From mboxrd@z Thu Jan 1 00:00:00 1970 From: Wolfgang Denk Subject: Re: Forcing PIO mode instead of DMA via DT property Date: Mon, 23 Jul 2012 07:47:58 +0200 Message-ID: <20120723054758.C622C200263@gemini.denx.de> References: <201207222328.27008.marex@denx.de> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: In-reply-to: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: devicetree-discuss-bounces+gldd-devicetree-discuss=m.gmane.org-uLR06cmDAlY/bJ5BZ2RsiQ@public.gmane.org Sender: "devicetree-discuss" To: Aggrwal Poonam-B10812 Cc: Marek Vasut , "pavel-ynQEQJNshbs@public.gmane.org" , "devicetree-discuss-uLR06cmDAlY/bJ5BZ2RsiQ@public.gmane.org" List-Id: devicetree@vger.kernel.org Dear Poonam, In message you wrote: > > > I'm trying to implement a driver that can do both DMA and PIO, and it > > would be nice if the user was able to select the mode (on a per-bus > > basis) using the DT. > > The PIO mode can reduce the overhead in some cases and therefore be > > better choice than the DMA (for example when most transfers move only > > very few data, or when board-specific hardware properties kick in). > > > > I was thinking about using some "manf,use-pio" DT property, but I haven't > > found any such example yet, so I wonder if this is a good idea. > Not sure, but as far as I understand device tree is to publish the hardware > capabilities, not the user choice/configuration. > One option would be to put a compile time flag in the driver, which prefers > PIO than DMA. > I believe by default DMA is the first preference. We just had such a situation, where PIO had to be used on a specific bus, because due to a hardware problem (board design error) DMA would crash the system. Yet the same driver/kernel would work fine with DMA on other buses / boards. OK, this was on x86, so DT would not have helped, but I think a bus-specific property of being able to do DMA (or not) might indeed make sense for the DT. Best regards, Wolfgang Denk -- DENX Software Engineering GmbH, MD: Wolfgang Denk & Detlev Zundel HRB 165235 Munich, Office: Kirchenstr.5, D-82194 Groebenzell, Germany Phone: (+49)-8142-66989-10 Fax: (+49)-8142-66989-80 Email: wd-ynQEQJNshbs@public.gmane.org "I haven't lost my mind - it's backed up on tape somewhere."