From mboxrd@z Thu Jan 1 00:00:00 1970 From: Stephen Warren Date: Thu, 26 Jun 2014 10:47:55 -0600 Subject: [U-Boot] [PATCH 1/3] i2c: tegra: use repeated start for reads In-Reply-To: References: <1403715449-2177-1-git-send-email-swarren@wwwdotorg.org> Message-ID: <53AC4EBB.8030108@wwwdotorg.org> List-Id: MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: u-boot@lists.denx.de On 06/26/2014 02:11 AM, Joakim Tjernlund wrote: >> From: Stephen Warren >> To: u-boot at lists.denx.de, Heiko Schocher , >> Cc: Stephen Warren , Tom Warren >> Date: 2014/06/25 19:05 >> Subject: [U-Boot] [PATCH 1/3] i2c: tegra: use repeated start for reads >> Sent by: u-boot-bounces at lists.denx.de >> >> From: Stephen Warren >> >> I2C read transactions are typically implemented as follows: >> >> START(write) address REPEATED_START(read) data... STOP >> >> However, Tegra's I2C driver currently implements reads as follows: >> >> START(write) address STOP START(read) data... STOP >> >> This sequence confuses at least the AS3722 PMIC on the Jetson TK1 board, >> leading to corrupted read data in some cases. Fix the driver to chain >> the transactions together using repeated starts to solve this. > > While I agree to use Repeated START I just wanted to share this: > A common reason for STOP START(read) sequence not working sometimes is > that > the driver initializes STOP but does not wait for the STOP to complete > before issuing a START. I don't believe that's the case here, since all this patch does is set a flag to indicate whether the write transaction (to set the intra-chip register address) generates STOP or REPEATED_START at the end. If the code or HW wasn't waiting for the STOP to complete, I see no reason it would wait for the REPEATED_START to complete either, so I think the subsequent register read transaction would be corrupted in either case. There's certainly code in tegra_i2c.c:wait_for_transfer_complete() to poll until each transaction completes before starting the next, and there's even error handling to detect any problems there:-)