linux-kernel - Re: [PATCH] clocksource/fsl: Fix errata A-007728 for flextimer

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Date:   Thu, 08 Sep 2016 08:22:39 +0200
From:   Alexander Stein <alexander.stein@...tec-electronic.com>
To:     linux-kernel@...r.kernel.org
Cc:     Meng Yi <meng.yi@....com>, daniel.lezcano@...aro.org,
        tglx@...utronix.de
Subject: Re: [PATCH] clocksource/fsl: Fix errata A-007728 for flextimer

On Thursday 08 September 2016 10:04:55, Meng Yi wrote:
> If the FTM counter reaches the FTM_MOD value between the reading of the
> TOF bit and the writing of 0 to the TOF bit, the process of clearing the
> TOF bit does not work as expected when FTMx_CONF[NUMTOF] != 0 and the
> current TOF count is less than FTMx_CONF[NUMTOF]. If the above condition
> is met, the TOF bit remains set. If the TOF interrupt is enabled
> (FTMx_SC[TOIE] = 1), the TOF interrupt also remains asserted.
> 
> Above is the errata discription
> 
> The workaround is clearing TOF bit until it is cleaned(FTM counter doesn't
> always reache the FTM_MOD anyway),which may cost some cycles.
> 
> Signed-off-by: Meng Yi <meng.yi@....com>
> ---
>  drivers/clocksource/fsl_ftm_timer.c | 9 ++++-----
>  1 file changed, 4 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/clocksource/fsl_ftm_timer.c
> b/drivers/clocksource/fsl_ftm_timer.c index 738515b..ade26e5 100644
> --- a/drivers/clocksource/fsl_ftm_timer.c
> +++ b/drivers/clocksource/fsl_ftm_timer.c
> @@ -83,11 +83,10 @@ static inline void ftm_counter_disable(void __iomem
> *base)
> 
>  static inline void ftm_irq_acknowledge(void __iomem *base)
>  {
> -	u32 val;
> -
> -	val = ftm_readl(base + FTM_SC);
> -	val &= ~FTM_SC_TOF;
> -	ftm_writel(val, base + FTM_SC);
> +	/*read and clean the FTM_SC_TOF bit until its cleared*/
> +	while (FTM_SC_TOF & ftm_readl(base + FTM_SC))
> +		ftm_writel(ftm_readl(base + FTM_SC) & (~FTM_SC_TOF),
> +			   base + FTM_SC);
>  }

So you are essentially polling hardware in interrupt context. Please add a 
sensible timeout to abort this loop in case of defective hardware, and maybe 
disable the interrupt in that case.

Best regards,
Alexander