RE: [PATCH] USB: Fix configuration selection issues introduced in v4.20.0

["Gopal, Saranya" <saranya.gopal@intel.com>]

Hi Yakimov,

As per UAC3 configuration, the first configuration will always be backward-compatible.
So, there cannot be any UAC3-compatible device which has first configuration as UAC3.
And secondly, the commit ff2a8c532c14 does not break the pre-existing logic.
I also thought so much about moving the code above Ethernet check while preparing ff2a8c532c14.
But, then, it doesn't break the existing logic and so decided against moving it.

Thanks,
Saranya
> -----Original Message-----
> From: Nikolay Yakimov [mailto:root@livid.pp.ru]
> Sent: Tuesday, January 15, 2019 9:44 PM
> To: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> Cc: Nikolay Yakimov <root@livid.pp.ru>; Gopal, Saranya
> <saranya.gopal@intel.com>; linux-usb@vger.kernel.org; linux-
> kernel@vger.kernel.org
> Subject: [PATCH] USB: Fix configuration selection issues introduced in v4.20.0
> 
> Commit f13912d3f014a introduced changes to the usb_choose_configuration
> function
> to better support USB Audio UAC3-compatible devices. However, there are a
> few
> problems with this patch. First of all, it adds new "if" clauses in the middle
> of an existing "if"/"else if" tree, which obviously breaks pre-existing logic.
> Secondly, since it continues iterating over configurations in one of the branches,
> other code in the loop can choose an unintended configuration. Finally,
> if an audio device's first configuration is UAC3-compatible, and there
> are multiple UAC3 configurations, the second one would be chosen, due to
> the first configuration never being checked for UAC3-compatibility.
> 
> Commit ff2a8c532c14 tries to fix the second issue, but it goes about it in a
> somewhat unnecessarily convoluted way, in my opinion, and does nothing
> to fix the first or the last one.
> 
> This patch tries to rectify problems described by essentially rewriting
> code introduced in f13912d3f014a. Notice the code was moved to *before*
> the "if"/"else if" tree.
> 
> Signed-off-by: Nikolay Yakimov <root@livid.pp.ru>
> ---
>  drivers/usb/core/generic.c | 44 ++++++++++++++++++++++----------------
>  1 file changed, 25 insertions(+), 19 deletions(-)
> 
> diff --git a/drivers/usb/core/generic.c b/drivers/usb/core/generic.c
> index f713cecc1f41..1ac9c1e5f773 100644
> --- a/drivers/usb/core/generic.c
> +++ b/drivers/usb/core/generic.c
> @@ -118,6 +118,31 @@ int usb_choose_configuration(struct usb_device
> *udev)
>  			continue;
>  		}
> 
> +		/*
> +		 * Select first configuration as default for audio so that
> +		 * devices that don't comply with UAC3 protocol are supported.
> +		 * But, still iterate through other configurations and
> +		 * select UAC3 compliant config if present.
> +		 */
> +		if (desc && is_audio(desc)) {
> +			/* Always prefer the first found UAC3 config */
> +			if (is_uac3_config(desc)) {
> +				best = c;
> +				break;
> +			}
> +
> +			/* If there is no UAC3 config, prefer the first config */
> +			else if (i == 0)
> +				best = c;
> +
> +			/* Unconditional continue, because the rest of the code
> +			 * in the loop is irrelevant for audio devices, and
> +			 * because it can reassign best, which for audio devices
> +			 * we don't want.
> +			 */
> +			continue;
> +		}
> +
>  		/* When the first config's first interface is one of Microsoft's
>  		 * pet nonstandard Ethernet-over-USB protocols, ignore it
> unless
>  		 * this kernel has enabled the necessary host side driver.
> @@ -132,25 +157,6 @@ int usb_choose_configuration(struct usb_device
> *udev)
>  #endif
>  		}
> 
> -		/*
> -		 * Select first configuration as default for audio so that
> -		 * devices that don't comply with UAC3 protocol are supported.
> -		 * But, still iterate through other configurations and
> -		 * select UAC3 compliant config if present.
> -		 */
> -		if (i == 0 && num_configs > 1 && desc && is_audio(desc)) {
> -			best = c;
> -			continue;
> -		}
> -
> -		if (i > 0 && desc && is_audio(desc)) {
> -			if (is_uac3_config(desc)) {
> -				best = c;
> -				break;
> -			}
> -			continue;
> -		}
> -
>  		/* From the remaining configs, choose the first one whose
>  		 * first interface is for a non-vendor-specific class.
>  		 * Reason: Linux is more likely to have a class driver
> --
> 2.20.1