linux-kernel - Re: [PATCH v2 13/13] tty: pruss SUART driver

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Date:	Tue, 22 Feb 2011 17:34:15 +0100
From:	Arnd Bergmann <arnd@...db.de>
To:	"Subhasish Ghosh" <subhasish@...tralsolutions.com>
Cc:	linux-arm-kernel@...ts.infradead.org,
	"Thomas Gleixner" <tglx@...utronix.de>,
	"Alan Cox" <alan@...rguk.ukuu.org.uk>, sachi@...tralsolutions.com,
	davinci-linux-open-source@...ux.davincidsp.com,
	"Greg Kroah-Hartman" <gregkh@...e.de>, nsekhar@...com,
	"open list" <linux-kernel@...r.kernel.org>, m-watkins@...com
Subject: Re: [PATCH v2 13/13] tty: pruss SUART driver

On Tuesday 22 February 2011, Subhasish Ghosh wrote:

> @@ -122,13 +122,10 @@ static void omapl_pru_tx_chars(struct omapl_pru_suart 
> *soft_uart, u32 uart_no)
>         if (!(suart_get_duplex(soft_uart, uart_no) & ePRU_SUART_HALF_TX))
>                 return;
> 
> -       if (down_trylock(&soft_uart->port_sem[uart_no]))
> -               return;
> -
>         if (uart_circ_empty(xmit) ||
>                         uart_tx_stopped(&soft_uart->port[uart_no])) {
>                 pruss_suart_stop_tx(&soft_uart->port[uart_no]);
> -               up(&soft_uart->port_sem[uart_no]);
> +               soft_uart->tx_empty[uart_no] = true;
>                 return;
>         }
> 
> @@ -259,7 +256,6 @@ static irqreturn_t pruss_suart_interrupt(s32 irq, void 
> *dev_id)
>                         pru_intr_clr_isrstatus(dev, uart_num, PRU_TX_INTR);
>                         pru_softuart_clr_tx_status(dev, &soft_uart->suart_hdl
>                                                  [port->line]);
> -                       up(&soft_uart->port_sem[port->line]);
>                         omapl_pru_tx_chars(soft_uart, port->line);
>                 }
>         } while (txrx_flag & (PRU_RX_INTR | PRU_TX_INTR));
> @@ -294,7 +290,10 @@ static void pruss_suart_start_tx(struct uart_port 
> *port)
> 
>         suart_intr_setmask(dev, soft_uart->suart_hdl[port->line].uart_num,
>                            PRU_TX_INTR, CHN_TXRX_IE_MASK_CMPLT);
> -       omapl_pru_tx_chars(soft_uart, port->line);
> +       if (soft_uart->tx_empty[port->line] == true) {
> +               soft_uart->tx_empty[port->line] = false;
> +               omapl_pru_tx_chars(soft_uart, port->line);
> +       }
>  }

This looks racy, and I think you at least need to take the spinlock in
pruss_suart_start_tx(), but I don't fully understand the intention of the
code.

I guess you could also use a bitmask for tx_empty and use test_and_clear_bit()
on that to guarantee atomicity.

	Arnd
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