Re: serial8250: bogus low_latency destabilizes kernel, need sanitycheck

[Pavel Roskin]

Hi Alan,

Quoting One Thousand Gnomes <gnomes@xxxxxxxxxxxxxxxxxxx>:

> > 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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