linux-kernel - Re: [PATCH 02/14] serial: tegra: add support to ignore read

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Date:   Tue, 13 Aug 2019 11:42:08 +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 02/14] serial: tegra: add support to ignore read

On Mon, Aug 12, 2019 at 04:58:11PM +0530, Krishna Yarlagadda wrote:
> From: Shardar Shariff Md <smohammed@...dia.com>
> 
> Add support to ignore read characters if CREAD flag is not set.
> 
> Signed-off-by: Shardar Shariff Md <smohammed@...dia.com>
> Signed-off-by: Krishna Yarlagadda <kyarlagadda@...dia.com>
> ---
>  drivers/tty/serial/serial-tegra.c | 12 ++++++++++++
>  1 file changed, 12 insertions(+)
> 
> diff --git a/drivers/tty/serial/serial-tegra.c b/drivers/tty/serial/serial-tegra.c
> index 19f4c24..93d299e 100644
> --- a/drivers/tty/serial/serial-tegra.c
> +++ b/drivers/tty/serial/serial-tegra.c
> @@ -542,6 +542,9 @@ static void tegra_uart_handle_rx_pio(struct tegra_uart_port *tup,
>  		ch = (unsigned char) tegra_uart_read(tup, UART_RX);
>  		tup->uport.icount.rx++;
>  
> +		if (tup->uport.ignore_status_mask & UART_LSR_DR)
> +			continue;
> +
>  		if (!uart_handle_sysrq_char(&tup->uport, ch) && tty)
>  			tty_insert_flip_char(tty, ch, flag);

Is it a good idea to ignore even sysrq characters if CREAD is not set?
According to termios, CREAD enables the receiver, so technically if it
isn't set you can't even receive sysrq characters. But I don't know if
there are any rules regarding this.

Is this the same way that other drivers work?

Thierry

>  	} while (1);
> @@ -562,6 +565,10 @@ static void tegra_uart_copy_rx_to_tty(struct tegra_uart_port *tup,
>  		dev_err(tup->uport.dev, "No tty port\n");
>  		return;
>  	}
> +
> +	if (tup->uport.ignore_status_mask & UART_LSR_DR)
> +		return;
> +
>  	dma_sync_single_for_cpu(tup->uport.dev, tup->rx_dma_buf_phys,
>  				TEGRA_UART_RX_DMA_BUFFER_SIZE, DMA_FROM_DEVICE);
>  	copied = tty_insert_flip_string(tty,
> @@ -1190,6 +1197,11 @@ static void tegra_uart_set_termios(struct uart_port *u,
>  	tegra_uart_write(tup, tup->ier_shadow, UART_IER);
>  	tegra_uart_read(tup, UART_IER);
>  
> +	tup->uport.ignore_status_mask = 0;
> +	/* Ignore all characters if CREAD is not set */
> +	if ((termios->c_cflag & CREAD) == 0)
> +		tup->uport.ignore_status_mask |= UART_LSR_DR;
> +
>  	spin_unlock_irqrestore(&u->lock, flags);
>  }
>  
> -- 
> 2.7.4
> 

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