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

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Message-ID: <4C90C4E6.9000703@ru.mvista.com>
Date:	Wed, 15 Sep 2010 17:06:46 +0400
From:	Sergei Shtylyov <sshtylyov@...sta.com>
To:	Ming Lei <tom.leiming@...il.com>
CC:	Sergei Shtylyov <sshtylyov@...sta.com>, balbi@...com,
	"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: Re: [Discussion] USB: musb-gadget: how to fix ZLP issue in musb_g_tx

Ming Lei wrote:
> 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) {
>            ....
>       }

    I'm not sure -- it seems erratic.

> 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.

    This is not for ZLP -- this is here to set TXPktRdy for the last short 
packet in the Inventra DMA mode 0 that doesn't set TXPktRdy in such case.

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

WBR, Sergei

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