netdev - Re: [PATCH] USBNET: fix handling padding packet

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Date:	Wed, 18 Sep 2013 19:06:24 +0200
From:	Oliver Neukum <oneukum@...e.de>
To:	Bjørn Mork <bjorn@...k.no>
Cc:	Ming Lei <ming.lei@...onical.com>,
	"David S. Miller" <davem@...emloft.net>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Network Development <netdev@...r.kernel.org>,
	linux-usb <linux-usb@...r.kernel.org>
Subject: Re: [PATCH] USBNET: fix handling padding packet

On Wed, 2013-09-18 at 17:52 +0200, Bjørn Mork wrote:

> No modern device should need the padding.  No old device will be able to
> use the SG feature as implemented. You only enable it on USB3, don't

On XHCI.

> you? If this feature is restricted to USB3 capable devices, then it most
> certainly can be restricted to ZLP capable devices with absolutely no
> difference in the resulting set of supported devices.

No, USB 3.0 uses no companion controllers, so you can have devices
of any speed connected to it.

> Anyway, if you want to keep the padding for SG then maybe this will work
> and allow you to drop the extra struct usbnet field and allocation:
> 
>                         if (skb_tailroom(skb) && !dev->can_dma_sg) {
>                                skb->data[skb->len] = 0;
>                                __skb_put(skb, 1);
>                         } else if (dev->can_dma_sg) {
>                               sg_set_buf(&urb->sg[urb->num_sgs++], skb->data, 1);
>                         }
> 
> I.e. cheat and use the skb->data buffer twice, if that is allowed?  The
> actual value of the padding byte should not matter, I believe?

That makes me immediately suspect a violation of the DMA rules.

	Regards
		Oliver


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