linux-kernel - [Discussion] USB: musb-gadget: how to fix ZLP issue in musb_g

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Date:	Wed, 15 Sep 2010 18:53:10 +0800
From:	Ming Lei <tom.leiming@...il.com>
To:	Sergei Shtylyov <sshtylyov@...sta.com>, balbi@...com
Cc:	"greg@...ah.com" <greg@...ah.com>,
	"linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
	"linux-omap@...r.kernel.org" <linux-omap@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	David Brownell <dbrownell@...rs.sourceforge.net>,
	"Gadiyar, Anand" <gadiyar@...com>,
	Mike Frysinger <vapier@...too.org>
Subject: [Discussion] USB: musb-gadget: how to fix ZLP issue in musb_g_tx

Hi All,

In another thread, Sergei pointed out there is a ZLP issue in musb_g_tx:

Sergei Shtylyov <sshtylyov@...sta.com> wrote:
> Hello.
>
> On 15-09-2010 14:05, Felipe Balbi wrote:
>
> >> I didn't say it was duplicate for DMA, just too late.
>
> > how come ? we need to send ZLP before giving back the request.
>
>     Well, look at the code ionce again. We need to send ZLP *after*
> request->actual == request->length, but as the check is inserted
> after the ZLP send, ZLP *may* be sent once the first DMA completes,
> not the last.
>
> WBR, Sergei

balbi also confirmed that is is really a problem.

I also have two related questions below for the problem:

1),  why is the check for "is_dma" needed here?

      if (is_dma || request->actual == request->length) {
           ....
      }

2), why is a zlp needed in the case below?

      #ifdef CONFIG_USB_INVENTRA_DMA
                  || (is_dma && (!dma->desired_mode ||
                     (request->actual & (musb_ep->packet_sz - 1))))
     #endif

Seems no request->zero is set to ask for zlp explicitly in
the case above.

IMO, it is not difficult to give a good fix for the ZLP problem
if the two questions are clear.

-- 
Lei Ming
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