linux-kernel - Re: [PATCH 1/2] serial: 8250: Don't service RX FIFO if interrupts are disabled

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Date:   Thu, 8 Feb 2018 17:16:07 +0200
From:   Andy Shevchenko <andy.shevchenko@...il.com>
To:     Vignesh R <vigneshr@...com>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jslaby@...e.com>, linux-serial@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux OMAP Mailing List <linux-omap@...r.kernel.org>,
        linux-arm Mailing List <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH 1/2] serial: 8250: Don't service RX FIFO if interrupts are disabled

On Thu, Feb 8, 2018 at 2:55 PM, Vignesh R <vigneshr@...com> wrote:
> Currently, data in RX FIFO is read based on UART_LSR register state even
> if RDI and RLSI interrupts are disabled in UART_IER register.
> This is because when IRQ handler is called due to TX FIFO empty event,
> RX FIFO is serviced based on UART_LSR register status instead of
> UART_IIR status. This defeats the purpose of disabling UART RX
> FIFO interrupts during throttling(see, omap_8250_throttle()) as IRQ
> handler continues to drain UART RX FIFO resulting in overflow of buffer
> at tty layer.
> Fix this by making sure that driver drains UART RX FIFO only when
> UART_IIR_RDI is set along with UART_LSR_BI or UART_LSR_DR bits.
>
> Signed-off-by: Vignesh R <vigneshr@...com>

> -       if (status & (UART_LSR_DR | UART_LSR_BI)) {
> +       if (status & (UART_LSR_DR | UART_LSR_BI) &&
> +           iir & UART_IIR_RDI) {

>                 if (!up->dma || handle_rx_dma(up, iir))

handle_rx_dma() checks for IRQ status as well.

But for now it seems we are on safe side since checks are done versus
IRQ status with bit 2 set, meaning that iir & RDI will be true.

>                         status = serial8250_rx_chars(up, status);
>         }

Anyway, thanks for the patch, though I need some time to test it on
non-OMAP hardware with DMA enabled.

-- 
With Best Regards,
Andy Shevchenko