linux-kernel - Re: [PATCH v2] usb: hub: try old enumeration scheme first for high speed devices

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Date:   Fri, 17 Aug 2018 10:19:19 -0400 (EDT)
From:   Alan Stern <stern@...land.harvard.edu>
To:     Zeng Tao <prime.zeng@...ilicon.com>
cc:     gregkh@...uxfoundation.org, <mathias.nyman@...ux.intel.com>,
        <drinkcat@...omium.org>, <felipe.balbi@...ux.intel.com>,
        <drake@...lessm.com>, <mike.looijmans@...ic.nl>, <joe@...ches.com>,
        Jonathan Corbet <corbet@....net>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
        Marc Zyngier <marc.zyngier@....com>,
        Kai-Heng Feng <kai.heng.feng@...onical.com>,
        Thymo van Beers <thymovanbeers@...il.com>,
        Frederic Weisbecker <frederic@...nel.org>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        David Rientjes <rientjes@...gle.com>,
        <linux-doc@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <linux-usb@...r.kernel.org>
Subject: Re: [PATCH v2] usb: hub: try old enumeration scheme first for high
 speed devices

On Fri, 17 Aug 2018, Zeng Tao wrote:

> The new scheme is required just to support legacy low and full-speed
> devices. For high speed devices, it will slower the enumeration speed.
> So in this patch we try the "old" enumeration scheme first for high speed
> devices, and this is what Windows does since Windows 8.
> 
> Signed-off-by: Zeng Tao <prime.zeng@...ilicon.com>
> ---
> Changes in v2:
> 1. As suggested by Alan, mention in the commit log that this change is
> follow what the Window does.
> 2. As suggested by Roger, update the kernel-parameter description.
> ---
>  Documentation/admin-guide/kernel-parameters.txt | 3 ++-
>  drivers/usb/core/hub.c                          | 5 ++++-
>  2 files changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index 533ff5c..95db23c 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -4453,7 +4453,8 @@
>  
>  	usbcore.old_scheme_first=
>  			[USB] Start with the old device initialization
> -			scheme (default 0 = off).
> +			scheme,  applies only to low and full-speed devices
> +			 (default 0 = off).
>  
>  	usbcore.usbfs_memory_mb=
>  			[USB] Memory limit (in MB) for buffers allocated by
> diff --git a/drivers/usb/core/hub.c b/drivers/usb/core/hub.c
> index 1fb2668..d265b19 100644
> --- a/drivers/usb/core/hub.c
> +++ b/drivers/usb/core/hub.c
> @@ -2661,10 +2661,13 @@ static bool use_new_scheme(struct usb_device *udev, int retry,
>  	int old_scheme_first_port =
>  		port_dev->quirks & USB_PORT_QUIRK_OLD_SCHEME;
>  
> +	int quick_enumeration = (udev->speed == USB_SPEED_HIGH);

Minor style point: the preceding blank line should be eliminated.

> +
>  	if (udev->speed >= USB_SPEED_SUPER)
>  		return false;
>  
> -	return USE_NEW_SCHEME(retry, old_scheme_first_port || old_scheme_first);
> +	return USE_NEW_SCHEME(retry, old_scheme_first_port || old_scheme_first
> +			      || quick_enumeration);
>  }
>  
>  /* Is a USB 3.0 port in the Inactive or Compliance Mode state?

Aside from that:

Acked-by: Alan Stern <stern@...land.harvard.edu>