linux-kernel - Re: [PATCH v1 2/3] usb: dwc3: gadget: Add support for snps,reserved-endpoints property

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Message-ID: <Z5kL_w0yqUHMRDzQ@smile.fi.intel.com>
Date: Tue, 28 Jan 2025 18:55:27 +0200
From: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To: Thinh Nguyen <Thinh.Nguyen@...opsys.com>
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Felipe Balbi <balbi@...nel.org>,
	"linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Rob Herring <robh@...nel.org>,
	Krzysztof Kozlowski <krzk+dt@...nel.org>,
	Conor Dooley <conor+dt@...nel.org>, Ferry Toth <fntoth@...il.com>
Subject: Re: [PATCH v1 2/3] usb: dwc3: gadget: Add support for
 snps,reserved-endpoints property

On Tue, Jan 28, 2025 at 02:39:50AM +0000, Thinh Nguyen wrote:
> On Wed, Jan 22, 2025, Andy Shevchenko wrote:
> > On Wed, Jan 22, 2025 at 01:44:02AM +0000, Thinh Nguyen wrote:
> > > On Fri, Jan 17, 2025, Andy Shevchenko wrote:
> > > > On Thu, Jan 16, 2025 at 11:35:19PM +0000, Thinh Nguyen wrote:
> > > > > On Thu, Jan 16, 2025, Andy Shevchenko wrote:

...

> > > > > >  	for (epnum = 0; epnum < total; epnum++) {
> > > > > > -		int			ret;
> > > > > > +		for (num = 0; num < count; num++) {
> > > > > > +			if (epnum == eps[num])
> > > > > > +				break;
> > > > > > +		}
> > > > > > +		if (num < count)
> > > > > > +			continue;
> > > > > 
> > > > > You can probably rewrite this logic better.
> > > > 
> > > > Any suggestions?
> > > > 
> > > > Thanks for the review!
> > > 
> > > From the first look, is the list sorted? If so, you don't need another
> > > for-loop.
> > 
> > Even if it's sorted it's not 1:1 mapped by indices. I do not understand how we
> > can avoid the second loop. The only possibility is indeed to sort the list and
> > sparse it in accordance to the endpoint numbers, but if we are going this way,
> > it's much easier to switch to bitmap and the respective bitops.
> 
> If it's sorted, it can be something like this. Just a quick logic and not tested:
> 
> num = 0
> for (epnum = 0; epnum < total; epnum++) {
> 	if (num < count && epnum == eps[num]) {
> 		num++;
> 		continue;
> 	}
> 
> 	...
> }

Ah, okay, I have got the idea. Let me try to mock up something working
out of it.

> > > Also, we loop over the number of endpoints throughout the driver, but
> > > you only skip it here during init. Please double check for invalid
> > > accessing of endpoints in other places.
> > > 
> > > Perhaps set the dwc->eps[reserved_ep] to ERR_PTR(-EINVAL) or something
> > > when you parse the reserved endpoints so you can skip them in your loop.
> > 
> > Note, this is only for UDC, in USB host these are okay to be used.
> > Does your suggestion imply that?
> 
> No. We track the total num_eps in dwc->num_eps. Then we do for-loop to
> dwc->eps[i] and access the endpoint. Often we check if the endpoint is
> NULL before accessing dwc->eps[i]. However, we don't do it everywhere.
> So I ask for you to review to make sure that this change doesn't break
> elsewhere where we may try to access dwc->eps[i] to an uninit endpoint
> (Note I see at least 1 place e.g. dwc3_gadget_clear_tx_fifos that may
> break)

I see, so having my code as is also requiring to check all users of
the eps array in the _gadget part_ of the driver to see if they won't
crash due to NULL pointer dereference. Is it what you want?
If so, definitely I will revisit that.

-- 
With Best Regards,
Andy Shevchenko