From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Grant Likely" Subject: Re: ALSA vs. non coherent DMA Date: Wed, 7 May 2008 09:44:01 -0600 Message-ID: References: <1210032508.21644.129.camel@pasglop> <4821BB0E.80000@freescale.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: Received: from an-out-0708.google.com (an-out-0708.google.com [209.85.132.243]) by alsa0.perex.cz (Postfix) with ESMTP id 1E1BE24395 for ; Wed, 7 May 2008 17:44:04 +0200 (CEST) Received: by an-out-0708.google.com with SMTP id d31so95112and.80 for ; Wed, 07 May 2008 08:44:01 -0700 (PDT) In-Reply-To: <4821BB0E.80000@freescale.com> Content-Disposition: inline List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: alsa-devel-bounces@alsa-project.org Errors-To: alsa-devel-bounces@alsa-project.org To: Timur Tabi Cc: Takashi Iwai , benh@kernel.crashing.org, alsa-devel@alsa-project.org, Linux Kernel list , linuxppc-dev list List-Id: alsa-devel@alsa-project.org On Wed, May 7, 2008 at 8:22 AM, Timur Tabi wrote: > Takashi Iwai wrote: > > > This is a mmap of the data record to be shared in realtime with apps. > > The app updates its data pointer (appl_ptr) on the mmapped buffer > > while the driver updates the data (e.g. DMA position, called hwptr) on > > the fly on the mmapped record. Due to its real-time nature, it has to > > be coherent -- at least, it was a problem on ARM. > > This doesn't sound like a coherency problem to me, and least not one you'd find > on PowerPC. Both the driver and the application run on the host CPU, so there > shouldn't be any coherency problem. My understanding is that a "non coherent" > platform is one where the host CPU isn't aware when a *hardware device* writes > directly to memory, e.g. via DMA. IIRC, some ARMs have a different situation because the dcache is virtually instead of physically tagged. Therefore, the kernel mapping may not see data that has not been flushed out of the user space mappings. (Someone please correct me if I'm wrong). Cheers, g. -- Grant Likely, B.Sc., P.Eng. Secret Lab Technologies Ltd.