linux-kernel - Re: Resend Re: [PATCH v6] usb:serial:pl2303: add GPIOs interface on PL2303

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Message-ID: <20140829103845.GG4894@localhost>
Date:	Fri, 29 Aug 2014 12:38:45 +0200
From:	Johan Hovold <johan@...nel.org>
To:	Wang YanQing <udknight@...il.com>
Cc:	Johan Hovold <johan@...nel.org>, gregkh@...uxfoundation.org,
	linus.walleij@...aro.org, jhovold@...il.com, andi@...as.de,
	dforsi@...il.com, gnomes@...rguk.ukuu.org.uk,
	linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: Resend Re: [PATCH v6] usb:serial:pl2303: add GPIOs interface on
 PL2303

On Thu, Aug 28, 2014 at 07:38:10AM +0800, Wang YanQing wrote:
> On Tue, Aug 12, 2014 at 04:02:59PM +0200, Johan Hovold wrote:
> > On Sat, Aug 09, 2014 at 02:46:56AM +0800, Wang YanQing wrote:
> > > On Fri, Aug 08, 2014 at 09:54:42AM +0200, Johan Hovold wrote:
> > > > On Fri, Aug 08, 2014 at 03:10:34AM +0800, Wang YanQing wrote:

> > > "
> > > You must allocate the buffer dynamically as some platforms cannot do
> > > DMA to the stack.
> > > "
> > > Thanks very much for point out it, could you clarify it? 
> > > I want to know the reason.
> > 
> > The memory where the stack resides might not be available for DMA, and
> > even if it is, there could still be problems with cache coherency.
> 
> It is still vague:
> stack memory maybe resident higher place than normal memory,
> but I don't think kmalloc could be immune from this problem, unless
> we use GFP_DMA?

No, you don't need to use GFP_DMA (unless implementing a driver for an
ISA device on x86).

Have a look at Documentation/DMA-API-HOWTO.txt, specifically the section
"What memory is DMA'able?".

Johan
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