linux-kernel - Re: [PATCH v2 3/4] serial: samsung: introduce s3c24xx_serial_rx_drain

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Date:	Fri, 11 Sep 2015 15:15:51 +0900
From:	Krzysztof Kozlowski <k.kozlowski@...sung.com>
To:	Robert Baldyga <r.baldyga@...sung.com>, gregkh@...uxfoundation.org
Cc:	jslaby@...e.com, linux-serial@...r.kernel.org,
	linux-kernel@...r.kernel.org, m.szyprowski@...sung.com
Subject: Re: [PATCH v2 3/4] serial: samsung: introduce
 s3c24xx_serial_rx_drain_fifo() function

On 10.09.2015 22:41, Robert Baldyga wrote:
> This patch introduces s3c24xx_serial_rx_drain_fifo() which reads data
> from RX FIFO and writes it to tty buffer. It also checks for special
> conditions (such as 'break') and handles it. This function has been
> separated from s3c24xx_serial_rx_chars_pio() as it contains code which
> can be used also in DMA mode.

Much better, thanks! Now it is also easier to spot the difference (see
below).

> 
> Signed-off-by: Robert Baldyga <r.baldyga@...sung.com>
> ---
>  drivers/tty/serial/samsung.c | 23 +++++++++++++----------
>  1 file changed, 13 insertions(+), 10 deletions(-)
> 
> diff --git a/drivers/tty/serial/samsung.c b/drivers/tty/serial/samsung.c
> index dc4be54..1d7dd86 100644
> --- a/drivers/tty/serial/samsung.c
> +++ b/drivers/tty/serial/samsung.c
> @@ -621,16 +621,12 @@ finish:
>  	return IRQ_HANDLED;
>  }
>  
> -static irqreturn_t s3c24xx_serial_rx_chars_pio(void *dev_id)
> +static void s3c24xx_serial_rx_drain_fifo(struct s3c24xx_uart_port *ourport)
>  {
> -	struct s3c24xx_uart_port *ourport = dev_id;
>  	struct uart_port *port = &ourport->port;
>  	unsigned int ufcon, ch, flag, ufstat, uerstat;
> -	unsigned long flags;
>  	int max_count = port->fifosize;
>  
> -	spin_lock_irqsave(&port->lock, flags);
> -
>  	while (max_count-- > 0) {
>  		ufcon = rd_regl(port, S3C2410_UFCON);
>  		ufstat = rd_regl(port, S3C2410_UFSTAT);
> @@ -654,9 +650,7 @@ static irqreturn_t s3c24xx_serial_rx_chars_pio(void *dev_id)
>  					ufcon |= S3C2410_UFCON_RESETRX;
>  					wr_regl(port, S3C2410_UFCON, ufcon);
>  					rx_enabled(port) = 1;
> -					spin_unlock_irqrestore(&port->lock,
> -							flags);
> -					goto out;
> +					return;
>  				}
>  				continue;
>  			}
> @@ -702,10 +696,19 @@ static irqreturn_t s3c24xx_serial_rx_chars_pio(void *dev_id)
>  				 ch, flag);
>  	}
>  
> -	spin_unlock_irqrestore(&port->lock, flags);
>  	tty_flip_buffer_push(&port->state->port);

Here is a difference - previously this was outside of spinlock. I think
moving it inside spin lock is okay, just the interrupts won't be
disabled before unlock and queue_work() from tty_flip_buffer_push().

However after testing this patchset (entire) on:
next-20150910 + my dt-for-next branch (dma for serial) + this patchset
you can see quite complicated lockdep warning:

[    3.568657] =========================================================
[    3.575079] [ INFO: possible irq lock inversion dependency detected ]
[    3.581506] 4.2.0-next-20150910-00009-g65fd5a9cff54 #218 Not tainted
[    3.587838] ---------------------------------------------------------
[    3.594263] swapper/0/0 just changed the state of lock:
[    3.599470]  (&port_lock_key){..-...}, at: [<c02a8fac>]
s3c24xx_serial_tx_dma_complete+0x8c/0xfc
[    3.608237] but this lock took another, SOFTIRQ-unsafe lock in the past:
[    3.614919]  (&(&ctx->lock)->rlock){+.+...}
[    3.614919]
[    3.614919] and interrupts could create inverse lock ordering between
them.
[    3.614919]
[    3.625076]
[    3.625076] other info that might help us debug this:
[    3.631586]  Possible interrupt unsafe locking scenario:
[    3.631586]
[    3.638356]        CPU0                    CPU1
[    3.642870]        ----                    ----
[    3.647382]   lock(&(&ctx->lock)->rlock);
[    3.651376]                                local_irq_disable();
[    3.657278]                                lock(&port_lock_key);
[    3.663267]                                lock(&(&ctx->lock)->rlock);
[    3.669777]   <Interrupt>
[    3.672381]     lock(&port_lock_key);


Config: exynos, disabled MMC_CLKGATE, enabled usual testing stuff
Board: Trats2

Didn't you notice it?


Additionally the SysRq "Show backtrace of all active CPUs" on this
linux-next (without additional patches, pure next) has significant delay
(like 5 seconds) and a:
[  169.221223] s3c-i2c 138d0000.i2c: timeout waiting for bus idle

That's weird. But as I said this occurs on pure next as well.

Dmesgs and config attached.

Best regards,
Krzysztof


> +}
> +
> +static irqreturn_t s3c24xx_serial_rx_chars_pio(void *dev_id)
> +{
> +	struct s3c24xx_uart_port *ourport = dev_id;
> +	struct uart_port *port = &ourport->port;
> +	unsigned long flags;
> +
> +	spin_lock_irqsave(&port->lock, flags);
> +	s3c24xx_serial_rx_drain_fifo(ourport);
> +	spin_unlock_irqrestore(&port->lock, flags);
>  
> -out:
>  	return IRQ_HANDLED;
>  }
>  
> 


View attachment "trats2-backtrace-stall-dmesg.txt" of type "text/plain" (6459 bytes)

View attachment "trats2-serial-lockdep-issue-config.txt" of type "text/plain" (92777 bytes)

View attachment "trats2-serial-lockdep-issue-dmesg.txt" of type "text/plain" (37811 bytes)