linux-kernel - Re: [PATCH 2/2] usb: misc: onboard_hub: add support for XMOS XVF3500

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Date: Tue, 30 Jan 2024 17:26:42 +0000
From: Matthias Kaehlcke <mka@...omium.org>
To: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc: Javier Carrasco <javier.carrasco@...fvision.net>,
	Liam Girdwood <lgirdwood@...il.com>,
	Mark Brown <broonie@...nel.org>, Rob Herring <robh+dt@...nel.org>,
	Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
	Conor Dooley <conor+dt@...nel.org>, linux-sound@...r.kernel.org,
	devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-usb@...r.kernel.org
Subject: Re: [PATCH 2/2] usb: misc: onboard_hub: add support for XMOS XVF3500

On Tue, Jan 30, 2024 at 08:19:40AM -0800, Greg Kroah-Hartman wrote:
> On Tue, Jan 30, 2024 at 04:11:29PM +0000, Matthias Kaehlcke wrote:
> > Hi Javier,
> > 
> > I understand your motivation for using the onboard_usb_hub driver
> > for powering up a non-hub device, it feels a bit hacky to use it
> > as is though. Re-using the driver might be the right thing to do,
> > but then it should probably be renamed to onboard_usb_dev (or
> > similar) and do the hub specific bits as special case.
> > 
> > Greg, do you have any thoughts on this?
> 
> Yeah, this worries me, adding non-hub support to this driver feels odd.

It is odd as long as this driver claims to be hub-specific, but truth
is that the hub-specific bits are a small part of the driver, I think
it might be worthwhile to consider adapting the driver to other devices
if there is no clear better solution.

A possible alternative could be a separate onboard_usb_dev driver for
non-hub devices, with a similar structure as the onboard_hub driver,
but without the hub-specific bits.

> Why can't this all just be done in an individual driver for this device
> itself?

I suppose the reason is the good old chicken-egg situation that the (USB)
driver is only instantiated after the device has bee powered on, which is
what the driver is supposed to take care of. For the onboard_hub driver
this was solved by having a platform driver that is instantiated by the
parent hub if the onboard hub has a device tree entry. Probably something
similar would be needed for an individual driver, and the generic hub
driver would have to call the equivalent of onboard_hub_create_pdevs()
for all drivers of this type (or a wrapper that does this).

m.