linux-kernel - Re: [PATCH v4 2/2] USB: misc: Add onboard_usb

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Date:   Tue, 29 Sep 2020 12:00:36 -0400
From:   Alan Stern <stern@...land.harvard.edu>
To:     Matthias Kaehlcke <mka@...omium.org>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Rob Herring <robh+dt@...nel.org>,
        Frank Rowand <frowand.list@...il.com>,
        linux-kernel@...r.kernel.org, linux-usb@...r.kernel.org,
        Bastien Nocera <hadess@...ess.net>,
        Stephen Boyd <swboyd@...omium.org>,
        Douglas Anderson <dianders@...omium.org>,
        Ravi Chandra Sadineni <ravisadineni@...omium.org>,
        Krzysztof Kozlowski <krzk@...nel.org>,
        devicetree@...r.kernel.org, Peter Chen <peter.chen@....com>,
        "Alexander A. Klimov" <grandmaster@...klimov.de>,
        "David S. Miller" <davem@...emloft.net>,
        Johan Hovold <johan@...nel.org>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Mauro Carvalho Chehab <mchehab+huawei@...nel.org>,
        Rob Herring <robh@...nel.org>
Subject: Re: [PATCH v4 2/2] USB: misc: Add onboard_usb_hub driver

On Mon, Sep 28, 2020 at 06:43:55PM -0700, Matthias Kaehlcke wrote:
> > Have you tried manually unbinding and rebinding the two drivers a few
> > times to make sure they will still work?
> 
> I went through a few dozen bund/unbind cycles for both drivers and things
> looked good overall, but then last minute I found that determining whether
> wakeup capable devices are connected doesn't always work as (I) expected.
> I didn't see this earlier, it seems to be reproduce more easily after
> unbinding and rebinding the platform driver.
> 
> During development I already noticed that usb_wakeup_enabled_descendants()
> returns a cached value, which was a problem for an earlier version of the
> driver. The values are updated by hub_suspend(), my (flawed) assumption
> was that the USB driver would always suspend before the platform driver.
> This generally seems to be the case on my development platform after boot,
> but not necessarily after unbinding and rebinding the driver. Using the
> _suspend_late hook instead of _suspend seems to be a reliable workaround.

Yes, for unrelated (i.e., not in a parent-child relation) devices, the 
PM subsystem doesn't guarantee ordering of suspend and resume callbacks.  
You can enforce the ordering by using device_pm_wait_for_dev().  But the 
suspend_late approach seems like a better solution in this case.

> > I'm a little concerned about  all the devm_* stuff in here; does that
> > get released when the driver is unbound from the device or when the device
> > is unregistered?  And if the latter, what happens if you have multiple
> > sysfs attribute groups going at the same time?
> 
> The memory gets released when the device is unbound:
> 
> device_release_driver
>   device_release_driver_internal
>     __device_release_driver
>       devres_release_all
> 
> Anyway, if you prefer I can change the driver to use kmalloc/kfree.

No, that's fine.  I just wasn't sure about this and wanted to check.

Alan Stern