linux-kernel - Re: [PATCH v5 2/4] pinctrl: qcom: No need to read-modify-write the interrupt status

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Date:   Thu, 14 Jan 2021 10:33:24 -0600
From:   Bjorn Andersson <bjorn.andersson@...aro.org>
To:     Douglas Anderson <dianders@...omium.org>
Cc:     Marc Zyngier <maz@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Jason Cooper <jason@...edaemon.net>,
        Linus Walleij <linus.walleij@...aro.org>,
        Neeraj Upadhyay <neeraju@...eaurora.org>,
        Rajendra Nayak <rnayak@...eaurora.org>,
        Stephen Boyd <swboyd@...omium.org>,
        Maulik Shah <mkshah@...eaurora.org>,
        linux-gpio@...r.kernel.org,
        Srinivas Ramana <sramana@...eaurora.org>,
        linux-arm-msm@...r.kernel.org, Andy Gross <agross@...nel.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 2/4] pinctrl: qcom: No need to read-modify-write the
 interrupt status

On Fri 08 Jan 11:35 CST 2021, Douglas Anderson wrote:

> When the Qualcomm pinctrl driver wants to Ack an interrupt, it does a
> read-modify-write on the interrupt status register.  On some SoCs it
> makes sure that the status bit is 1 to "Ack" and on others it makes
> sure that the bit is 0 to "Ack".  Presumably the first type of
> interrupt controller is a "write 1 to clear" type register and the
> second just let you directly set the interrupt status register.
> 
> As far as I can tell from scanning structure definitions, the
> interrupt status bit is always in a register by itself.  Thus with
> both types of interrupt controllers it is safe to "Ack" interrupts
> without doing a read-modify-write.  We can do a simple write.
> 
> It should be noted that if the interrupt status bit _was_ ever in a
> register with other things (like maybe status bits for other GPIOs):
> a) For "write 1 clear" type controllers then read-modify-write would
>    be totally wrong because we'd accidentally end up clearing
>    interrupts we weren't looking at.
> b) For "direct set" type controllers then read-modify-write would also
>    be wrong because someone setting one of the other bits in the
>    register might accidentally clear (or set) our interrupt.
> I say this simply to show that the current read-modify-write doesn't
> provide any sort of "future proofing" of the code.  In fact (for
> "write 1 clear" controllers) the new code is slightly more "future
> proof" since it would allow more than one interrupt status bits to
> share a register.
> 
> NOTE: this code fixes no bugs--it simply avoids an extra register
> read.
> 
> Signed-off-by: Douglas Anderson <dianders@...omium.org>

Reviewed-by: Bjorn Andersson <bjorn.andersson@...aro.org>

Regards,
Bjorn

> ---
> 
> Changes in v5:
> - ("pinctrl: qcom: No need to read-modify-write the ...") new for v5.
> 
>  drivers/pinctrl/qcom/pinctrl-msm.c | 23 ++++++++---------------
>  1 file changed, 8 insertions(+), 15 deletions(-)
> 
> diff --git a/drivers/pinctrl/qcom/pinctrl-msm.c b/drivers/pinctrl/qcom/pinctrl-msm.c
> index 1d2a78452c2d..1787ada6bfab 100644
> --- a/drivers/pinctrl/qcom/pinctrl-msm.c
> +++ b/drivers/pinctrl/qcom/pinctrl-msm.c
> @@ -792,16 +792,13 @@ static void msm_gpio_irq_clear_unmask(struct irq_data *d, bool status_clear)
>  
>  	raw_spin_lock_irqsave(&pctrl->lock, flags);
>  
> -	if (status_clear) {
> -		/*
> -		 * clear the interrupt status bit before unmask to avoid
> -		 * any erroneous interrupts that would have got latched
> -		 * when the interrupt is not in use.
> -		 */
> -		val = msm_readl_intr_status(pctrl, g);
> -		val &= ~BIT(g->intr_status_bit);
> -		msm_writel_intr_status(val, pctrl, g);
> -	}
> +	/*
> +	 * clear the interrupt status bit before unmask to avoid
> +	 * any erroneous interrupts that would have got latched
> +	 * when the interrupt is not in use.
> +	 */
> +	if (status_clear)
> +		msm_writel_intr_status(0, pctrl, g);
>  
>  	val = msm_readl_intr_cfg(pctrl, g);
>  	val |= BIT(g->intr_raw_status_bit);
> @@ -906,11 +903,7 @@ static void msm_gpio_irq_ack(struct irq_data *d)
>  
>  	raw_spin_lock_irqsave(&pctrl->lock, flags);
>  
> -	val = msm_readl_intr_status(pctrl, g);
> -	if (g->intr_ack_high)
> -		val |= BIT(g->intr_status_bit);
> -	else
> -		val &= ~BIT(g->intr_status_bit);
> +	val = (g->intr_ack_high) ? BIT(g->intr_status_bit) : 0;
>  	msm_writel_intr_status(val, pctrl, g);
>  
>  	if (test_bit(d->hwirq, pctrl->dual_edge_irqs))
> -- 
> 2.29.2.729.g45daf8777d-goog
>