From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Martin K. Petersen" Subject: Re: [PATCH v2 3/3] scsi/ncr5380: Improve interrupt latency during PIO tranfers Date: Fri, 09 Sep 2016 07:28:54 -0400 Message-ID: References: <20160827022959.303563262@telegraphics.com.au> <20160831044453.413698369@telegraphics.com.au> Mime-Version: 1.0 Content-Type: text/plain Return-path: Received: from aserp1040.oracle.com ([141.146.126.69]:48449 "EHLO aserp1040.oracle.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753715AbcIIL3F (ORCPT ); Fri, 9 Sep 2016 07:29:05 -0400 In-Reply-To: <20160831044453.413698369@telegraphics.com.au> (Finn Thain's message of "Wed, 31 Aug 2016 14:44:56 +1000") Sender: linux-m68k-owner@vger.kernel.org List-Id: linux-m68k@vger.kernel.org To: Finn Thain Cc: "James E.J. Bottomley" , "Martin K. Petersen" , Geert Uytterhoeven , linux-scsi@vger.kernel.org, linux-m68k@vger.kernel.org, linux-kernel@vger.kernel.org, Michael Schmitz >>>>> "Finn" == Finn Thain writes: Finn> Large PIO transfers are broken up into chunks to try to avoid Finn> disabling local IRQs for long periods. But IRQs are still disabled Finn> for too long and this causes SCC FIFO overruns during serial port Finn> transfers. Finn> This patch reduces the PIO chunk size to reduce interrupt latency Finn> to something on the order of milliseconds, at the expense of Finn> additional CPU overhead from extra iterations of the Finn> NCR5380_main() loop. Finn> That CPU overhead is a problem for slow machines (e.g. mac_scsi on Finn> 25 MHz Finn> 68030) but these machines generally use PDMA not PIO. This patch Finn> doesn't Finn> make the overhead any worse on my Mac LC III (because it only gets Finn> about 510 accesses per ms). Finn> This patch decreases disk performance by a fraction of one percent Finn> for dmx3191d on my 333 MHz PowerPC 750. Other affected hardware Finn> (such as g_NCR5380 on x86) was not tested but 5380 ISA cards Finn> generally use PDMA and not PIO. Geert? -- Martin K. Petersen Oracle Linux Engineering