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Message-ID: <aa9fdfc04c3b6a3bba688bac244a157242faab82.camel@icenowy.me>
Subject: Re: [PATCH v2 1/2] usb: misc: onboard_usb_hub: Don't create
 platform devices for DT nodes without 'vdd-supply'
From:   Icenowy Zheng <uwu@icenowy.me>
To:     Doug Anderson <dianders@chromium.org>,
        Matthias Kaehlcke <mka@chromium.org>
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        linux-kernel@vger.kernel.org, linux-usb@vger.kernel.org,
        Stefan Wahren <stefan.wahren@i2se.com>, stable@vger.kernel.org,
        Ravi Chandra Sadineni <ravisadineni@chromium.org>,
        Alexander Stein <alexander.stein@ew.tq-group.com>
Date:   Fri, 23 Dec 2022 15:46:45 +0800
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References: <20221222022605.v2.1.If5e7ec83b1782e4dffa6ea759416a27326c8231d@changeid>
         <CAD=FV=XNxZ3iDYAAqKWqDVLihJ63Du4L7kDdKO55avR9nghc5A@mail.gmail.com>
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=E5=9C=A8 2022-12-22=E6=98=9F=E6=9C=9F=E5=9B=9B=E7=9A=84 11:26 -0800=EF=BC=
=8CDoug Anderson=E5=86=99=E9=81=93=EF=BC=9A
> Hi,
>=20
> On Wed, Dec 21, 2022 at 6:26 PM Matthias Kaehlcke <mka@chromium.org>
> wrote:
> >=20
> > The primary task of the onboard_usb_hub driver is to control the
> > power of an onboard USB hub. The driver gets the regulator from the
> > device tree property "vdd-supply" of the hub's DT node. Some boards
> > have device tree nodes for USB hubs supported by this driver, but
> > don't specify a "vdd-supply". This is not an error per se, it just
> > means that the onboard hub driver can't be used for these hubs, so
> > don't create platform devices for such nodes.
> >=20
> > This change doesn't completely fix the reported regression. It
> > should fix it for the RPi 3 B Plus and boards with similar hub
> > configurations (compatible DT nodes without "vdd-supply"), boards
> > that actually use the onboard hub driver could still be impacted
> > by the race conditions discussed in that thread. Not creating the
> > platform devices for nodes without "vdd-supply" is the right
> > thing to do, independently from the race condition, which will
> > be fixed in future patch.
> >=20
> > Fixes: 8bc063641ceb ("usb: misc: Add onboard_usb_hub driver")
> > Link:
> > https://lore.kernel.org/r/d04bcc45-3471-4417-b30b-5cf9880d785d@i2se.com=
/
> > Reported-by: Stefan Wahren <stefan.wahren@i2se.com>
> > Signed-off-by: Matthias Kaehlcke <mka@chromium.org>
> > ---
> >=20
> > Changes in v2:
> > - don't create platform devices when "vdd-supply" is missing,
> > =C2=A0 rather than returning an error from _find_onboard_hub()
> > - check for "vdd-supply" not "vdd" (Johan)
> > - updated subject and commit message
> > - added 'Link' tag (regzbot)
> >=20
> > =C2=A0drivers/usb/misc/onboard_usb_hub_pdevs.c | 13 +++++++++++++
> > =C2=A01 file changed, 13 insertions(+)
>=20
> I'm a tad bit skeptical.
>=20
> It somehow feels a bit too much like "inside knowledge" to add this
> here. I guess the "onboard_usb_hub_pdevs.c" is already pretty
> entangled with "onboard_usb_hub.c", but I'd rather the "pdevs" file
> keep the absolute minimum amount of stuff in it and all of the
> details
> be in the other file.
>=20
> If this was the only issue though, I'd be tempted to let it slide. As
> it is, I'm kinda worried that your patch will break Alexander Stein,
> who should have been CCed (I've CCed him now) or Icenowy Zheng (also
> CCed now). I believe those folks are using the USB hub driver
> primarily to drive a reset GPIO. Looking at the example in the
> bindings for one of them (genesys,gl850g.yaml), I even see that the
> reset-gpio is specified but not a vdd-supply. I think you'll break
> that?

Well technically in my final DT a regulator is included (to have the
Vbus enabled when enabling the hub), however I am still against this
patch, because the driver should work w/o vdd-supply (or w/o reset-
gpios), and changing this behavior is a DT binding stability breakage.

In addition the kernel never fails because of a lacking regulator
unless explicitly forbid dummy regulators.

BTW USB is a discoverable bus, and if a hub do not need special
handlement, it just does not need to appear in the DT, thus no onboard
hub DT node.

>=20
> In general, it feels like it should actually be fine to create the
> USB
> hub driver even if vdd isn't supplied. Sure, it won't do a lot, but
> it
> shouldn't actively hurt anything. You'll just be turning off and on
> bogus regulators and burning a few CPU cycles. I guess the problem is
> some race condition that you talk about in the commit message. I'd
> rather see that fixed... That being said, if we want to be more
> efficient and not burn CPU cycles and memory in Stefan Wahren's case,
> maybe the USB hub driver itself could return a canonical error value
> from its probe when it detects that it has no useful job and then
> "onboard_usb_hub_pdevs" could just silently bail out?

I agree.