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

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Date:   Tue, 27 Aug 2019 09:29:21 +0000
From:   Krishna Yarlagadda <kyarlagadda@...dia.com>
To:     Thierry Reding <thierry.reding@...il.com>
CC:     "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
        "robh+dt@...nel.org" <robh+dt@...nel.org>,
        "mark.rutland@....com" <mark.rutland@....com>,
        Jonathan Hunter <jonathanh@...dia.com>,
        "Laxman Dewangan" <ldewangan@...dia.com>,
        "jslaby@...e.com" <jslaby@...e.com>,
        "linux-serial@...r.kernel.org" <linux-serial@...r.kernel.org>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "linux-tegra@...r.kernel.org" <linux-tegra@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Shardar Mohammed" <smohammed@...dia.com>
Subject: RE: [PATCH 02/14] serial: tegra: add support to ignore read

> -----Original Message-----
> From: linux-tegra-owner@...r.kernel.org <linux-tegra-
> owner@...r.kernel.org> On Behalf Of Thierry Reding
> Sent: Tuesday, August 13, 2019 3:12 PM
> To: Krishna Yarlagadda <kyarlagadda@...dia.com>
> Cc: gregkh@...uxfoundation.org; robh+dt@...nel.org;
> mark.rutland@....com; Jonathan Hunter <jonathanh@...dia.com>; Laxman
> Dewangan <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 Mohammed
> <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
> 
Looked into few drivers and sysrq characters are not ignored. This driver does
not support console driver. So this might not be needed.

KY
> >  	} 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
> >