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Date: Mon, 17 Feb 2025 09:53:34 +0100
From: Greg KH <gregkh@linuxfoundation.org>
To: Hsin-chen Chuang <chharry@google.com>
Cc: linux-bluetooth@vger.kernel.org, luiz.dentz@gmail.com,
	chromeos-bluetooth-upstreaming@chromium.org,
	Hsin-chen Chuang <chharry@chromium.org>,
	"David S. Miller" <davem@davemloft.net>,
	Eric Dumazet <edumazet@google.com>,
	Jakub Kicinski <kuba@kernel.org>,
	Johan Hedberg <johan.hedberg@gmail.com>,
	Marcel Holtmann <marcel@holtmann.org>,
	Paolo Abeni <pabeni@redhat.com>, Simon Horman <horms@kernel.org>,
	Ying Hsu <yinghsu@chromium.org>, linux-kernel@vger.kernel.org,
	netdev@vger.kernel.org
Subject: Re: [PATCH v5] Bluetooth: Fix possible race with userspace of sysfs
 isoc_alt
Message-ID: <2025021717-prepay-sharpener-37fb@gregkh>
References: <20250214191615.v5.1.If6f14aa2512336173a53fc3552756cd8a332b0a3@changeid>
 <2025021425-surgical-wackiness-0940@gregkh>
 <CADg1FFd3H0DLV-WX8jTB1VGyOZYEzchP99QvYxWmg1XCOo1ttg@mail.gmail.com>
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In-Reply-To: <CADg1FFd3H0DLV-WX8jTB1VGyOZYEzchP99QvYxWmg1XCOo1ttg@mail.gmail.com>

On Mon, Feb 17, 2025 at 04:44:35PM +0800, Hsin-chen Chuang wrote:
> On Fri, Feb 14, 2025 at 7:37 PM Greg KH <gregkh@linuxfoundation.org> wrote:
> >
> > On Fri, Feb 14, 2025 at 07:16:17PM +0800, Hsin-chen Chuang wrote:
> > > From: Hsin-chen Chuang <chharry@chromium.org>
> > >
> > > Expose the isoc_alt attr with device group to avoid the racing.
> > >
> > > Now we create a dev node for btusb. The isoc_alt attr belongs to it and
> > > it also becomes the parent device of hci dev.
> > >
> > > Fixes: b16b327edb4d ("Bluetooth: btusb: add sysfs attribute to control USB alt setting")
> >
> > Wait, step back, why is this commit needed if you can change the alt
> > setting already today through usbfs/libusb without needing to mess with
> > the bluetooth stack at all?
> 
> In short: We want to configure the alternate settings without
> detaching the btusb driver, while detaching seems necessary for
> libusb_set_interface_alt_setting to work (Please correct me if I'm
> wrong!)

I think changing the alternate setting should work using usbfs as you
would send that command to the device, not the interface, so the driver
bound to the existing interface would not need to be removed.

Try it out and see yourself to verify this before you continue down any
of this.  There's no need to use libfs for just a single usbfs command,
right?

thanks,

greg k-h