linux-kernel - Re: [PATCH v2 1/2] usb: misc: onboard_usb_hub: Don't create platform devices for DT nodes without 'vdd-supply'

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Message-ID: <3724284.kQq0lBPeGt@steina-w>
Date:   Mon, 02 Jan 2023 10:20:10 +0100
From:   Alexander Stein <alexander.stein@...tq-group.com>
To:     Doug Anderson <dianders@...omium.org>,
        Matthias Kaehlcke <mka@...omium.org>,
        Icenowy Zheng <uwu@...nowy.me>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-kernel@...r.kernel.org, linux-usb@...r.kernel.org,
        Stefan Wahren <stefan.wahren@...e.com>, stable@...r.kernel.org,
        Ravi Chandra Sadineni <ravisadineni@...omium.org>
Subject: Re: [PATCH v2 1/2] usb: misc: onboard_usb_hub: Don't create platform devices for DT nodes without 'vdd-supply'

Hi everybody,

Am Freitag, 23. Dezember 2022, 08:46:45 CET schrieb Icenowy Zheng:
> 在 2022-12-22星期四的 11:26 -0800，Doug Anderson写道：
> 
> > Hi,
> > 
> > On Wed, Dec 21, 2022 at 6:26 PM Matthias Kaehlcke <mka@...omium.org>
> > 
> > wrote:
> > > 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.
> > > 
> > > 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.
> > > 
> > > 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@...e.com>
> > > Signed-off-by: Matthias Kaehlcke <mka@...omium.org>
> > > ---
> > > 
> > > Changes in v2:
> > > - don't create platform devices when "vdd-supply" is missing,
> > >   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)
> > > 
> > >  drivers/usb/misc/onboard_usb_hub_pdevs.c | 13 +++++++++++++
> > >  1 file changed, 13 insertions(+)
> > 
> > I'm a tad bit skeptical.
> > 
> > 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.
> > 
> > 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.

I second that. The bindings don't require neither vdd-supply nor reset-gpios.

But I have to admit I lack to understand the purpose of this series in the 
first place. What is the benefit or fix?

Best regards,
Alexader

> 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.
> 
> > 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.