From mboxrd@z Thu Jan 1 00:00:00 1970 From: Mika Westerberg Subject: Re: [PATCH 2/2] i2c: designware: fix rx fifo depth tracking Date: Mon, 21 Nov 2016 12:40:32 +0200 Message-ID: <20161121104032.GH1446@lahna.fi.intel.com> References: <20161118193542.GO1041@n2100.armlinux.org.uk> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Received: from mga14.intel.com ([192.55.52.115]:37869 "EHLO mga14.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753657AbcKUKo6 (ORCPT ); Mon, 21 Nov 2016 05:44:58 -0500 Content-Disposition: inline In-Reply-To: Sender: linux-i2c-owner@vger.kernel.org List-Id: linux-i2c@vger.kernel.org To: Russell King Cc: Andrew Jackson , Liviu Dudau , Wolfram Sang , Jarkko Nikula , Andy Shevchenko , linux-i2c@vger.kernel.org, linux-arm-kernel@lists.infradead.org On Fri, Nov 18, 2016 at 07:40:10PM +0000, Russell King wrote: > When loading the TX fifo to receive bytes on the I2C bus, we incorrectly > count the number of bytes: > > rx_limit = dev->rx_fifo_depth - dw_readl(dev, DW_IC_RXFLR); > > while (buf_len > 0 && tx_limit > 0 && rx_limit > 0) { > if (rx_limit - dev->rx_outstanding <= 0) > break; > rx_limit--; > dev->rx_outstanding++; > } > > DW_IC_RXFLR indicates how many bytes are available to be read in the > FIFO, dev->rx_fifo_depth is the FIFO size, and dev->rx_outstanding is > the number of bytes that we've requested to be read so far, but which > have not been read. > > Firstly, increasing dev->rx_outstanding and decreasing rx_limit and then > comparing them results in each byte consuming "two" bytes in this > tracking, so this is obviously wrong. > > Secondly, the number of bytes that _could_ be received into the FIFO at > any time is the number of bytes we have so far requested but not yet > read from the FIFO - in other words dev->rx_outstanding. > > So, in order to request enough bytes to fill the RX FIFO, we need to > request dev->rx_fifo_depth - dev->rx_outstanding bytes. > > Modifying the code thusly results in us reaching the maximum number of > bytes outstanding each time we queue more "receive" operations, provided > the transfer allows that to happen. > > Signed-off-by: Russell King Reviewed-by: Mika Westerberg