linux-kernel - Re: [PATCH 2/2] reset: simple: read back to make sure changes are applied

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Message-ID: <c046c1c1-7a14-5052-fbf1-d1efab6c9d9f@arm.com>
Date:   Wed, 8 Mar 2017 14:04:27 +0000
From:   Andre Przywara <andre.przywara@....com>
To:     Philipp Zabel <p.zabel@...gutronix.de>,
        linux-kernel@...r.kernel.org
Cc:     Maxime Coquelin <mcoquelin.stm32@...il.com>,
        Alexandre Torgue <alexandre.torgue@...com>,
        Maxime Ripard <maxime.ripard@...e-electrons.com>,
        Chen-Yu Tsai <wens@...e.org>
Subject: Re: [PATCH 2/2] reset: simple: read back to make sure changes are
 applied

Hi,

On 08/03/17 09:54, Philipp Zabel wrote:
> Read back the register after setting or clearing a reset bit to make
> sure that the changes are applied to the reset controller hardware.
> Theoretically, this avoids the write to stay stuck in a store buffer
> during the delay of an assert-delay-deassert sequence, and makes sure
> that the reset really is asserted for the specified duration.
>
> Signed-off-by: Philipp Zabel <p.zabel@...gutronix.de>
> ---
>  drivers/reset/reset-simple.c | 14 ++++++++++----
>  1 file changed, 10 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/reset/reset-simple.c b/drivers/reset/reset-simple.c
> index 2160e84fe216b..e32289bdaf0c6 100644
> --- a/drivers/reset/reset-simple.c
> +++ b/drivers/reset/reset-simple.c
> @@ -33,13 +33,16 @@ static int reset_simple_clear(struct reset_controller_dev *rcdev,
>  	int reg_width = sizeof(u32);
>  	int bank = id / (reg_width * BITS_PER_BYTE);
>  	int offset = id % (reg_width * BITS_PER_BYTE);
> +	void __iomem *addr = data->membase + (bank * reg_width);
>  	unsigned long flags;
>  	u32 reg;
>  
>  	spin_lock_irqsave(&data->lock, flags);
>  
> -	reg = readl(data->membase + (bank * reg_width));
> -	writel(reg & ~BIT(offset), data->membase + (bank * reg_width));
> +	reg = readl(addr);
> +	writel(reg & ~BIT(offset), addr);
> +	/* Read back to make sure the write doesn't linger in a store buffer */
> +	readl(addr);

Nit: "Read back to make sure the write doesn't linger in a store
buffer", sounds somewhat dodgy.
What about: "Read back to make sure the write has reached the device."?

And I wonder if we actually need to check the returned value to make
sure the device has actually changed the state? Or is this too paranoid?

Cheers,
Andre.

>  	spin_unlock_irqrestore(&data->lock, flags);
>  
> @@ -53,13 +56,16 @@ static int reset_simple_set(struct reset_controller_dev *rcdev,
>  	int reg_width = sizeof(u32);
>  	int bank = id / (reg_width * BITS_PER_BYTE);
>  	int offset = id % (reg_width * BITS_PER_BYTE);
> +	void __iomem *addr = data->membase + (bank * reg_width);
>  	unsigned long flags;
>  	u32 reg;
>  
>  	spin_lock_irqsave(&data->lock, flags);
>  
> -	reg = readl(data->membase + (bank * reg_width));
> -	writel(reg | BIT(offset), data->membase + (bank * reg_width));
> +	reg = readl(addr);
> +	writel(reg | BIT(offset), addr);
> +	/* Read back to make sure the write doesn't linger in a store buffer */
> +	readl(addr);
>  
>  	spin_unlock_irqrestore(&data->lock, flags);
>  
>