I guess if we look through all
> sys/bus/platform/devices/*/of_node/compatbile strings for the desired
> "snps,dwc3", then find which of the directories have the desired
> address in the string? (The suggestion for looking at reg seems
> better, but I don't get a char value out of the dwc3 of_node/reg
> file).

The algorithm is simple:
1) You know what USB controllers you have on your platform. They are
supposed to be compatible with snps,dwc3 string and have some pre-defined
base address.
2) Find all the files in the directory /sys/class/udc/.
3) Walk through all the directories in /sys/bus/platform/devices/ with
names found in 2) and stop on the device with matching compatible/base
address defined in 1).

In my case the strings could be retrieved like this:
USB_NAME_COMPAT=$(/sys/bus/platform/devices/1f100000.usb/of_node/compatible | tr '\0' '\t' | cut -f1)
USB_DEVICE_ADDR="$(head -c 4 /sys/bus/platform/devices/1f100000.usb/of_node/reg | hexdump -ve '/1 "%02x"' | sed -e 's/^0*//g')"

Regards,
-Sergey

> 
> It seems much more straightforward to do an `ls /sys/class/udc/$GADGET_ADDR.*`
> 
> But again, if folks decide the names can be rearranged to usb.<addr>
> in the future, that would break too.
> 
> thanks
> -john