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From: Johan Hovold <johan@kernel.org>
To: Wang YanQing <udknight@gmail.com>
Cc: Johan Hovold <johan@kernel.org>,
	gregkh@linuxfoundation.org, linus.walleij@linaro.org,
	jhovold@gmail.com, andi@lisas.de, dforsi@gmail.com,
	gnomes@lxorguk.ukuu.org.uk, linux-usb@vger.kernel.org,
	linux-kernel@vger.kernel.org
Subject: Re: Resend Re: [PATCH v6] usb:serial:pl2303: add GPIOs interface on PL2303
Date: Fri, 29 Aug 2014 12:38:45 +0200	[thread overview]
Message-ID: <20140829103845.GG4894@localhost> (raw)
In-Reply-To: <20140827233810.GA418@localhost.localdomain>

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

      reply	other threads:[~2014-08-29 10:40 UTC|newest]

Thread overview: 11+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2014-07-29 16:57 [PATCH v6] usb:serial:pl2303: add GPIOs interface on PL2303 Wang YanQing
2014-07-29 20:31 ` Johan Hovold
2014-08-04 14:00 ` Johan Hovold
2014-08-04 17:15   ` Resend " Wang YanQing
2014-08-05 13:54     ` Johan Hovold
2014-08-07 19:10       ` Wang YanQing
2014-08-08  7:54         ` Johan Hovold
2014-08-08 18:46           ` Wang YanQing
2014-08-12 14:02             ` Johan Hovold
2014-08-27 23:38               ` Wang YanQing
2014-08-29 10:38                 ` Johan Hovold [this message]

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