From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from gate.crashing.org (gate.crashing.org [63.228.1.57]) (using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits)) (Client did not present a certificate) by ozlabs.org (Postfix) with ESMTPS id B3DF8DDED7 for ; Tue, 6 May 2008 10:08:41 +1000 (EST) Subject: ALSA vs. non coherent DMA From: Benjamin Herrenschmidt To: Takashi Iwai Content-Type: text/plain Date: Tue, 06 May 2008 10:08:28 +1000 Message-Id: <1210032508.21644.129.camel@pasglop> Mime-Version: 1.0 Cc: linuxppc-dev list , alsa-devel@alsa-project.org, Linux Kernel list Reply-To: benh@kernel.crashing.org List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Hi Takashi ! I'm bringing up an old thread as I'm just discovering that the problem still hasn't been fixed. There seem to be a few issues with ALSA current usage of mmap vs. non cache coherent architecture, such as embedded PowerPC's. I can see at least two with a quick look to pcm-native.c, one I don't understand and one I think I do: - The control/status mapping. Can you elaborate a bit on what this is actually doing and why it shouldn't be done on "non coherent" architectures ? Currently this -is- done on all powerpc's, whether they are coherent or not and I want to understand what the underlying issue is. - The mmap of DMA pages. Here, the problem appears two fold: * Use of virt_to_page() on virtual addresses returned by dma_alloc_coherent(). * No using the appropriate page protection for a DMA coherent mapping to userspace. It seems like you have solved that in part with implementing a generic dma_mmap_coherent() in the past that for some reason you never merged upstream (I can track that to about 2 years ago). Is there a reason ? I think we need to at least apply a band-aid today as it's becoming a nasty issue for several non-coherent powerpc platforms. It could be in the form of implementing dma_mmap_coherent() and changing Alsa to use it with the appropriate ifdef, or just adding an ifdef CONFIG_PPC with the right code in there for now until a better solution is found. It should be trivial though. Getting the PFN from the DMA address is easy if we have the dma handle and the virtual address, though that -is- definitely platform specific. I can implement a function for that if you need. As for the pgprot, we can come up with something like pgprot_mmap_dma(). Either that or I can fold it all in a powerpc wide implementation of a dma_mmap_coherent() like we envisioned initially. Let me know what approach is preferred here and I'll come up with patches ASAP. As far as I'm concerned, this is a bug and thus must be fixed now for .26 and possibly backported to stable even if we can come up with a non invasive solution). I'm annoyed because it represents a trivial amount of code, this problem should have been fixed a long time ago. Cheers, Ben.