Re: [PATCH] ARM: i.MX53: Set pll3 directly to 216MHz.

[Mogens Lauridsen <mlauridsen171@gmail.com>]

On Thu, Jun 28, 2018 at 10:30 AM, Sascha Hauer <s.hauer@pengutronix.de> wrote:
> On Wed, Jun 27, 2018 at 04:07:11PM +0200, Mogens Lauridsen wrote:
>> PLL3 was first set to 400MHz and then some peripheral was switched
>> to PLL3. Finally PLL3 was set to 216MHz. This could make some
>> i.MX538 hang in a dead loop in the boot process.
>
> Let's see what the code currently does:
>
> By reset default the clock path to the DDR is:
>
> PLL2 (192MHz) -> periph_clk -> main_bus_clk -> axi_a_podf -> axi_a (/1 = 192MHz) -> ddr_clk_root
>
> PLL2 is running at 192MHz. The code now tries to switch PLL2 from 192MHz
> to 400MHz. This requires that the RAM is driven by some other clock
> during
> the PLL reconfiguration. The code switches the clock path to:
>
> PLL3 (400MHz) -> periph_clk -> main_bus_clk -> axi_a_podf -> axi_a (/2 = 200MHz) -> ddr_clk_root
>
> Then PLL2 is reconfigured to 400MHz and the DDR clock path is switched
> back to the original path, this time with the PLL runnning at 400MHz, so
> RAM is then running at the desired speed.
>
> The code configures PLL3 to 400MHz probably to keep the DDR frequency
> nearly constant during the transition.
>
> I have no idea why your change helps you. When I understand correctly
> then you are running nearly at half the frequency during the transition
> (400/216).
>
> I'm afraid to merge such change as long as we do not fully understand
> what we are doing and why it helps.
I see your point.
I don't know exactly why it helps. But to me it looks suspicious to
set a clock to 400MHz
which in the end only is suppose to run at 216 MHz. I fear that there are small
differences in the individual iMX538 and in some of them the pll might
not always be able
to get a lock at the high frequency. The problem seem to have some
relationship with the temperature of the chip. (Higher temperature also seem
to solve the problem).
>
> BTW the current code is the same as on U-Boot which is derived from the
> original Freescale code, so this is not exactly new or barebox specific.
Yes, I did see that the code has been in barebox since 2011. I am also
puzzled why we see this, and nobody else has seen/reported it. We
are using the i.MX538 which is in a pop package with the DDR mounted
on the top of the i.MX. This chip might not be widely used.
>
> Do you have anything special in your dcd table that influences the
> clocks in an unexpected way?
Not as far as I know. Could you give me a hint of what I should look for?

Best regards,
Mogens

>
> Sascha
>
>
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