From mboxrd@z Thu Jan 1 00:00:00 1970 From: Stefan Richter Subject: Scatter-gather segment merges by IOMMU? Date: Fri, 08 Aug 2008 21:44:31 +0200 Message-ID: <489CA21F.1030604@s5r6.in-berlin.de> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Return-path: Received: from einhorn.in-berlin.de ([192.109.42.8]:50239 "EHLO einhorn.in-berlin.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752541AbYHHToe (ORCPT ); Fri, 8 Aug 2008 15:44:34 -0400 Sender: linux-scsi-owner@vger.kernel.org List-Id: linux-scsi@vger.kernel.org To: linux-scsi@vger.kernel.org Cc: linux-kernel@vger.kernel.org Hi all, the block layer usually tries to merge s/g segments if consecutive segments combined fit into the queue's max_segment_size. When such a scatter gather list is DMA-mapped, can it happen that an IOMMU collapses the elements even further, so that sg_dma_len() of a DMA-mapped s/g segment exceeds max_segment_size? As I understood some discussions in the past, this could indeed happen, which is a nuisance. But I may have misunderstood something, or something may have changed in the meantime... -- Stefan Richter -=====-==--- =--- -=--- http://arcgraph.de/sr/