linux-kernel - Re: [PATCH 16/26] serial: msm_serial: drop low-latency workaround

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Date:   Wed, 21 Apr 2021 12:44:17 -0500
From:   Bjorn Andersson <bjorn.andersson@...aro.org>
To:     Johan Hovold <johan@...nel.org>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jirislaby@...nel.org>,
        linux-serial@...r.kernel.org, linux-kernel@...r.kernel.org,
        Andy Gross <agross@...nel.org>
Subject: Re: [PATCH 16/26] serial: msm_serial: drop low-latency workaround

On Wed 21 Apr 04:54 CDT 2021, Johan Hovold wrote:

> Commit f77232dab25b ("tty: serial: msm: drop uart_port->lock before
> calling tty_flip_buffer_push()") claimed to address a locking
> issue but only provided a dubious lockdep splat from an unrelated
> driver, which in the end turned out to be due a broken local change
> carried by the author.
> 
> Unfortunately these patches were merged before the issue had been
> analysed properly so the commit messages makes no sense whatsoever.
> 
> The real issue was first seen on RT which at the time effectively always
> set the low_latency flag for all serial drivers by patching
> tty_flip_buffer_push(). This in turn revealed that many drivers did not
> handle the infamous low_latency behaviour which meant that data was
> pushed immediately to the line discipline instead of being deferred to a
> work queue.
> 
> Since commit a9c3f68f3cd8 ("tty: Fix low_latency BUG"),
> tty_flip_buffer_push() always schedules a work item to push data to the
> line discipline and there's no need to keep any low_latency hacks
> around.
> 
> Link: https://lore.kernel.org/linux-serial/cover.1376923198.git.viresh.kumar@linaro.org/
> Cc: Andy Gross <agross@...nel.org>
> Cc: Bjorn Andersson <bjorn.andersson@...aro.org>

Acked-by: Bjorn Andersson <bjorn.andersson@...aro.org>

Regards,
Bjorn

> Signed-off-by: Johan Hovold <johan@...nel.org>
> ---
>  drivers/tty/serial/msm_serial.c | 4 ----
>  1 file changed, 4 deletions(-)
> 
> diff --git a/drivers/tty/serial/msm_serial.c b/drivers/tty/serial/msm_serial.c
> index 770c182e2208..fcef7a961430 100644
> --- a/drivers/tty/serial/msm_serial.c
> +++ b/drivers/tty/serial/msm_serial.c
> @@ -757,9 +757,7 @@ static void msm_handle_rx_dm(struct uart_port *port, unsigned int misr)
>  		count -= r_count;
>  	}
>  
> -	spin_unlock(&port->lock);
>  	tty_flip_buffer_push(tport);
> -	spin_lock(&port->lock);
>  
>  	if (misr & (UART_IMR_RXSTALE))
>  		msm_write(port, UART_CR_CMD_RESET_STALE_INT, UART_CR);
> @@ -819,9 +817,7 @@ static void msm_handle_rx(struct uart_port *port)
>  			tty_insert_flip_char(tport, c, flag);
>  	}
>  
> -	spin_unlock(&port->lock);
>  	tty_flip_buffer_push(tport);
> -	spin_lock(&port->lock);
>  }
>  
>  static void msm_handle_tx_pio(struct uart_port *port, unsigned int tx_count)
> -- 
> 2.26.3
>