From mboxrd@z Thu Jan 1 00:00:00 1970 From: Arnd Bergmann Subject: Re: [PATCH] asm-generic: add dma-mapping-linear.h Date: Mon, 1 Jun 2009 11:41:32 +0100 Message-ID: <200906011141.33463.arnd@arndb.de> References: <20090601082943.GA5550@flint.arm.linux.org.uk> <20090601183323V.fujita.tomonori@lab.ntt.co.jp> <20090601101405.GA19806@flint.arm.linux.org.uk> Mime-Version: 1.0 Content-Type: Text/Plain; charset="iso-8859-1" Content-Transfer-Encoding: 7bit Return-path: Received: from moutng.kundenserver.de ([212.227.126.171]:52309 "EHLO moutng.kundenserver.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1757520AbZFAKln (ORCPT ); Mon, 1 Jun 2009 06:41:43 -0400 In-Reply-To: <20090601101405.GA19806@flint.arm.linux.org.uk> Sender: linux-arch-owner@vger.kernel.org List-ID: To: Russell King Cc: FUJITA Tomonori , linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org On Monday 01 June 2009, Russell King wrote: > So, on a non-DMA coherent cache architecture, when DMA is normally > performed the data ends up in RAM with the cache flushed for that > region. If, instead dma_map_single uses a bounce buffer to do that > DMA, then the same needs to be true of the original buffer - the > data needs to be in RAM with the cache flushed. While this seems logical from a correctness perspective, I would like to understand why it actually matters. Flushing the cache on the original buffer will impact performance but doesn't generally make a difference to users. In cases where you need the cache to be flushed for aliasing reasons (VIPT caches...), the architecture specific code should flush that buffer somewhere, but do we really need to flush it for all architectures? Arnd <><