[U-Boot] [PATCH 1/3] i2c: tegra: use repeated start for reads

[Stephen Warren]

On 06/26/2014 02:11 AM, Joakim Tjernlund wrote:
>> From: Stephen Warren <swarren at wwwdotorg.org>
>> To: u-boot at lists.denx.de, Heiko Schocher <hs at denx.de>, 
>> Cc: Stephen Warren <swarren at nvidia.com>, Tom Warren <twarren at nvidia.com>
>> 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 <swarren at nvidia.com>
>>
>> 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:-)