linux-kernel - Re: [PATCH tty-next v3 1/6] serial: 8250: Adjust the timeout for FIFO mode

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Message-ID: <848cbe1c-d84d-4377-8709-bb98d1d83146@kernel.org>
Date: Mon, 2 Dec 2024 07:12:11 +0100
From: Jiri Slaby <jirislaby@...nel.org>
To: John Ogness <john.ogness@...utronix.de>,
 "Maciej W. Rozycki" <macro@...am.me.uk>
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
 Petr Mladek <pmladek@...e.com>, Sergey Senozhatsky
 <senozhatsky@...omium.org>, Steven Rostedt <rostedt@...dmis.org>,
 Thomas Gleixner <tglx@...utronix.de>, Esben Haabendal <esben@...nix.com>,
 linux-serial@...r.kernel.org, linux-kernel@...r.kernel.org,
 Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
 Rengarajan S <rengarajan.s@...rochip.com>,
 Jeff Johnson <quic_jjohnson@...cinc.com>,
 Serge Semin <fancer.lancer@...il.com>,
 Lino Sanfilippo <l.sanfilippo@...bus.com>,
 Wander Lairson Costa <wander@...hat.com>
Subject: Re: [PATCH tty-next v3 1/6] serial: 8250: Adjust the timeout for FIFO
 mode

On 04. 11. 24, 15:13, John Ogness wrote:
> On 2024-11-04, Jiri Slaby <jirislaby@...nel.org> wrote:
>> Instead of looping fifosize multiplied by random timeout, can we
>> re-use port->frame_time?
> 
> Rather than 10k loops, we could loop
> 
> 	(port->frame_time * some_scaled_padding) / 1000
> 
> times. The padding is important because we should not timeout in the
> normal scenario. Perhaps using ~2 as @some_padding. Something like:
> 
> 	port->frame_time >> 9
> 
> ?

No, spell it out as you did above:
   port->frame_time * 2 / NSEC_PER_USEC

>>>    The difference between THRE and TEMT is the state of the shift register
>>> only[2]:
>>>
>>> "In the FIFO mode, TEMT is set when the transmitter FIFO and shift
>>> register are both empty."
>>
>> But what's the purpose of spinning _here_? The kernel can run and
>> FIFO too. Without the kernel waiting for the FIFO.
>>
>> If we want to wait for fifo to empty, why not *also* the TSR. Meaning:
>>
>> Did you want UART_LSR_TEMT?
> 
> Let us assume we have a line with 640 characters and a FIFO of 64
> bytes. For this line, we must wait for the FIFO to empty 10 times. It is
> enough to wait for THRE for each of the 64-byte blocks because we are
> only interested in refilling the FIFO at this point. Only at the very
> end (in the caller...  serial8250_console_write()) do we need to wait
> for everything to flush to the wire (TEMT).
> 
> By waiting on TEMT for each of the 64-byte blocks, we are waiting longer
> than necessary. This creates a small window where the FIFO is empty and
> there is nothing being transmitted.
> 
> I did a simple test on my beaglebone-black hardware, sending 100 lines
> of 924 bytes at 9600 bps. Since my hardware uses a 64-byte FIFO, each
> line would have 14 such windows.
> 
> And indeed, waiting for TEMT rather than only THRE for the 64-byte
> blocks resulted in an extra 30ms total transfer for all 92400
> bytes. That is about 20us lost in each window by unnecessarily waiting
> for TEMT.

Sure -- you still misunderstand me. I am still asking why do you want to 
wait for the TX machinery at the *end* (for the last 64 B of the 640 B 
line) of transmission at all? It occurs to me as wasted cycles.

thanks,
-- 
js
suse labs