Subject: [PATCH v3] serial: mxs-auart: keep the AUART unit in reset state when not in use

From: [email-masked] (Peter Hurley)
Date: Fri, 17 Jul 2015 09:10:06 -0400

Hi Juergen,

On 07/16/2015 03:40 AM, Juergen Borleis wrote:
> Whenever the UART device driver gets closed from userland, the driver
> disables the UART unit and then stops its clock to save power.
> 
> The bit which disabled the UART unit is described as:
> 
>   "UART Enable. If this bit is set to 1, the UART is enabled. Data
>   transmission and reception occurs for the UART signals. When the
>   UART is disabled in the middle of transmission or reception, it
>   completes the current character before stopping."
> 
> The important part is the "it completes the current character". Whenever
> a reception is ongoing when the UART gets disabled (including the clock
> off) the statemachine freezes and "remembers" this state on the next
> open() and re-enabling of the unit's clock.
> 
> In this case we end up receiving an additional bogus character
> immediately.
> 
> The solution in this change is to move the AUART unit into its reset
> state on close() and only release it from its reset state on the next
> open().
> 
> Signed-off-by: Juergen Borleis <[email-masked]>
> ---
> 
> Notes:
>     v3:
>      - missing newline added to an error message
>     
>     v2:
>      - rename mxs_auart_do_reset() to mxs_auart_keep_reset() to better reflect
>        what it really does
>      - adapt the delay in mxs_auart_keep_reset() to wait for the reset of the
>        AURAT unit to what is used in mxs_auart_reset()

The function names and semantics are not clear. See below.

>      - typo fixed
> 
>  drivers/tty/serial/mxs-auart.c | 38 ++++++++++++++++++++++++++++++--------
>  1 file changed, 30 insertions(+), 8 deletions(-)
> 
> diff --git a/drivers/tty/serial/mxs-auart.c b/drivers/tty/serial/mxs-auart.c
> index 13cf773..135ee6c 100644
> --- a/drivers/tty/serial/mxs-auart.c
> +++ b/drivers/tty/serial/mxs-auart.c
> @@ -858,6 +858,30 @@ static void mxs_auart_reset(struct uart_port *u)
>  	writel(AUART_CTRL0_CLKGATE, u->membase + AUART_CTRL0_CLR);
>  }
>  
> +static void mxs_auart_keep_reset(struct uart_port *u)
> +{
> +	int i;
> +	u32 reg;
> +
> +	reg = readl(u->membase + AUART_CTRL0);
> +	/* if already in reset state, keep it untouched */
> +	if (reg & AUART_CTRL0_SFTRST)
> +		return;
> +
> +	writel(AUART_CTRL0_CLKGATE, u->membase + AUART_CTRL0_CLR);
> +	writel(AUART_CTRL0_SFTRST, u->membase + AUART_CTRL0_SET);
> +
> +	for (i = 0; i < 10000; i++) {
> +		reg = readl(u->membase + AUART_CTRL0);
> +		/* reset is finished when the clock is gated */
> +		if (reg & AUART_CTRL0_CLKGATE)
> +			return;
> +		udelay(3);
> +	}
> +
> +	dev_err(u->dev, "Failed to reset the unit.\n");
> +}
> +
>  static int mxs_auart_startup(struct uart_port *u)
>  {
>  	int ret;
> @@ -867,7 +891,10 @@ static int mxs_auart_startup(struct uart_port *u)
>  	if (ret)
>  		return ret;
>  
> -	writel(AUART_CTRL0_CLKGATE, u->membase + AUART_CTRL0_CLR);
> +	/* reset the unit if not aleady done */
> +	mxs_auart_keep_reset(u);

Why is this performing a soft-reset on open now? Didn't mxs_auart_shutdown()
already leave it in this state?

Because if you're performing a soft-reset deliberately as part of the
initial condition, you make no mention of that in the changelog.

> +	/* bring it out of reset now */
> +	mxs_auart_reset(u);

mxs_auart_reset() is really not resetting but simply performing a block enable.
Isn't there a generic block enable for the iMX.2x SoCs? (Maybe there should be)

The names of these functions don't match expected operations of startup().
Start up should be enabling the device, not keeping it in reset.

And "keep reset" immediately followed by "reset" makes no sense.

Regards,
Peter Hurley

>  	writel(AUART_CTRL2_UARTEN, u->membase + AUART_CTRL2_SET);
>  
> @@ -899,13 +926,8 @@ static void mxs_auart_shutdown(struct uart_port *u)
>  	if (auart_dma_enabled(s))
>  		mxs_auart_dma_exit(s);
>  
> -	writel(AUART_CTRL2_UARTEN, u->membase + AUART_CTRL2_CLR);
> -
> -	writel(AUART_INTR_RXIEN | AUART_INTR_RTIEN | AUART_INTR_CTSMIEN,
> -			u->membase + AUART_INTR_CLR);
> -
> -	writel(AUART_CTRL0_CLKGATE, u->membase + AUART_CTRL0_SET);
> -
> +	/* reset the unit and keep it in reset state */
> +	mxs_auart_keep_reset(u);
>  	clk_disable_unprepare(s->clk);
>  }
>  
>