linux-kernel - Re: [PATCH] usb: dwc2: host: fix isoc urb actual length

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Date:   Tue, 7 Nov 2017 10:18:41 -0500 (EST)
From:   Alan Stern <stern@...land.harvard.edu>
To:     wlf <wulf@...k-chips.com>
cc:     Minas Harutyunyan <Minas.Harutyunyan@...opsys.com>,
        William Wu <william.wu@...k-chips.com>,
        "John.Youn@...opsys.com" <John.Youn@...opsys.com>,
        "balbi@...nel.org" <balbi@...nel.org>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
        "heiko@...ech.de" <heiko@...ech.de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
        "linux-rockchip@...ts.infradead.org" 
        <linux-rockchip@...ts.infradead.org>,
        "frank.wang@...k-chips.com" <frank.wang@...k-chips.com>,
        "huangtao@...k-chips.com" <huangtao@...k-chips.com>,
        "dianders@...gle.com" <dianders@...gle.com>,
        "daniel.meng@...k-chips.com" <daniel.meng@...k-chips.com>,
        "fml@...k-chips.com" <fml@...k-chips.com>
Subject: Re: [PATCH] usb: dwc2: host: fix isoc urb actual length

On Tue, 7 Nov 2017, wlf wrote:

> > That sounds like a good idea.  Minas, does the following patch fix your
> > problem?
> > 
> > In theory we could do this calculation for every isochronous URB, not
> > just those coming from usbfs.  But I don't think there's any point,
> > since the USB class drivers don't use it.
> >
> > Alan Stern
> >
> >
> >
> > Index: usb-4.x/drivers/usb/core/devio.c
> > ===================================================================
> > --- usb-4.x.orig/drivers/usb/core/devio.c
> > +++ usb-4.x/drivers/usb/core/devio.c
> > @@ -1828,6 +1828,18 @@ static int proc_unlinkurb(struct usb_dev
> >   	return 0;
> >   }
> >   
> > +static void compute_isochronous_actual_length(struct urb *urb)
> > +{
> > +	unsigned i;
> > +
> > +	if (urb->number_of_packets > 0) {
> > +		urb->actual_length = 0;
> > +		for (i = 0; i < urb->number_of_packets; i++)
> > +			urb->actual_length +=
> > +					urb->iso_frame_desc[i].actual_length;
> > +	}
> > +}
> > +
> >   static int processcompl(struct async *as, void __user * __user *arg)
> >   {
> >   	struct urb *urb = as->urb;
> > @@ -1835,6 +1847,8 @@ static int processcompl(struct async *as
> >   	void __user *addr = as->userurb;
> >   	unsigned int i;
> >   
> > +	compute_isochronous_actual_length(urb);
> > +
> >   	if (as->userbuffer && urb->actual_length) {
> >   		if (copy_urb_data_to_user(as->userbuffer, urb))
> >   			goto err_out;
> > @@ -2003,6 +2017,8 @@ static int processcompl_compat(struct as
> >   	void __user *addr = as->userurb;
> >   	unsigned int i;
> >   
> > +	compute_isochronous_actual_length(urb);
> > +
> >   	if (as->userbuffer && urb->actual_length) {
> >   		if (copy_urb_data_to_user(as->userbuffer, urb))
> >   			return -EFAULT;
> >
> >
> Yeah,  it's a good idea to calculate the urb actual length in the devio 
> driver.
> Your patch seems good,  and I think we can do a small optimization base 
> your patch,
> like the following patch:
> 
> diff --git a/drivers/usb/core/devio.c b/drivers/usb/core/devio.c
> index bd94192..a2e7b97 100644
> --- a/drivers/usb/core/devio.c
> +++ b/drivers/usb/core/devio.c
> @@ -1664,6 +1664,9 @@ static int processcompl(struct async *as, void 
> __user * __user *arg)
>          void __user *addr = as->userurb;
>          unsigned int i;
> 
> +       if (usb_endpoint_xfer_isoc(&urb->ep->desc))
> +               compute_isochronous_actual_length(urb);
> +
>          if (as->userbuffer && urb->actual_length) {
>                  if (copy_urb_data_to_user(as->userbuffer, urb))
>                          goto err_out;
> @@ -1833,6 +1836,9 @@ static int processcompl_compat(struct async *as, 
> void __user * __user *arg)
>          void __user *addr = as->userurb;
>          unsigned int i;
> 
> +       if (usb_endpoint_xfer_isoc(&urb->ep->desc))
> +               compute_isochronous_actual_length(urb);
> +

Well, this depends on whether you want to optimize for space or for 
speed.  I've been going for space.  And since usbfs is inherently 
rather slow, I don't think this will make any significant speed 
difference.  So I don't think adding the extra tests is worthwhile.

(Besides, if you really wanted to do it this way, you should have moved 
the test for "urb->number_of_packets > 0" into the callers instead of 
adding an additional test of the endpoint type.)

However, nobody has answered my original question: Does the patch 
actually fix the problem?

Alan Stern