From mboxrd@z Thu Jan 1 00:00:00 1970 From: Marek Vasut Subject: Re: Forcing PIO mode instead of DMA via DT property Date: Mon, 23 Jul 2012 05:31:02 +0200 Message-ID: <201207230531.02305.marex@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: "pavel-ynQEQJNshbs@public.gmane.org" , "devicetree-discuss-uLR06cmDAlY/bJ5BZ2RsiQ@public.gmane.org" , Wolfgang Denk List-Id: devicetree@vger.kernel.org Dear Aggrwal Poonam-B10812, > > -----Original Message----- > > From: devicetree-discuss [mailto:devicetree-discuss- > > bounces+poonam.aggrwal=freescale.com-uLR06cmDAlY/bJ5BZ2RsiQ@public.gmane.org] On Behalf Of Marek > > Vasut > > Sent: Monday, July 23, 2012 2:58 AM > > To: devicetree-discuss-uLR06cmDAlY/bJ5BZ2RsiQ@public.gmane.org > > Cc: pavel-ynQEQJNshbs@public.gmane.org; Wolfgang Denk > > Subject: Forcing PIO mode instead of DMA via DT property > > > > Hi guys, > > > > 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. This might actually be one of the things I've been missing about DT, so thanks for stating that clearly :-) > 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. Certainly, but please consider you might want to use PIO only on selected instances of the driver (aka. selected busses, I wasn't probably clear enough, this is a bus driver), not degrade performance on all of them. Possibly due to a specific hardware wiring of the particular system. > Regards > Poonam Best regards, Marek Vasut