[U-Boot] [PATCH] mx6: Disable Power Down Bit of watchdog

[Fabio Estevam]

On Thu, Feb 7, 2013 at 12:39 PM, Otavio Salvador
<otavio at ossystems.com.br> wrote:
> On Thu, Feb 7, 2013 at 12:28 PM, Fabio Estevam
> <fabio.estevam at freescale.com> wrote:
>> On a mx6qsabresd revision C board with rev1.2 mx6q, the system gets resetted and it is not able
>> to reach the Linux prompt.
>>
>> Clearing the PDE - Power Down Enable bit fixes the problem.
>>
>> Signed-off-by: Fabio Estevam <fabio.estevam at freescale.com>
>
> What is not clear to me is why this affects only revision C of the
> board. Is it clear for you?

Yes, I investigated this further and I see that on mx6qsabresd revB
the watchdog does not cut power to the mx6.

For example:

- On mx6qsabresd revB:

U-Boot > reset
resetting ...


U-Boot 2013.01-10524-g432a3aa-dirty (Feb 07 2013 - 13:34:46)

CPU:   Freescale i.MX6Q rev1.1 at 792 MHz
Reset cause: WDOG
Board: MX6Q-SabreSD
DRAM:  1 GiB
MMC:   FSL_SDHC: 0, FSL_SDHC: 1, FSL_SDHC: 2
*** Warning - bad CRC, using default environment
In:    serial
Out:   serial
Err:   serial
Net:   FEC [PRIME]
Warning: FEC using MAC address from net device

Hit any key to stop autoboot:  0
U-Boot >

- On mx6qsabresd revC:

U-Boot > reset
resetting ...


U-Boot 2013.01-10524-g432a3aa-dirty (Feb 07 2013 - 13:34:46)

CPU:   Freescale i.MX6Q rev1.1 at 792 MHz
Reset cause: POR
Board: MX6Q-SabreSD
DRAM:  1 GiB
MMC:   FSL_SDHC: 0, FSL_SDHC: 1, FSL_SDHC: 2
*** Warning - bad CRC, using default environment
In:    serial
Out:   serial
Err:   serial
Net:   FEC [PRIME]
Warning: FEC using MAC address from net device

Hit any key to stop autoboot:  0
U-Boot >

So the watchdog reset happens on both revisions, but only on revC it
causes a POR due to its POR/watchdog circuitry.

So in order to fix this:

1. We can always clear Power Down Enable (PDE) bit as I did on this
patch and how it is done on FSL U-boot.

2. Check the board revision and only clear (PDE) for revC board.

I would prefer number 1. as if someone copies the schematics of a revC
board, but does not burn the fuses to indicate that their hardware
behaves as a mx6qsabresd revC, then the reset problem will occur on
their own board.

I can also improve the commit log of the original patch to provide more details.