From mboxrd@z Thu Jan 1 00:00:00 1970 From: Oliver Neukum Subject: Re: USB transfer_buffer allocations on 64bit systems Date: Fri, 9 Apr 2010 08:04:35 +0200 Message-ID: <201004090804.36213.oliver@neukum.org> References: Mime-Version: 1.0 Content-Type: Text/Plain; charset="iso-8859-1" Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: Sender: linux-kernel-owner@vger.kernel.org To: Alan Stern Cc: Daniel Mack , linux-kernel@vger.kernel.org, Pedro Ribeiro , akpm@linux-foundation.org, Greg KH , alsa-devel@alsa-project.org, linux-usb@vger.kernel.org List-Id: alsa-devel@alsa-project.org Am Freitag, 9. April 2010 00:20:36 schrieb Alan Stern: > > > That would work, but it doesn't match the way existing drivers use the > > > interface. For example, the audio driver allocates a 16-byte coherent > > > buffer and then uses four bytes from it for each of four different > > > URBs. > > > > That will not work with any fallback that does not yield a coherent buffer. > > What you mean isn't entirely clear. But it certainly does work in > various circumstances that don't yield coherent buffers. For example, > it works if the controller uses PIO instead of DMA. It also works if > the controller uses DMA and the URBs have to be bounced. It'll work on x86. On incoherent architectures this violates the cacheline rules for DMA-mapping if you have to bounce. So it seems to me that if you want to share a buffer between URBs, it must be coherent. Regards Oliver