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 20:20:37 +0200
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
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 2018-05-16 15:58:10 [+0300], Andy Shevchenko wrote:
> > > 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.

My point was simply to clarify if this entirely broken and requires
backports or if this makes things more complicated and could be avoided.

> > 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_.

Yes, full context is helpful :)

Sebastian