linux-kernel - Re: [PATCH v1 3/3] serial: 8250_port: Disable DMA operations for kernel console

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Date:   Wed, 16 May 2018 15:58:10 +0300
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        linux-kernel@...r.kernel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jslaby@...e.com>, linux-serial@...r.kernel.org,
        Arnd Bergmann <arnd@...db.de>
Subject: Re: [PATCH v1 3/3] serial: 8250_port: Disable DMA operations for
 kernel console

On Wed, 2018-05-16 at 12:55 +0200, Sebastian Andrzej Siewior wrote:
> On 2018-05-15 21:34:09 [+0300], Andy Shevchenko wrote:

Thanks for review, my answers below.

> > It will be too tricky and error prone to allow DMA operations on
> > kernel
> > console.
> 
> Why is it tricky and error prone? I had it working…

On OMAP only? Had you tested this on let's say Intel Cherrytrail where
DMA controller is a separate PCI device which needs to be handled
separately from UART IP.

> But I don't mind dropping the DMA on the kernel console because I
> doubt
> that we lose something here by disabling it. I would even imagine that
> it gets "simpler" (maybe what you tried to say by "error prone") to
> print something in the NMI case by writing directly to the FIFO
> register
> instead setting up a DMA transfer and so on.

"error prone" mostly refers to patch 2 commit message. Here it seems I
need to put something like above to explain why DMA case tricky _as
well_.

> > Disable any kind of DMA operations in such case.
> > 
> > Signed-off-by: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
> > ---
> >  drivers/tty/serial/8250/8250_port.c | 5 ++++-
> >  1 file changed, 4 insertions(+), 1 deletion(-)
> > 
> > diff --git a/drivers/tty/serial/8250/8250_port.c
> > b/drivers/tty/serial/8250/8250_port.c
> > index db97222a1bf4..9a4696ea728b 100644
> > --- a/drivers/tty/serial/8250/8250_port.c
> > +++ b/drivers/tty/serial/8250/8250_port.c
> > @@ -2371,7 +2371,10 @@ int serial8250_do_startup(struct uart_port
> > *port)
> >  	 * Request DMA channels for both RX and TX.
> >  	 */
> >  	if (up->dma) {
> > -		retval = serial8250_request_dma(up);
> > +		if (uart_console(port))
> > +			retval = -ENXIO;
> > +		else
> > +			retval = serial8250_request_dma(up);
> >  		if (retval)
> >  			pr_warn_ratelimited("ttyS%d - failed to
> > request DMA\n",
> >  					    serial_index(port));
> 
> I would appreciate if you note that it is disabled on purpose and not
> because something failed.
> Other than those minor things I an board :)

Got it for v2.

> 
> > -- 
> > 2.17.0
> 
> Sebastian

-- 
Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Intel Finland Oy