Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757652AbcDMLWv (ORCPT ); Wed, 13 Apr 2016 07:22:51 -0400 Received: from mail-wm0-f47.google.com ([74.125.82.47]:37940 "EHLO mail-wm0-f47.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753318AbcDMLWs (ORCPT ); Wed, 13 Apr 2016 07:22:48 -0400 Message-ID: <1460546565.19152.148.camel@nexus-software.ie> Subject: Re: [PATCH v1 12/12] serial: 8250_lpss: enable DMA on Intel Quark UART From: "Bryan O'Donoghue" To: Andy Shevchenko , Andy Shevchenko Cc: Vinod Koul , "linux-kernel@vger.kernel.org" , dmaengine , Greg Kroah-Hartman , "Puustinen, Ismo" , Heikki Krogerus , "linux-serial@vger.kernel.org" Date: Wed, 13 Apr 2016 12:22:45 +0100 In-Reply-To: <1460479824.6620.121.camel@linux.intel.com> References: <1460061433-63750-1-git-send-email-andriy.shevchenko@linux.intel.com> <1460061433-63750-13-git-send-email-andriy.shevchenko@linux.intel.com> <1460388795.19152.38.camel@nexus-software.ie> <1460478320.19152.92.camel@nexus-software.ie> <1460479824.6620.121.camel@linux.intel.com> Organization: Nexus Software Content-Type: text/plain; charset="UTF-8" X-Mailer: Evolution 3.16.5-1ubuntu3.1 Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 782 Lines: 24 On Tue, 2016-04-12 at 19:50 +0300, Andy Shevchenko wrote: > > I haven't read your V2 yet but on this, I'd suggest raising the > burst > > size to 32 bytes for UART (no higher) we found during bringup that > > larger sizes "fall-over and die" but, anything up to 32 bytes is OK > - > > and therefore you should be able to reduce the number of > > bursts/interrupts etc. > > It can't be more that FIFO size and recommendation as far as I know > is > FIFO/2, which is exactly 8 bytes. Why not ? We went as high as 32 bytes previously in the BSP with no obvious errors. At 8 bytes or 1/2 of the FIFO size I'd ask the question is DMA even worth it i.e. does it take more time to setup and execute a DMA transaction @ 1/2 FIFO size than just writing straight into the FIFO ? --- bod