[PATCH] ARM: tegra: fix ulpi regression on tegra20

[Dmitry Osipenko]

Hi Marcel,

On 24.04.2018 01:05, Marcel Ziswiler wrote:
> Hi Dmitry
> 
> On Fri, 2018-04-20 at 13:50 +0300, Dmitry Osipenko wrote:
>> ...
>> I managed to find CDEV clocks in TRM this time.
> 
> And where exactly in which TRM? In all my attempts at finding anything
> CDEV2 is just a pad group which includes the DAP_MCLK2 pad in question. 

That's the DEV2 clock you're talking about below.

>> Seems assigning CDEV2 clock to
>> "ulpi-link" was correct
> 
> Sorry, but I do really not see how you can get to any such conclusion.
> 
> Whatever that CDEV2 clock exactly is. Its offset 93 points towards the
> CLK_RST_CONTROLLER_CLK_OUT_ENB_U_0 register which has CLK_ENB_DEV2_OUT
> at bit position 29 reading "Enable clock to DEV2 pad". While the TRM
> misses further documenting what exactly that DEV2 pad should be I guess
> it may point towards our suspected DAP_MCLK2. So for some reason
> besides PLL_P_OUT4 which is what that pad is actually muxed to also
> some additional DEV2 pad clock needs enabling. In addition there would
> be a CLK_RST_CONTROLLER_MISC_CLK_ENB_0 register where one could also
> specify a DEV2_OSC_DIV_SEL but I would assume that one only applies if
> the pad in question being muxed to OSC which is not the case at least
> in all device trees I have looked at.
> 
>> and both CDEV2 and PLL_P_OUT4 should be enabled,
> 
> Agreed, basically the DAP_MCLK2 pad seems gated by an additional enable
> called CLK_ENB_DEV2_OUT.
> 
>> CDEV2
>> should gate the PLL_P_OUT4 that feeds USB HW and PLL_P_OUT4 should be
>> always-enabled because it is enabled by init_table, but apparently it
>> is getting
>> disabled erroneously.
> 
> At least that was the case back when I had trouble getting ULPI to work
> on T20. Strangely latest -next right now does no longer seem to suffer
> from that same issue even if my patch is reverted but as mentioned
> before nobody stops the required PLL_P_OUT4 which is what is actually
> driving DAP_MCLK2 to not be changed behind the scenes breaking the
> whole thing again.
> 
>> Marcel, could you please revert your patch, add
>> "trace_event=clk_enable,clk_disable,clk_set_parent tp_printk" to
>> kernels cmdline
>> and post the log?
> 
> Yes, the only difference in those traces is whether or not that non-
> existent CDEV2 clock is enabled or not:
> 
> [    1.897521] clk_enable: cdev2_fixed
> [    1.901008] clk_enable: cdev2
> 
> I also do have an explanation why it kept working in my case. Probably
> the boot ROM or U-Boot is already setting that bit.
> 
>> It looks like there is some clk framework bug,
> 
> My conclusion is that there should be a DAP_MCLK2 clock which is gated
> by that CLK_ENB_DEV2_OUT but really outputs a T20 internal clock
> according to its pin muxing which if set to OSC may be further divided
> down by DEV2_OSC_DIV_SEL.

Could be that DEV2_OSC_DIV_SEL is only relevant when DAP_MCLK2 is mux'ed to OSC.
Maybe Peter could clarify everything.

