serial: clk: bcm2835: Strange effects when using aux-uart in console

Tue Feb 16 10:57:37 PST 2016

Hi Martin & Stefan,

Quoting Martin Sperl (2016-02-13 03:24:43)
> 
> > On 13.02.2016, at 11:01, Stefan Wahren <stefan.wahren at i2se.com> wrote:
> > 
> > Hi Martin,
> > 
> >> Martin Sperl <kernel at martin.sperl.org> hat am 12. Februar 2016 um 20:44
> >> geschrieben:
> >> 
> >> So the issue is triggered as soon as the plld_per
> >> pll divider gets disabled/reenabled.
> >> 
> >> This happens because the clk_hw.core.prepare_count
> >> drops down to 0 and then unprepare is called.
> >> 
> >> So we need to increase the ref-count for the pll
> >> and pll_dividers to at least 1 so that these never
> >> get disabled - at least for now until we can come
> >> up with a better contract with the firmware.
> >> 
> >> Obviously this may impact other drivers as well
> >> where a pll is used for the first time - if nothing
> >> else uses it and the clock gets released, then
> >> the clock would trigger a unprepare of the whole
> >> branch of the clock tree.
> >> 
> >> The question is: how can we solve it in an acceptable
> >> manner?
> >> 
> >> Do we need a driver that just holds a reference to
> >> those clocks? Or should we just prepare the clock
> >> after registering it in clk-bcm2835.c?
> >> 
> >> As for why does this not show up when compiled in?
> >> It seems that in that case the amba_pl011 driver
> >> never gets removed and then probed again.
> >> 
> >> This is possibly related to the optional use of DMA,
> >> with the amba-pl011 driver that retries the install,
> >> which is not supported on the bcm2835 - at least that
> >> is what the datasheet says. And DMA is (probably) not
> >> enabled during the early boot stages, so it does not
> >> fail once when it tries to register DMA.
> >> 
> >> Thanks,
> >> Martin
> >> 
> > 
> > i think i must correct myself. The fixed apb should stay since there is no
> > dynamic replacement and a proper disabling of plld shouldn't cause this freeze.
> > 
> > I have the suspicion the freeze is caused by a clock glitch or lock-up.
> > 
> > Could you please revert your changes and apply the attached patch? Maybe we can
> > see more.
> 
> I will give it a try.
> 
> But to me it seems as if the disabling of PLLD produces a hickup in the 
> firmware (which relies on PLLD-DIV and thus PLL).
> 
> This would also explain the 3-4 second bar/blanking pattern seen on HDMI,
> which I would guess is related to a reset loop...
> 
> I got a simple fix for now, that just prepares a few clocks/pll/pll_divs
> during probing, so that they will never get unprepared/fully stopped):
> 
> diff --git a/drivers/clk/bcm/clk-bcm2835.c b/drivers/clk/bcm/clk-bcm2835.c
> index 015e687..fe0c401 100644
> --- a/drivers/clk/bcm/clk-bcm2835.c
> +++ b/drivers/clk/bcm/clk-bcm2835.c
> @@ -37,6 +37,7 @@
>   * generator).
>   */
> 
> +#include <linux/clk.h>
>  #include <linux/clk-provider.h>
>  #include <linux/clkdev.h>
>  #include <linux/clk/bcm2835.h>
> @@ -1504,6 +1505,7 @@ static int bcm2835_clk_probe(struct platform_device *pdev)
>         struct clk **clks;
>         struct bcm2835_cprman *cprman;
>         struct resource *res;
> +       int ret;
> 
>         cprman = devm_kzalloc(dev, sizeof(*cprman), GFP_KERNEL);
>         if (!cprman)
> @@ -1597,6 +1599,37 @@ static int bcm2835_clk_probe(struct platform_device *pdev
>         clks[BCM2835_CLOCK_PWM] =
>                 bcm2835_register_clock(cprman, &bcm2835_clock_pwm_data);
> 
> +       /*
> +        * Several PLLs, PLL-dividers and clocks need to get prepared
> +        * so that they are never unprepared and released.
> +        * We run this separately as there may be dependencies that would
> +        * need to be fullfilled first...
> +        * Note: Especially unpreparing PLLD_PER will kill the system
> +        *       (even if it is prepared again almost immediately
> +        *        the HDMI display will fail)
> +        */
> +#define CLK_PREPARE_INITIAL(clkid)                                     \
> +       if (clks[clkid] && (ret = clk_prepare(clks[clkid])))            \
> +               dev_err(dev, "could not prepare clock %pC - %d\n",      \
> +                       clks[clkid], ret);

You should not only prepare the clock, but enable it as well, even if
your .enable callback is no-op. s/clk_prepare/clk_prepare_enable/ should
do it. But instead of all of this stuff below...

