Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753938Ab0DIOuc (ORCPT ); Fri, 9 Apr 2010 10:50:32 -0400 Received: from smtp-out003.kontent.com ([81.88.40.217]:58535 "EHLO smtp-out003.kontent.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753638Ab0DIOua (ORCPT ); Fri, 9 Apr 2010 10:50:30 -0400 From: Oliver Neukum To: Alan Stern Subject: Re: USB transfer_buffer allocations on 64bit systems Date: Fri, 9 Apr 2010 16:50:31 +0200 User-Agent: KMail/1.12.4 (Linux/2.6.34-rc3-0.1-default; KDE/4.3.5; x86_64; ; ) 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 References: In-Reply-To: MIME-Version: 1.0 Content-Type: Text/Plain; charset="iso-8859-1" Content-Transfer-Encoding: 7bit Message-Id: <201004091650.31488.oliver@neukum.org> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1181 Lines: 26 Am Freitag, 9. April 2010 16:41:48 schrieb Alan Stern: > > It'll work on x86. On incoherent architectures this violates the cacheline > > rules for DMA-mapping if you have to bounce. > > Not true. Consider: The driver allocates a 16-byte buffer (xbuf) > divided up into four sets of four bytes, and sets > > urb[i].transfer_buffer_dma = xbuf_dma + 4*i; > > Then usb_submit_urb(urb[i]) will copy the appropriate four bytes to a > bounce buffer and map the bounce buffer. Accesses to the other parts > of xbuf won't violate the cacheline rules, because xbuf isn't mapped > for DMA -- only the bounce buffer is. When urb[i] completes, the > bounce buffer contents will be copied back to the original four bytes > in xbuf. Again, there is no violation of cacheline rules. I think you are assuming that either every or no part of the buffer is mapped for DMA in place. I don't think you can assume that. Regards Oliver -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/