linux-kernel - Re: [PATCH v2] usb: gadget: udc: core: Offload usb_udc_vbus

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Date:   Fri, 19 May 2023 11:07:25 -0400
From:   Alan Stern <stern@...land.harvard.edu>
To:     Badhri Jagan Sridharan <badhri@...gle.com>
Cc:     gregkh@...uxfoundation.org, colin.i.king@...il.com,
        xuetao09@...wei.com, quic_eserrao@...cinc.com,
        water.zhangjiantao@...wei.com, peter.chen@...escale.com,
        balbi@...com, francesco@...cini.it, alistair@...stair23.me,
        stephan@...hold.net, bagasdotme@...il.com, luca@...tu.xyz,
        linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org,
        stable@...r.kernel.org,
        Francesco Dolcini <francesco.dolcini@...adex.com>
Subject: Re: [PATCH v2] usb: gadget: udc: core: Offload usb_udc_vbus_handler
 processing

On Fri, May 19, 2023 at 10:49:49AM -0400, Alan Stern wrote:
> On Fri, May 19, 2023 at 04:30:41AM +0000, Badhri Jagan Sridharan wrote:
> > chipidea udc calls usb_udc_vbus_handler from udc_start gadget
> > ops causing a deadlock. Avoid this by offloading usb_udc_vbus_handler
> > processing.
> 
> Look, this is way overkill.
> 
> usb_udc_vbus_handler() has only two jobs to do: set udc->vbus and call 
> usb_udc_connect_control().  Furthermore, it gets called from only two 
> drivers: chipidea and max3420.
> 
> Why not have the callers set udc->vbus themselves and then call 
> usb_gadget_{dis}connect() directly?  Then we could eliminate 
> usb_udc_vbus_handler() entirely.  And the unnecessary calls -- the ones 
> causing deadlocks -- from within udc_start() and udc_stop() handlers can 
> be removed with no further consequence.
> 
> This approach simplifies and removes code.  Whereas your approach 
> complicates and adds code for no good reason.

I changed my mind.

After looking more closely, I found the comment in gadget.h about 
->disconnect() callbacks happening in interrupt context.  This means we 
cannot use a mutex to protect the associated state, and therefore the 
connect_lock _must_ be a spinlock, not a mutex.

This also probably means that udc_start and udc_stop callbacks should 
not be invoked with the lock held.  In fact, you might want to avoid 
using the lock at all with gadget_bind_driver() and 
gadget_unbind_driver() -- use it only in the functions that these 
routines call.

So it appears the whole connect_lock thing needs to be redesigned with 
these ideas in mind.  However, it's still true that the UDC drivers 
shouldn't try to set the connection state from within their udc_start 
and udc_stop callbacks, because the core takes care of this 
automatically.

Alan Stern