linux-kernel - Re: [PATCH V3 1/4] i2c: tegra: make sure register writes completes

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Message-ID: <4FD8B7FC.3060708@wwwdotorg.org>
Date:	Wed, 13 Jun 2012 09:55:40 -0600
From:	Stephen Warren <swarren@...dotorg.org>
To:	Laxman Dewangan <ldewangan@...dia.com>
CC:	khali@...ux-fr.org, w.sang@...gutronix.de, ben-linux@...ff.org,
	olof@...om.net, linux-i2c@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-tegra@...r.kernel.org
Subject: Re: [PATCH V3 1/4] i2c: tegra: make sure register writes completes

On 06/13/2012 04:12 AM, Laxman Dewangan wrote:
> The Tegra PPSB (an peripheral bus) queues writes transactions.
> In order to guarantee that writes have completed before a
> certain time, a read transaction to a register on the same
> bus must be executed.
> This is necessary in situations such as when clearing an
> interrupt status or enable, so that when returning from an
> interrupt handler, the HW has already de-asserted its
> interrupt status output, which will avoid spurious interrupts.

> diff --git a/drivers/i2c/busses/i2c-tegra.c b/drivers/i2c/busses/i2c-tegra.c

> @@ -165,6 +165,10 @@ static void i2c_writel(struct tegra_i2c_dev *i2c_dev, u32 val,
>  	unsigned long reg)
>  {
>  	writel(val, i2c_dev->base + tegra_i2c_reg_addr(i2c_dev, reg));
> +
> +	/* Read back register to make sure that register writes completed */
> +	if (reg != I2C_TX_FIFO)
> +		readl(i2c_dev->base + tegra_i2c_reg_addr(i2c_dev, reg));

I guess that's fine, but it sure does seem rather heavy-weight. Don't
you only need to do the readback if you just wrote to the IRQ status or
mask registers, rather than if you wrote to /any/ register other than
the FIFO?
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