linux-kernel - Re: [PATCH] usb: xhci: xhci_setup_port_arrays: early -ENODEV if maxports is 0.

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Message-ID: <ZwA-n56XlNkkLNXM@freebase>
Date: Fri, 4 Oct 2024 21:14:39 +0200
From: Olivier Dautricourt <olivierdautricourt@...il.com>
To: Michał Pecio <michal.pecio@...il.com>
Cc: gregkh@...uxfoundation.org, linux-kernel@...r.kernel.org,
	linux-usb@...r.kernel.org, mathias.nyman@...el.com
Subject: Re: [PATCH] usb: xhci: xhci_setup_port_arrays: early -ENODEV if
 maxports is 0.

Hello,

On Fri, Oct 04, 2024 at 12:57:16PM +0200, Michał Pecio wrote:
> Hi,
> 
> > If the controller reports HCSPARAMS1.maxports==0 then we can skip the
> > whole function: it would fail later after doing a bunch of unnecessary
> > stuff. It can occur on a buggy hardware (the value is driven by external
> > signals).
> 
> This function runs once during HC initialization, so what's the benefit
> of bypassing it? Does it take unusually long time? Does it panic?
> 
> It seems to alreday be written to handle such abnormal cases gracefully.

That is correct, the case is handled without panic, but the 0 value gets
silently propagated until it eventually fails on line 2220:
	if (xhci->usb2_rhub.num_ports == 0 && xhci->usb3_rhub.num_ports == 0) {
		xhci_warn(xhci, "No ports on the roothubs?\n");
		return -ENODEV;
	}
The benefits are only:
  - Reporting a more precise issue
  - Avoids iterating through the capability structures of the controller
  - failsafe if future changes

This is totally a nitpick as the case is unusual, if you think it is not
worth taking it upstream i'll understand.


Kr,
Olivier