From mboxrd@z Thu Jan 1 00:00:00 1970 From: Russell King - ARM Linux Subject: Re: [PATCH] ARM: PL011: Add support for Rx DMA buffer polling Date: Fri, 11 Jan 2013 16:18:28 +0000 Message-ID: <20130111161828.GD23505@n2100.arm.linux.org.uk> References: <006c01cdefc2$c7e1cb90$57a562b0$@min@lge.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Received: from caramon.arm.linux.org.uk ([78.32.30.218]:44869 "EHLO caramon.arm.linux.org.uk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754921Ab3AKQTC (ORCPT ); Fri, 11 Jan 2013 11:19:02 -0500 Content-Disposition: inline In-Reply-To: <006c01cdefc2$c7e1cb90$57a562b0$@min@lge.com> Sender: linux-serial-owner@vger.kernel.org List-Id: linux-serial@vger.kernel.org To: Chanho Min Cc: 'Alan Cox' , 'Greg Kroah-Hartman' , 'Linus Walleij' , linux-kernel@vger.kernel.org, linux-serial@vger.kernel.org On Fri, Jan 11, 2013 at 03:13:31PM +0900, Chanho Min wrote: > In DMA support, Received data is not triggered until the DMA buffer is filled. > In order to actually use Rx DMA, We would like to suggest the use of the timer > for polling DMA buffer. It makes possible character-level trigger. > In our test, no data loss occurred at high-baudrate as compared with interrupt > driven (We tested with 3Mbps). We know it will be because of the increased Rx > buffer rather than DMA effect. but,It is very useful for high speed uart device. > We changes: > > - CONFIG_SERIAL_AMBA_PL011_DMAPOLL is added to select this feature. > > - add timer and last_residue for polling. Every polling, timer handler checks > the residue in the dma buffer and transfer data to the tty. Also, last_residue > is updated for the next polling. > > - We use consistent DMA mappings instead of streaming DMA mappings to avoid from > the frequent cache operation. > > - pl011_dma_rx_chars is modified. the pending size is recalculated because data > can be taken by polling. > > - add module parameter for adjusting polling interval. Here's the question: what is the effect of having to poll every tick on the overall system? What I'm getting at is: what is the cost of that polling with DMA enabled over not having DMA on the receive side at all? What is the trade-off vs baud rate? Should we scale the polling interval according to baud rate?