>> but just in case please also try
>> to apply this patch:
>>
>> diff --git a/drivers/clk/tegra/clk-tegra-periph.c
>> b/drivers/clk/tegra/clk-tegra-periph.c
>> index 2acba2986bc6..407bd0c0ac2f 100644
>> --- a/drivers/clk/tegra/clk-tegra-periph.c
>> +++ b/drivers/clk/tegra/clk-tegra-periph.c
>> @@ -1024,7 +1024,7 @@ static void __init init_pllp(void __iomem
>> *clk_base, void
>> __iomem *pmc_base,
>>                 if (dt_clk) {
>>                         clk =
>> tegra_clk_register_pll_out("pll_p_out4",
>>                                         "pll_p_out4_div", clk_base +
>> PLLP_OUTB,
>> -                                       17, 16, CLK_IGNORE_UNUSED |
>> +                                       17, 16, CLK_IS_CRITICAL |
>>                                         CLK_SET_RATE_PARENT, 0,
>>                                         &PLLP_OUTB_lock);
>>                         *dt_clk = clk;
> 
> I did not have to do any such but maybe that would at least prevent any
> further issues on PLL_P_OUT4. However I still believe this is kind of
> wrong as PLL_P_OUT4 is what drives DAP_MCLK2 in its current muxing
> which is connected to the ULPI transceivers REFCLK pin.
> 
> BTW: That pin is usually to be driven at 24 MHz and not 26 MHz which
> CDEV2 claims to be at.

Pin is driven by the PLL_P_OUT4, which is set to 24 MHz via the init_table. The
clock tree is invalid in that regards because Tegra's clock driver defines
non-existent "cdev2_fixed" 26 MHz clock source and sets it as a parent for the
"cdev2" clock, while "cdev2" should be a gate (and maybe divider?) for the real
parent clock (PLL_P_OUT4 in our case).

> What do you think?

It looks to me that a clock signal coming from the mux'ed CDEV2 pin is routed to
the DEV2 of CLK controller (which can gate it) and then it goes out to DAP_MCLK2.

PLL_P_OUT4 ---> PINMUX_CDEV2 ---> CLK_DEV2 ---> DAP_MCLK2

But it also could be that CLK_ENB_DEV2_OUT is indeed some internal
pinmux-related clock-gate. I think Peter should have a clue.

Anyway the implementation details do not really matter for us, what matters is
that PLL_P_OUT4 clk and DEV2 clk-gate must be enabled.

Seems indeed ideally would be to have DAP_MCLK2 for the USB controller's
"ulpi-link" clock and then DEV2 will be enable bit for the DAP_MCLK2. But also
information about parent clock of the DAP_MCLK2 needs to be conducted to the clk
driver via DT, that probably would require backward-incompatible change of the
binding which is undesirable.

One tolerable solution could be to remove fake "cdev2_fixed" clock in the driver
and hardcode PLL_P_OUT4 for the parent of "cdev2". (Note that cdev1 also has a
fake parent)

index 0ee56dd04cec..4708653dedeb 100644
--- a/drivers/clk/tegra/clk-tegra20.c
+++ b/drivers/clk/tegra/clk-tegra20.c
@@ -838,8 +838,7 @@ static void __init tegra20_periph_clk_init(void)
        clks[TEGRA20_CLK_CDEV1] = clk;

        /* cdev2 */
-       clk = clk_register_fixed_rate(NULL, "cdev2_fixed", NULL, 0, 26000000);
-       clk = tegra_clk_register_periph_gate("cdev2", "cdev2_fixed", 0,
+       clk = tegra_clk_register_periph_gate("cdev2", "pll_p_out4", 0,
                                    clk_base, 0, 93, periph_clk_enb_refcnt);
        clks[TEGRA20_CLK_CDEV2] = clk;


Now the real problem is that PLL_P_OUT4 was getting disabled. We had CDEV2 clock
in the DT for "ulpi-link" and PLL_P_OUT4 was permanently enabled (supposedly) by
the Tegra's clock driver using init_table. Apparently PLL_P_OUT4 was disabled on
SCLK re-parenting, as you mentioned in the commit description. This should be
considered as a bug because one of two purposes (permanently enable and setup
default clock rate) of the init_table is defeated.

Could you please try to bisect to the point when erroneous PLL_P_OUT4 disable
issue is fixed->broken? It is quite important to know what commit changed the
behaviour. If it was some common clk issue that got fixed - that should be a
good case for us, if it was some unrelated change that obscured the issue -
that's not good and we should go back and fix the real bug.