[U-Boot] [PATCH v2] ARM: mxs: allow boards to select DC-DC switching clock source

[Michael Heimpold <mhei@heimpold.de>]

Hi Marek,

Am Saturday 12 December 2015, 02:42:47 schrieb Marek Vasut:
> On Friday, December 11, 2015 at 12:05:36 AM, Michael Heimpold wrote:
> > For some board designs, it might be useful to switch the DC-DC
> > clock source to something else rather the default 24 MHz, e.g.
> > for EMI reasons.
> 
> Can you elaborate on this ? Also, is there gonna be a user for this?

Sure. Every DC-DC switching frequency also generates some harmonics.
On usual board design, this is not a huge problem, but on our I2SE
Duckbill EnOcean devices, the EnOcean transceiver sits very close to the
i.MX because of stacked PCBs. Our hardware guys measured, that the
harmonics overlaps a little bit with the EnOcean spectrum. Changing
the DC-DC frequency results in others harmonics, which do not interfere
anymore.
I already sent a patch for adding Duckbill support and I'm preparing a v2
which would include this topic, so this would be the first user of this 
feature.

> 
> > For this, override the mxs_power_setup_dcdc_clocksource function
> > in your board support files.
> > 
> > Example:
> > void mxs_power_setup_dcdc_clocksource(void)
> > {
> > 
> >     mxs_power_select_dcdc_clocksource(POWER_MISC_FREQSEL_20MHZ);
> > 
> > }
> > 
> > Signed-off-by: Michael Heimpold <mhei@heimpold.de>
> > Cc: Marek Vasut <marex@denx.de>
> > Cc: Otavio Salvador <otavio@ossystems.com.br>
> > Cc: Fabio Estevam <fabio.estevam@freescale.com>
> > ---
> > 
> > Changes in v2:
> >   - use a weak function approach instead of ifdef'ery as suggested
> >   
> >     by Marek Vasut
> >  
> >  arch/arm/cpu/arm926ejs/mxs/spl_power_init.c | 31
> > 
> > +++++++++++++++++++++++++++++ arch/arm/include/asm/arch-mxs/sys_proto.h
> > 
> > |  2 ++
> >  
> >  2 files changed, 33 insertions(+)
> > 
> > diff --git a/arch/arm/cpu/arm926ejs/mxs/spl_power_init.c
> > b/arch/arm/cpu/arm926ejs/mxs/spl_power_init.c index 1972de8..64e215c
> > 100644
> > --- a/arch/arm/cpu/arm926ejs/mxs/spl_power_init.c
> > +++ b/arch/arm/cpu/arm926ejs/mxs/spl_power_init.c
> > @@ -248,6 +248,36 @@ static void mxs_power_setup_5v_detect(void)
> > 
> >  }
> >  
> >  /**
> > 
> > + * mxs_power_switch_dcdc_clocksource() - Switch PLL clock for DC-DC
> > converters + *
> > + * This function configures and then enables an alternative PLL clock
> > source + * for the DC-DC converters.
> > + */
> > +void mxs_power_select_dcdc_clocksource(uint32_t freqsel)
> > +{
> > +	struct mxs_power_regs *power_regs =
> > +		(struct mxs_power_regs *)MXS_POWER_BASE;
> > +
> > +	/* Select clocksource for DC-DC converters */
> > +	clrsetbits_le32(&power_regs->hw_power_misc,
> > +			POWER_MISC_FREQSEL_MASK,
> > +			freqsel);
> > +	setbits_le32(&power_regs->hw_power_misc,
> > +			POWER_MISC_SEL_PLLCLK);
> > +}
> > +
> > +/**
> > + * mxs_power_setup_dcdc_clocksource()
> 
> Please make sure this is compatible with kerneldoc.

Ah, yes. Will do in v3.

> 
> > + * Normally, there is no need to switch DC-DC clocksource. However,
> > boards
> > + * can implement this function when required.
> > + */
> > +__weak void mxs_power_setup_dcdc_clocksource(void)
> > +{
> > +	debug("SPL: Using default DC-DC clocksource\n");
> > +}
> > +
> > +/**
> > 
> >   * mxs_src_power_init() - Preconfigure the power block
> >   *
> >   * This function configures reasonable values for the DC-DC control loop
> > 
> > @@ -872,6 +902,7 @@ static void mxs_power_configure_power_source(void)
> > 
> >  	debug("SPL: Configuring power source\n");
> > 
> > +	mxs_power_setup_dcdc_clocksource();
> > 
> >  	mxs_src_power_init();
> >  	
> >  	if (readl(&power_regs->hw_power_sts) & POWER_STS_VDD5V_GT_VDDIO) {
> > 
> > diff --git a/arch/arm/include/asm/arch-mxs/sys_proto.h
> > b/arch/arm/include/asm/arch-mxs/sys_proto.h index 20ff101..4160e43 100644
> > --- a/arch/arm/include/asm/arch-mxs/sys_proto.h
> > +++ b/arch/arm/include/asm/arch-mxs/sys_proto.h
> > @@ -25,6 +25,8 @@ int mxsmmc_initialize(bd_t *bis, int id, int (*wp)(int),
> > int (*cd)(int)); void mxs_common_spl_init(const uint32_t arg, const
> > uint32_t *resptr, const iomux_cfg_t *iomux_setup,
> > 
> >  			 const unsigned int iomux_size);
> > 
> > +
> > +void mxs_power_select_dcdc_clocksource(uint32_t freqsel);
> 
> This function does not need to be exported if it's __weak I believe.

mxs_power_setup_dcdc_clocksource is the weak function, not 
mxs_power_select_dcdc_clocksource so I think this is needed.

Regards,
Michael