linux-kernel - Re: [PATCH v4] media: ucvideo: Add quirk for Logitech Rally Bar

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Date: Tue, 13 Feb 2024 12:47:25 +0200
From: Laurent Pinchart <laurent.pinchart@...asonboard.com>
To: Alan Stern <stern@...land.harvard.edu>
Cc: Oliver Neukum <oneukum@...e.com>,
	Ricardo Ribalda <ribalda@...omium.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Mauro Carvalho Chehab <mchehab@...nel.org>,
	linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-media@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH v4] media: ucvideo: Add quirk for Logitech Rally Bar

On Mon, Feb 12, 2024 at 02:04:31PM -0500, Alan Stern wrote:
> On Mon, Feb 12, 2024 at 01:22:42PM +0100, Oliver Neukum wrote:
> > On 04.02.24 11:52, Laurent Pinchart wrote:
> > > Hi Ricardo,
> > > 
> > > Thank you for the patch.
> > 
> > Hi,
> > 
> > sorry for commenting on this late, but this patch has
> > a fundamental issue. In fact this issue is the reason the
> > handling for quirks is in usbcore at all.
> > 
> > If you leave the setting/clearing of this flag to a driver you
> > are introducing a race condition. The driver may or may not be
> > present at the time a device is enumerated. And you have
> > no idea how long the autosuspend delay is on a system
> > and what its default policy is regarding suspending
> > devices.
> > That means that a device can have been suspended and
> > resumed before it is probed. On a device that needs
> > RESET_RESUME, we are in trouble.
> 
> Not necessarily.  If the driver knows that one of these devices may 
> already have been suspend and resumed, it can issue its own preemptive 
> reset at probe time.
> 
> > The inverse issue will arise if a device does not react
> > well to RESET_RESUME. You cannot rule out that a device
> > that must not be reset will be reset.
> 
> That's a separate issue, with its own list of potential problems.
> 
> > I am sorry, but it seems to me that the exceptions need
> > to go into usbcore.
> 
> If we do then we may want to come up with a better scheme for seeing 
> which devices need to have a quirk flag set.  A static listing probably 
> won't be good enough; the decision may have to be made dynamically.

I don't mind either way personally. Oliver, could you try to find a good
solution with Ricardo ? I'll merge the outcome.

-- 
Regards,

Laurent Pinchart