> +       CLK_PREPARE_INITIAL(BCM2835_PLLA);
> +       CLK_PREPARE_INITIAL(BCM2835_PLLB);
> +       CLK_PREPARE_INITIAL(BCM2835_PLLC);
> +       CLK_PREPARE_INITIAL(BCM2835_PLLD);
> +       CLK_PREPARE_INITIAL(BCM2835_PLLH);
> +       CLK_PREPARE_INITIAL(BCM2835_PLLA_CORE);
> +       CLK_PREPARE_INITIAL(BCM2835_PLLA_PER);
> +       CLK_PREPARE_INITIAL(BCM2835_PLLC_CORE0);
> +       CLK_PREPARE_INITIAL(BCM2835_PLLC_CORE1);
> +       CLK_PREPARE_INITIAL(BCM2835_PLLC_CORE2);
> +       CLK_PREPARE_INITIAL(BCM2835_PLLC_PER);
> +       CLK_PREPARE_INITIAL(BCM2835_PLLD_CORE);
> +       CLK_PREPARE_INITIAL(BCM2835_PLLD_PER);
> +       CLK_PREPARE_INITIAL(BCM2835_PLLH_RCAL);
> +       CLK_PREPARE_INITIAL(BCM2835_PLLH_AUX);
> +       CLK_PREPARE_INITIAL(BCM2835_PLLH_PIX);
> +       CLK_PREPARE_INITIAL(BCM2835_CLOCK_VPU);

... how about using the shiny new HAND_OFF clocks feature? See:

http://lkml.kernel.org/r/<1455225554-13267-1-git-send-email-mturquette@baylibre.com>

It seems like you need clocks to remain enabled from the time they are
registered until a clk consumer driver claims them and Does The Right
Thing. (e.g. only gates them when it is safe to do so, such as display
being turned off by user).

Let me know if those patches will help you. The first three patches in
the series introduce CRITICAL clocks, which never gate. Patches 4-6 are
a solution that I prefer, allowing the clocks to remain enabled by the
bootloader, and only gated later on when a consumer driver claims them.
No changes to clock consumer drivers are required, the framework handles
the transition of reference counts and the clock provider driver only
has to set the CLK_ENABLE_HAND_OFF flag for the affected clocks.

Regards,
Mike

> +
>         return of_clk_add_provider(dev->of_node, of_clk_src_onecell_get,
>                                    &cprman->onecell);
>  }
> 
> And that resolves the issue and also allows loading/unloading of the
> amba-pl011 module as well without any hickups.
> 
> I guess it would be better if we did make this configurable in the DT.
> (See patch 1 of my RFC patchset, which could get extended to handle this
> as well - at least a portion thereof).
> 
> But in more general terms: we need to come up with some agreement
> on ownership of the clocks between linux and the firmware and how
> to change them without impacting the other. 
> 
> One example is the “overclocking” that currently happens for
> downstream kernels in the firmware, which impacts some of the
> plls and PLLdivs as well as the sram control registers.
> 
> The reason for this seems to be that for overclocking the SRAM
> parameters also need to get changed and that can only get done
> reliably when running the code from L1/L2 caches - and that
> requirement of not touching SRAM while changing the SRAM parameters
> is probably hard to achieve cleanly on ARM alone.
> 
> Ciao,
>         Martin
> 
> P.s: unloading amba_pl011 gives the following error:
> root at raspcm:~# rmmod amba_pl011
> [  142.318263] Trying to free nonexistent resource <0000000020201000-0000000020201fff>
> but that is an issue not related to clock...
> 
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