imx6: PCIe imx6_pcie_assert_core_reset() hangs after watchdog reset

[Stefan Roese]

On 25.03.2015 10:39, Lucas Stach wrote:
> Am Mittwoch, den 25.03.2015, 10:29 +0100 schrieb Stefan Roese:
>> Hi!
>>
>> I'm currently facing a problem in the imx6 PCIe driver. The
>> problem happens, after the Linux imx6 internal watchdog has timed-out
>> and issues a reset (this is intended, as I'm testing this wdog right
>> now). But the next Linux boot hangs in imx6_pcie_assert_core_reset()
>> while accessing a register from the PCIe port logic memory:
>>
>> 		val = readl(pp->dbi_base + PCIE_PL_PFLR);
>>
>> The watchdog will issue a new reset once it times out again. But
>> Linux will hang here the next time as well. Only a power-cycle
>> helps. For a test purpose, I've removed the following part from
>> this function:
>>
>> 	if ((gpr1 & IMX6Q_GPR1_PCIE_REF_CLK_EN) &&
>> 	    (gpr12 & IMX6Q_GPR12_PCIE_CTL_2)) {
>> 		...
>> 	}
>>
>> Without it, Linux boots up fine. And PCIe also seems to be fully
>> functional.
>>
>> I have to admit that I have no real insight in the imx6 / DW PCIe
>> driver. So I'm checking with you experts, if you have any ideas why
>> this could be the case? And how this could be solved.
>>
>> BTW: This happens with v3.19 and v4.0-rc5 on a i.MX6Solo based board
>> (TQMa6S SoM). With no PCIe support enabled in the bootloader (U-Boot).
>
> The problem here is that the LTSSM state machine stays in "Link" state
> after the WD reset and no configuration of the core can happen in this
> state. The path inside the above if clause applies a workaround the get
> LTSSM back into the initial state.
>
> As this is a bit racy and not the intended way to do things we only
> apply this workaround if we are relatively certain that the PCIe link
> had been enabled without a reset to the core before.

Thanks for the explanation.

> So what you are saying is that after the WD reset the GPR registers are
> cleared?

No, they are not cleared.

After a power-on reset, these are the GPR values:
gpr1=48401005 gpr12=0f000000

Here the values after a "normal" reset issued via the reboot command:
gpr1=48441005 gpr12=0f004090

This is because of the shutdown handler calling
imx6_pcie_assert_core_reset().

Now after the watchdog reset, its a bit different:
gpr1=48411005 gpr12=0f004490

So this is only time (in my test cases), that the if clause is
entered (and hangs).

> If so we may need to make the logic to decide when to apply
> this WAR a bit smarter.

Any suggestions welcome.

Thanks,
Stefan