Re: [PATCH] Powerpc 8xx CPM_UART delay in receive

[leroy christophe]

Le 16/08/2012 17:21, Alan Cox a écrit :
> > MAX_IDL: Maximum idle characters. When a character is received, the
> > receiver begins counting idle characters. If MAX_IDL idle characters
> > are received before the next data character, an idle timeout occurs
> > and the buffer is closed,
> > generating a maskable interrupt request to the core to receive the
> > data from the buffer. Thus, MAX_IDL offers a way to demarcate frames.
> > To disable the feature, clear MAX_IDL. The bit length of an idle
> > character is calculated as follows: 1 + data length (5–9) + 1 (if
> > parity is used)
> > + number of stop bits (1–2). For 8 data bits, no parity, and 1 stop
> > bit, the character length is 10 bits
>
> So if you have slightly bursty high speed data as its quite typical
> before your change you would get one interrupt per buffer of 32 bytes,
> with it you'll get a lot more interrupts.
>
> You have two available hints about the way to set this - one of them is
> the baud rate (low baud rates mean the fifo isn't a big win and the
> latency is high), the other is the low_latency flag if the driver
> supports the low latency feature (and arguably you can still use a
> request for it as a hint even if you refuse the actual feature).
>
> So I think a reasonable approach would be set the idle timeout down for
> low baud rates or if low_latency is requested.
>
> > generated if there is at least one word in the FIFO and for a time
> > equivalent to the transmission of four characters
> Which is a bit more reasonable than one, although problematic at low
> speed (hence the fifo on/off).
What would then thing about:
* a value of 1 for all rates below 2400 (On 8250, fifo is set to 1 for 
such rates)
* a value of 2 for 2400 and 4800
* a value of 4 for 9600 (which is the default on the 8250 for all rates 
above 2400)
* a value of 8 for 19200
* a value of 16 for 38400 and above (on UCC_UART, maxidl is set to 16, 
never 32)

Christophe
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/