linux-kernel - Re: [PATCH 08/14] serial: tegra: check for FIFO mode enabled status

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Date:   Tue, 13 Aug 2019 12:03:57 +0200
From:   Thierry Reding <thierry.reding@...il.com>
To:     Krishna Yarlagadda <kyarlagadda@...dia.com>
Cc:     gregkh@...uxfoundation.org, robh+dt@...nel.org,
        mark.rutland@....com, jonathanh@...dia.com, ldewangan@...dia.com,
        jslaby@...e.com, linux-serial@...r.kernel.org,
        devicetree@...r.kernel.org, linux-tegra@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Shardar Shariff Md <smohammed@...dia.com>
Subject: Re: [PATCH 08/14] serial: tegra: check for FIFO mode enabled status

On Mon, Aug 12, 2019 at 04:58:17PM +0530, Krishna Yarlagadda wrote:
> Chips prior to Tegra186 needed delay of 3 UART clock cycles to avoid
> data loss. This issue is fixed in Tegra186 and a new flag is added to
> check if fifo mode is enabled. chip data updated to check if this flag
> is available for a chip. Tegra186 has new compatible to enable this
> flag.
> 
> Signed-off-by: Shardar Shariff Md <smohammed@...dia.com>
> Signed-off-by: Krishna Yarlagadda <kyarlagadda@...dia.com>
> ---
>  drivers/tty/serial/serial-tegra.c | 52 ++++++++++++++++++++++++++++++++++-----
>  1 file changed, 46 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/tty/serial/serial-tegra.c b/drivers/tty/serial/serial-tegra.c
> index 7ab81bb..e0379d9 100644
> --- a/drivers/tty/serial/serial-tegra.c
> +++ b/drivers/tty/serial/serial-tegra.c
> @@ -72,6 +72,8 @@
>  #define TEGRA_TX_PIO				1
>  #define TEGRA_TX_DMA				2
>  
> +#define TEGRA_UART_FCR_IIR_FIFO_EN		0x40
> +
>  /**
>   * tegra_uart_chip_data: SOC specific data.
>   *
> @@ -84,6 +86,7 @@ struct tegra_uart_chip_data {
>  	bool	tx_fifo_full_status;
>  	bool	allow_txfifo_reset_fifo_mode;
>  	bool	support_clk_src_div;
> +	bool	fifo_mode_enable_status;
>  };
>  
>  struct tegra_uart_port {
> @@ -263,6 +266,22 @@ static void tegra_uart_wait_sym_time(struct tegra_uart_port *tup,
>  			tup->current_baud));
>  }
>  
> +static int tegra_uart_is_fifo_mode_enabled(struct tegra_uart_port *tup)

I think this is a bad name. "is" makes it sound like this will return a
boolean value. Also, this doesn't really check whether FIFO mode is
enabled, but rather it waits for the FIFO mode to become enabled.
Perhaps, then, a better name would be

	tegra_uart_wait_fifo_mode_enabled()

?

> +{
> +	unsigned long iir;
> +	unsigned int tmout = 100;
> +
> +	do {
> +		iir = tegra_uart_read(tup, UART_IIR);
> +		if (iir & TEGRA_UART_FCR_IIR_FIFO_EN)
> +			return 0;
> +		udelay(1);
> +	} while (--tmout);
> +	dev_err(tup->uport.dev, "FIFO mode not enabled\n");

I'd push this out to callers. That way this function becomes useful in
situations where you don't want to output an error.

> +
> +	return -EIO;

-ETIMEDOUT?

Thierry

> +}
> +
>  static void tegra_uart_fifo_reset(struct tegra_uart_port *tup, u8 fcr_bits)
>  {
>  	unsigned long fcr = tup->fcr_shadow;
> @@ -282,6 +301,8 @@ static void tegra_uart_fifo_reset(struct tegra_uart_port *tup, u8 fcr_bits)
>  		tegra_uart_write(tup, fcr, UART_FCR);
>  		fcr |= UART_FCR_ENABLE_FIFO;
>  		tegra_uart_write(tup, fcr, UART_FCR);
> +		if (tup->cdata->fifo_mode_enable_status)
> +			tegra_uart_is_fifo_mode_enabled(tup);
>  	}
>  
>  	/* Dummy read to ensure the write is posted */
> @@ -918,12 +939,19 @@ static int tegra_uart_hw_init(struct tegra_uart_port *tup)
>  	/* Dummy read to ensure the write is posted */
>  	tegra_uart_read(tup, UART_SCR);
>  
> -	/*
> -	 * For all tegra devices (up to t210), there is a hardware issue that
> -	 * requires software to wait for 3 UART clock periods after enabling
> -	 * the TX fifo, otherwise data could be lost.
> -	 */
> -	tegra_uart_wait_cycle_time(tup, 3);
> +	if (tup->cdata->fifo_mode_enable_status) {
> +		ret = tegra_uart_is_fifo_mode_enabled(tup);
> +		if (ret < 0)
> +			return ret;
> +	} else {
> +		/*
> +		 * For all tegra devices (up to t210), there is a hardware
> +		 * issue that requires software to wait for 3 UART clock
> +		 * periods after enabling the TX fifo, otherwise data could
> +		 * be lost.
> +		 */
> +		tegra_uart_wait_cycle_time(tup, 3);
> +	}
>  
>  	/*
>  	 * Initialize the UART with default configuration
> @@ -1294,12 +1322,21 @@ static struct tegra_uart_chip_data tegra20_uart_chip_data = {
>  	.tx_fifo_full_status		= false,
>  	.allow_txfifo_reset_fifo_mode	= true,
>  	.support_clk_src_div		= false,
> +	.fifo_mode_enable_status	= false,
>  };
>  
>  static struct tegra_uart_chip_data tegra30_uart_chip_data = {
>  	.tx_fifo_full_status		= true,
>  	.allow_txfifo_reset_fifo_mode	= false,
>  	.support_clk_src_div		= true,
> +	.fifo_mode_enable_status	= false,
> +};
> +
> +static struct tegra_uart_chip_data tegra186_uart_chip_data = {
> +	.tx_fifo_full_status		= true,
> +	.allow_txfifo_reset_fifo_mode	= false,
> +	.support_clk_src_div		= true,
> +	.fifo_mode_enable_status	= true,
>  };
>  
>  static const struct of_device_id tegra_uart_of_match[] = {
> @@ -1310,6 +1347,9 @@ static const struct of_device_id tegra_uart_of_match[] = {
>  		.compatible	= "nvidia,tegra20-hsuart",
>  		.data		= &tegra20_uart_chip_data,
>  	}, {
> +		.compatible     = "nvidia,tegra186-hsuart",
> +		.data		= &tegra186_uart_chip_data,
> +	}, {
>  	},
>  };
>  MODULE_DEVICE_TABLE(of, tegra_uart_of_match);
> -- 
> 2.7.4
> 

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