From mboxrd@z Thu Jan 1 00:00:00 1970 From: Badari Pulavarty Subject: Re: [BUG] x86_64 pci_map_sg modifies sg list - fails multiple map/unmaps Date: Mon, 5 Jan 2004 11:41:52 -0800 Sender: linux-scsi-owner@vger.kernel.org Message-ID: <200401051141.52236.pbadari@us.ibm.com> References: <2938942704.1073325455@aslan.btc.adaptec.com> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7BIT Return-path: Received: from e2.ny.us.ibm.com ([32.97.182.102]:30367 "EHLO e2.ny.us.ibm.com") by vger.kernel.org with ESMTP id S265343AbUAEUAQ convert rfc822-to-8bit (ORCPT ); Mon, 5 Jan 2004 15:00:16 -0500 In-Reply-To: <2938942704.1073325455@aslan.btc.adaptec.com> List-Id: linux-scsi@vger.kernel.org To: "Justin T. Gibbs" , Kernel Mailinglist , linux-scsi@vger.kernel.org Cc: Berkley Shands On Monday 05 January 2004 09:57 am, Justin T. Gibbs wrote: > Berkley Shands recently tripped over this problem. The 2.6.X pci_map_sg > code for x86_64 modifies the passed in S/G list to compact it for mapping > by the GART. This modification is not reversed when pci_unmap_sg is > called. In the case of a retried SCSI command, this causes any attempt > to map the command a second time to fail with a BUG assertion since the > nseg parameter passed into the second map call is state. nseg comes from > the "use_sg" field in the SCSI command structure which is never touched > by the HBA drivers invoking pci_map_sg. > > DMA-API.txt doesn't seem to cover this issue. Should the low-level DMA > code restore the S/G list to its original state on unmap or should the > SCSI HBA drivers be changed to update "use_sg" with the segment count > reported by the pci_map_sg() API? If the latter, this seems to contradict > the mandate in DMA-API that the nseg parameter passed into the unmap call > be the same as that passed into the map call. Most of the kernel assumes > that an S/G list can be mapped an unmapped multiple times using the same > arguments. This doesn't seem to me to be an unreasonable expectation. Hi, I ran into the same thing a month ago. I talked to Andi Kleen about this. He seems to think that, it is okay to modify the sg-list in pci_map_sg(). pci_unmap_sg() can't restore it back, since we lost lot of information. One option is to recreate entire sg-list in case of a retry. I used a patch similar to following to do this. Is this acceptable ? Thanks, Badari --- scsi_lib.c 2004-01-04 19:53:56.000000000 -0800 +++ scsi_lib.c.new 2004-01-04 19:57:35.000000000 -0800 @@ -308,6 +308,13 @@ void scsi_setup_cmd_retry(struct scsi_cm cmd->cmd_len = cmd->old_cmd_len; cmd->sc_data_direction = cmd->sc_old_data_direction; cmd->underflow = cmd->old_underflow; + if (cmd->use_sg) { + struct request *req = cmd->request; + int count; + + count = blk_rq_map_sg(req->q, req, cmd->request_buffer); + BUG_ON(count != cmd->use_sg); + } }