linux-kernel - Re: serial8250: bogus low_latency destabilizes kernel, need sanity check

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Message-ID: <20140114112457.q54ujbz9c444s040-cebfxv@webmail.spamcop.net>
Date:	Tue, 14 Jan 2014 11:24:57 -0500
From:	Pavel Roskin <proski@....org>
To:	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Jiri Slaby <jslaby@...e.cz>, linux-kernel@...r.kernel.org
Subject: Re: serial8250: bogus low_latency destabilizes kernel, need sanity
	check

Hi Alan,

Quoting One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>:

>> Maybe we should unset the low_latency flag as soon as DMA fails?  There
>> are two flags, one is state->uart_port->flags and the other is
>> port->low_latency.  I guess we need to unset both.
>
> Well low latency and DMA are pretty much exclusive in the real world so
> probably DMA ports shouldn't allow low_latency to be set at all in DMA
> mode.

That's a useful insight.  I assumed exactly the opposite.

But how should a real low_latency device work?  Are they supported by  
8250_core?  Do they have hard IRQs?  Are those IRQs handled by  
serial8250_handle_irq()?

If DMA is not used, then serial8250_rx_chars() is the only way to  
receive data.  But serial8250_rx_chars() calls tty_flip_buffer_push()  
unconditionally, and the later should not be called from the IRQ  
context for low_latency devices, if the comment about it is to be  
trusted.

-- 
Regards,
Pavel Roskin
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