[PATCH] Powerpc 8xx CPM_UART delay in receive

[PATCH] Powerpc 8xx CPM_UART delay in receive

[Christophe Leroy]

Hello,

I'm not sure who to address this Patch to either

It fixes a delay issue with CPM UART driver on Powerpc MPC8xx.
The problem is that with the actual code, the driver waits 32 IDLE patterns before returning the received data to the upper level. It means for instance about 1 second at 300 bauds.
This fix limits to one byte the waiting period.

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>

--- linux-3.5-vanilla/drivers/tty/serial/cpm_uart/cpm_uart_core.c	2012-07-21 22:58:29.000000000 +0200
+++ linux-3.5/drivers/tty/serial/cpm_uart/cpm_uart_core.c	2012-08-09 17:38:37.000000000 +0200
@@ -798,7 +799,7 @@
 	cpm_set_scc_fcr(sup);
 
 	out_be16(&sup->scc_genscc.scc_mrblr, pinfo->rx_fifosize);
-	out_be16(&sup->scc_maxidl, pinfo->rx_fifosize);
+	out_be16(&sup->scc_maxidl, 1);
 	out_be16(&sup->scc_brkcr, 1);
 	out_be16(&sup->scc_parec, 0);
 	out_be16(&sup->scc_frmec, 0);
@@ -872,7 +873,7 @@
 
 	/* Using idle character time requires some additional tuning.  */
 	out_be16(&up->smc_mrblr, pinfo->rx_fifosize);
-	out_be16(&up->smc_maxidl, pinfo->rx_fifosize);
+	out_be16(&up->smc_maxidl, 1);
 	out_be16(&up->smc_brklen, 0);
 	out_be16(&up->smc_brkec, 0);
 	out_be16(&up->smc_brkcr, 1);

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

[Alan Cox]

On Tue, 14 Aug 2012 16:26:28 +0200
Christophe Leroy <christophe.leroy@c-s.fr> wrote:

> Hello,
> 
> I'm not sure who to address this Patch to either
> 
> It fixes a delay issue with CPM UART driver on Powerpc MPC8xx.
> The problem is that with the actual code, the driver waits 32 IDLE patterns before returning the received data to the upper level. It means for instance about 1 second at 300 bauds.
> This fix limits to one byte the waiting period.

Take a look how the 8250 does it - I think you want to set the value
based upon the data rate. Your patch will break it for everyone doing
high seed I/O.

Alan

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

[leroy christophe]

Le 14/08/2012 16:52, Alan Cox a écrit :
> On Tue, 14 Aug 2012 16:26:28 +0200
> Christophe Leroy <christophe.leroy@c-s.fr> wrote:
>
>> Hello,
>>
>> I'm not sure who to address this Patch to either
>>
>> It fixes a delay issue with CPM UART driver on Powerpc MPC8xx.
>> The problem is that with the actual code, the driver waits 32 IDLE patterns before returning the received data to the upper level. It means for instance about 1 second at 300 bauds.
>> This fix limits to one byte the waiting period.
> Take a look how the 8250 does it - I think you want to set the value
> based upon the data rate. Your patch will break it for everyone doing
> high seed I/O.
>
> Alan
>
I'm not sure I understand what you mean. As far as I can see 8250/16550 
is working a bit different, as it is based on a fifo and triggers an 
interrupt as soon as a given number of bytes is received. I also see 
that in case this amount is not reached, there is a receive-timeout 
which goes on after no byte is received for a duration of more than 4 bytes.

The PowerPC CPM is working differently. It doesn't use a fifo but 
buffers. Buffers are handed to the microprocessor only when they are 
full or after a timeout period which is adjustable. In the driver, the 
buffers are configured with a size of 32 bytes. And the timeout is set 
to the size of the buffer. That is this timeout that I'm reducing to 1 
byte in my proposed patch. I can't see what it would break for high 
speed I/O.

Christophe

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

[Alan Cox]

> The PowerPC CPM is working differently. It doesn't use a fifo but 
> buffers. Buffers are handed to the microprocessor only when they are 
> full or after a timeout period which is adjustable. In the driver, the 

Which is different how - remembering we empty the FIFO on an IRQ

> buffers are configured with a size of 32 bytes. And the timeout is set 
> to the size of the buffer. That is this timeout that I'm reducing to 1 
> byte in my proposed patch. I can't see what it would break for high 
> speed I/O.

How can a timeout be measured in "bytes". Can we have a bit more clarity
on how the hardware works and take it from there ?

Alan

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

[leroy christophe]

Le 16/08/2012 16:29, Alan Cox a écrit :
>> The PowerPC CPM is working differently. It doesn't use a fifo but
>> buffers. Buffers are handed to the microprocessor only when they are
>> full or after a timeout period which is adjustable. In the driver, the
> Which is different how - remembering we empty the FIFO on an IRQ
>
>> buffers are configured with a size of 32 bytes. And the timeout is set
>> to the size of the buffer. That is this timeout that I'm reducing to 1
>> byte in my proposed patch. I can't see what it would break for high
>> speed I/O.
> How can a timeout be measured in "bytes". Can we have a bit more clarity
> on how the hardware works and take it from there ?
>
> Alan
>
The reference manual of MPC885 says the following about the MAX_IDL 
parameter:

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

If the UART is receiving data and gets an idle character (all ones), the 
channel begins counting consecutive idle characters received. If MAX_IDL 
is reached, the buffer is closed and an RX interrupt is generated if not 
masked. If no buffer is open, this event does not generate an interrupt 
or any status information. The internal idle counter (IDLC) is reset 
every time a character is received. To disable the idle sequence 
function, clear MAX_IDL


The datasheet of the 16550 UART says:

Besides, for FIFO mode operation a time out mechanism is implemented. 
Independently of the trigger level of the FIFO, an interrupt will be 
generated if there is at least one word in the FIFO and for a time 
equivalent to the transmission of four characters
- no new character has been received and
- the microprocessor has not read the RHR
To compute the time out, the current total number of bits (start, data, 
parity and stop(s)) is used, together with the current baud rate (i.e., 
it depends on the contents of the LCR, DLL, DLM and PSD registers).


Christophe

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

[Alan Cox]

> 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).

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

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

[Alan Cox]

> * 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)

That would seem sensible.



Alan