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

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Date:   Fri, 10 Aug 2018 13:50:43 +0300
From:   Roger Quadros <rogerq@...com>
To:     Zeng Tao <prime.zeng@...ilicon.com>, <gregkh@...uxfoundation.org>,
        <stern@...land.harvard.edu>, <mathias.nyman@...ux.intel.com>,
        <drinkcat@...omium.org>, <felipe.balbi@...ux.intel.com>,
        <drake@...lessm.com>, <mike.looijmans@...ic.nl>, <joe@...ches.com>
CC:     <linux-usb@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] usb: hub: try old enumeration scheme first for high speed
 devices

Hi,

On 10/08/18 18:01, 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.

How slow does it get? Is it significant?
Do we risk breaking existing HS devices that work? I don't think we can
be sure till we run this through testing.

> 
> Signed-off-by: Zeng Tao <prime.zeng@...ilicon.com>
> ---
>  drivers/usb/core/hub.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> 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);
> +
>  	if (udev->speed >= USB_SPEED_SUPER)
>  		return false;

how about replacing the above if with

	if (udev->speed >= USB_SPEED_HIGH)
		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);
>  }

Now we no longer respect the "old_scheme_first" parameter for most of the devices.

It should be clarified in Documentation/admin/kernel-parameters.txt that
"old_scheme_first" is only applicable to LOW/FULL speed devices.

>  
>  /* Is a USB 3.0 port in the Inactive or Compliance Mode state?
> 

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