linux-kernel - Re: [PATCH v6] usb: Adding SuperSpeed support to dummy

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Message-ID: <Pine.LNX.4.44L0.1011181045430.1801-100000@iolanthe.rowland.org>
Date:	Thu, 18 Nov 2010 11:06:37 -0500 (EST)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Tatyana Brokhman <tlinder@...eaurora.org>
cc:	gregkh@...e.de, <linux-usb@...r.kernel.org>,
	<linux-arm-msm@...r.kernel.org>,
	David Brownell <dbrownell@...rs.sourceforge.net>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v6] usb: Adding SuperSpeed support to dummy_hcd

On Thu, 18 Nov 2010, Tatyana Brokhman wrote:

> USB 3.0 hub includes 2 hubs - HS and SS ones.
> Thus, when dummy_hcd enabled it will register 2 root hubs (SS and HS).

Hmm.  A quick comparison with the original source shows that you
neglected to add code for SuperSpeed transfers to periodic_bytes() and
show_urb().

> @@ -1352,6 +1544,9 @@ static void dummy_timer (unsigned long _dum)
>  	case USB_SPEED_HIGH:
>  		total = 512/*bytes*/ * 13/*packets*/ * 8/*uframes*/;
>  		break;
> +	case USB_SPEED_SUPER:
> +		total = 400 << 20; /* 400MB = 400*(2^20) bytes */
> +		break;

Somehow I doubt that SuperSpeed USB, quick though it is, can transfer
400 MB in one millisecond.  Where did you get that number from?

I can't find the appropriate table in the USB-3.0 spec (it might not be
there at all), but with a bus transfer rate of 500 million bytes/s it
seems clear that you can't transfer more than 500000 bytes per frame.  
The actual limit must be lower than that, maybe something like 1024
bytes/packet * 60 packets/uframe * 8 uframes/frame = 491520 
bytes/frame.

Alan Stern

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