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SubjectRe: [PATCH v1 3/3] serial: 8250_port: Disable DMA operations for kernel console
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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@linux.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@linux.intel.com>
Intel Finland Oy

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