Message-ID: <Y7cn6PKzA0Xss0MN@google.com>
Date:   Thu, 5 Jan 2023 19:41:28 +0000
From:   Matthias Kaehlcke <mka@...omium.org>
To:     Stefan Wahren <stefan.wahren@...e.com>
Cc:     Alexander Stein <alexander.stein@...tq-group.com>,
        Doug Anderson <dianders@...omium.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-kernel@...r.kernel.org, linux-usb@...r.kernel.org,
        stable@...r.kernel.org,
        Ravi Chandra Sadineni <ravisadineni@...omium.org>,
        Icenowy Zheng <uwu@...nowy.me>
Subject: Re: [PATCH v2 1/2] usb: misc: onboard_usb_hub: Don't create platform
 devices for DT nodes without 'vdd-supply'

On Thu, Jan 05, 2023 at 08:50:17AM +0100, Stefan Wahren wrote:
> Hi Matthias,
> 
> Am 04.01.23 um 20:37 schrieb Matthias Kaehlcke:
> > On Wed, Jan 04, 2023 at 10:00:43AM +0100, Alexander Stein wrote:
> > > Hi Matthias,
> > > 
> > > Am Dienstag, 3. Januar 2023, 18:12:24 CET schrieb Matthias Kaehlcke:
> > > > On Thu, Dec 22, 2022 at 11:26:26AM -0800, Doug Anderson wrote:
> > > > > 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?
> > > > Thanks for pointing that out. My assumption was that the regulator is
> > > > needed for the driver to do anything useful, but you are right, the
> > > > reset GPIO alone could be used in combination with an always-on regulator
> > > > to 'switch the hub on and off'.
> > > > 
> > > > > 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...
> > > > Yes, the race conditions needs to be fixed as well, I didn't have enough
> > > > time to write and test a patch before taking a longer break for the
> > > > holidays, so I only sent out this (supposed) partial mitigation.
> > > > 
> > > > > 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?
> > > > _probe() could return an error, however onboard_hub_create_pdevs() can't
> > > > rely on that, since the actual onboard_hub driver might not have been
> > > > loaded yet when the function is called.
> > > > 
> > > > It would be nice not to instantiate the pdev and onboard_hub USB instances
> > > > if the DT node has neither a 'vdd-supply' nor 'reset-gpios'. If we aren't
> > > > ok with doing that in onboard_hub_create_pdevs() then at least the 'raw'
> > > > platform device would be created. onboard_hub_probe() could still
> > > > bail if both properties are absent, _find_onboard_hub() would have to
> > > > check it again to avoid the deferred probing 'loop' for the USB instances.
> > > I'm not really fond of checking for optional features like 'vdd-supply' and
> > > 'reset-gpios'. This issue will pop up again if some new optional feature is
> > > added again, e.g. power-domains.
> > It's not just any optional feature, it needs to be involved in controlling
> > power. I'm not super-exited about it either, but if we prefer not to
> > instantiate the drivers for certain DT nodes (TBD if that's a preference), we
> > need some sort of sentinel since the compatible string alone doesn't provide
> > enough information.
> > 
> > > > Alternatively we could 'just' fix the race condition involving the 'attach
> > > > work' and the onboard_hub driver is fully instantiated even on (certain)
> > > > boards where it does nothing. It's relatively rare that USB hub nodes are
> > > > specified in the DT (unless the intention is to use this driver) and
> > > > CONFIG_USB_ONBOARD_HUB needs to be set for the instances to be created,
> > > > so maybe creating the useless instances is not such a big deal.
> > > IMHO creating a pdev shouldn't harm in any case. It's similar to having a DT
> > > node without a corresponding driver enabled or even existing.
> > If we only want a 'raw' pdev (no instantiation of the onboard hub platform and
> > USB drivers) then a similar logic will be needed in the onboard hub driver(s).
> > 
> > So if we don't want any logic checking that at least one power related property
> > is defined then we have to accept that the onboard hub driver will be fully
> > instantiated even when it effectively does nothing.
> > 
> > If we add logic to the driver it needs to be checked in both the platform and
> > the USB driver (in the latter to avoid a deferred probe loop). It would be
> > simpler to just skip the creation of the 'raw' platform device in the first
> > place.
> > 
> > > Also adding USB devices to DT is something which is to be expected.
> > > usb-device.yaml exists since 2020 and the txt version since 2016.
> > Yes it it perfectly legal, so we need to handle this case somehow, and we
> > are currently discussing how to best do that :)
> > 
> > I still don't expect the combo of supported hub in the DT +
> > CONFIG_USB_ONBOARD_HUB=y/m to become super-popular, which could be an
> > argument for the option "just instantiate the drivers even if they do
> > nothing". Not my favorite option, but probably not that bad either.
> 
> i disagree in this point. The driver becomes more and more popular [1] and
> this breaks arm64 for RPi 3B+ too. So it's only a question of time until
> distributions run into this problem.

There seems to be a misunderstanding, the above option doesn't break
anything (as long as the attach race is fixed, which needs to be done
anyway). It impacts boards that specify a hub in the DT but *don't*
intend to use the driver (neither specify 'vdd-supply' nor 'reset-gpios').
I expect the number of such boards to remain low, since a USB hub is
usually not specified in the DT, unless the intention is to use the
onboard_hub driver and a few other cases.

There are two separate issues/questions:

1) fix the attach race

2) what to do with hubs for which the driver does nothing

 Possible options:

 a) instantiate the drivers regardless (current situation)
 b) don't create 'raw' pdev if the DT node doesn't have certain properties
    (a evolution of this patch)
 c) don't create instantiate the onboard_hub pdev and USB devices if the
    DT node doesn't have certain properties

> I willing to help in debugging the real issue, but i need a little bit
> guidance here.
> 
> [1] -
> https://lore.kernel.org/linux-arm-kernel/2188024.ZfL8zNpBrT@steina-w/T/
> 
> > 
> > > Unfortunately I'm not able to reproduce this issue on a different platform
> > > where the same HUB but no reset-gpios is required. I also noticed that
> > > onboard-usb-hub raises the error
> > > > Failed to attach USB driver: -22
> > > for each hub it is supposed to support.
> > Interesting
> > 
> > I also see the error with v6.2-rc1 but not a downstream v5.15 kernel which
> > runs most of the time on my boards. It turns out that with v6.2-rc1 the 'bus'
> > field of 'onboard_hub_usbdev_driver.drvwrap.driver' (passed to driver_attach())
> > is NULL, which causes driver_attach() / bus_for_each_dev() to return -EINVAL.
> > 
> > I did some testing (unbind/bind, unloading/reloading the driver) around the
> > 'attach work' independently from your report. I couldn't repro a situation
> > where the onboard_hub USB devices aren't probed by the driver, which is what
> > the 'attach work' is supposed to solve. At some point I observed issues with
> > that in the past, which is why driver_attach() is called. The driver_attach()
> > call was added to the onboard_hub series in early 2021, by that time I was
> > probably testing with a v5.4 kernel, it's not unconceivable that the issue I
> > saw back then is fixed in newer kernels.
> > 
> > With that I was already considering to remove the 'attach work', the error you
> > reported reinforces that, since the driver_attach() call from the onboard_hub
> > driver does nothing in more recent kernels due to 'bus' being NULL.
> > 
> > Thanks
> > 
> > Matthias

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