Messages in this thread | | | Date | Thu, 28 Oct 1999 09:08:46 -0400 | Subject | Re: serial.c (half duplex support) | From | tytso@mit ... |
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Date: Wed, 27 Oct 1999 17:00:37 +0200 (MEST) From: R.E.Wolff@BitWizard.nl (Rogier Wolff)
Ok, let me clarify my point-of-view in this discussion:
I'd expect Linux to get the most out of the hardware. In this case, transmissions achieving the full 115k2 on hardware that is capable of that. I'd -=like=- this to be achieved. If not, too bad. As long as it works.
Well, let's see if we aren't not achieving that goal before we start getting our knickers in a twist. I did my original performance tuning and testing on an 40 MHz 386 with kermit transfers at 115kbps, and some of the games in the driver (like not touching the ISR register since on an 8 MHz ISA bus each I/O instruction really costs you) were done to get performance as high as possible.
If we're having problems getting full 115k bps on hardware, especially on today's hardware (i.e. Pentiums with IDE DMA), I'd be a little bit surprised, quite frankly. But I'd really like to see some evidence that that's the case before we add all sorts of complexity to the serial driver.
David is in a position where the gaps violate the spec, and he might get caught in a situation where things actually don't work due to the gaps.
If we have interrupt latency of > 4 character times at 9600 or 19200 bps, some other driver (or some hardware --- like the cheap sh*t IDE contrllers that some boxes 2-3 years ago were inflicted with) has serious bugs, and we should fix that driver first. That kind of interrupt latency causes other problems besides ones for the serial driver, and after a certain point it doesn't make sense putting huge amounts of effort into making the serial driver more complicated than it already is.
We optimize for the common case, after all, and it's becoming clear that the Profibus case is not a friendly case; it looks like you will need to disable the FIFO's in order to make it meet the spec anyway. And without Profibus, if you have substandard hardware that requires turning off the interrupts for huge amounts of time, I consider it acceptable that you might not get full 115kbps transmission --- heck, sometimes on such boxes you end up losing characters off the receive FIFO in any case, so why bother optimizing the transmit side? Sometimes, it's just not worth it.
- Ted
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