RE: [PATCH] clk: imx: lpcg-scu: SW workaround for errata (e10858)

[Peng Fan]

> Subject: Re: [PATCH] clk: imx: lpcg-scu: SW workaround for errata (e10858)
> 
> On 20-09-08 17:14:47, peng.fan@xxxxxxx wrote:
> > From: Peng Fan <peng.fan@xxxxxxx>
> >
> > Back-to-back LPCG writes can be ignored by the LPCG register due to a
> > HW bug. The writes need to be separated by at least 4 cycles of the
> > gated clock.
> >
> > The workaround is implemented as follows:
> > 1. For clocks running greater than or equal to 24MHz, a read followed
> > by the write will provide sufficient delay.
> > 2. For clocks running below 24MHz, add a delay of 4 clock cylces after
> > the write to the LPCG register.
> >
> > Signed-off-by: Peng Fan <peng.fan@xxxxxxx>
> > ---
> >  drivers/clk/imx/clk-lpcg-scu.c | 31 +++++++++++++++++++++++++++++--
> >  1 file changed, 29 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/clk/imx/clk-lpcg-scu.c
> > b/drivers/clk/imx/clk-lpcg-scu.c index 1f0e44f921ae..6ee9d2caedf2
> > 100644
> > --- a/drivers/clk/imx/clk-lpcg-scu.c
> > +++ b/drivers/clk/imx/clk-lpcg-scu.c
> > @@ -6,6 +6,7 @@
> >
> >  #include <linux/bits.h>
> >  #include <linux/clk-provider.h>
> > +#include <linux/delay.h>
> >  #include <linux/err.h>
> >  #include <linux/io.h>
> >  #include <linux/slab.h>
> > @@ -38,6 +39,31 @@ struct clk_lpcg_scu {
> >
> >  #define to_clk_lpcg_scu(_hw) container_of(_hw, struct clk_lpcg_scu,
> > hw)
> >
> > +/* e10858 -LPCG clock gating register synchronization errata */
> > +static void do_lpcg_workaround(u32 rate, void __iomem *reg, u32 val)
> > +{
> > +	writel(val, reg);
> > +
> > +	if (rate >= 24000000 || rate == 0) {
> > +		u32 reg1;
> > +
> > +		/*
> > +		 * The time taken to access the LPCG registers from the AP core
> > +		 * through the interconnect is longer than the minimum delay
> > +		 * of 4 clock cycles required by the errata.
> > +		 * Adding a readl will provide sufficient delay to prevent
> > +		 * back-to-back writes.
> > +		 */
> > +		reg1 = readl(reg);
> > +	} else {
> > +		/*
> > +		 * For clocks running below 24MHz, wait a minimum of
> > +		 * 4 clock cycles.
> > +		 */
> > +		ndelay(4 * (DIV_ROUND_UP(1000000000, rate)));
> > +	}
> 
> Just to make sure this is totally understood, if the lpcg consumer needs to two
> enables/disables in less than 4 multiplied by the clock period, the second
> enable/disable will be delayed.

You surely wanna this following errata description.

Thanks,
Peng.

> 
> If we're fine with this, then here is my R-b.
> 
> Reviewed-by: Abel Vesa <abel.vesa@xxxxxxx>
> 
> > +}
> > +
> >  static int clk_lpcg_scu_enable(struct clk_hw *hw)  {
> >  	struct clk_lpcg_scu *clk = to_clk_lpcg_scu(hw); @@ -54,7 +80,8 @@
> > static int clk_lpcg_scu_enable(struct clk_hw *hw)
> >  		val |= CLK_GATE_SCU_LPCG_HW_SEL;
> >
> >  	reg |= val << clk->bit_idx;
> > -	writel(reg, clk->reg);
> > +
> > +	do_lpcg_workaround(clk_hw_get_rate(hw), clk->reg, reg);
> >
> >  	spin_unlock_irqrestore(&imx_lpcg_scu_lock, flags);
> >
> > @@ -71,7 +98,7 @@ static void clk_lpcg_scu_disable(struct clk_hw *hw)
> >
> >  	reg = readl_relaxed(clk->reg);
> >  	reg &= ~(CLK_GATE_SCU_LPCG_MASK << clk->bit_idx);
> > -	writel(reg, clk->reg);
> > +	do_lpcg_workaround(clk_hw_get_rate(hw), clk->reg, reg);
> >
> >  	spin_unlock_irqrestore(&imx_lpcg_scu_lock, flags);  }
> > --
> > 2.28.0
> >