linux-kernel - Re: Real time USB2Serial devices and behaivor

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Message-ID: <20080327210036.GA20027@csclub.uwaterloo.ca>
Date:	Thu, 27 Mar 2008 17:00:36 -0400
From:	lsorense@...lub.uwaterloo.ca (Lennart Sorensen)
To:	David Brownell <david-b@...bell.net>
Cc:	mgross@...ux.intel.com, Greg KH <greg@...ah.com>,
	lkml <linux-kernel@...r.kernel.org>,
	linux-rt-users@...r.kernel.org, linux-usb@...r.kernel.org
Subject: Re: Real time USB2Serial devices and behaivor

On Thu, Mar 27, 2008 at 12:48:40PM -0700, David Brownell wrote:
> I happened across this the other day, and saved the URL since
> the price was relatively sane ... $15 vs the $60 I saw at
> another site, although ISTR Fry's used to have them at $5:
> 
>   http://www.wrighthobbies.net/catalog/index.php?cPath=47
> 
> Yeah, I know.  "It's getting very hard to find real PCI slots
> any more."  :(

Well digi (www.digi.com) makes some nice PCI serial cards, that work
with the jsm driver.  Not sure how many models are supported, but
certainly I have used the 2 port card with DE-9 connectors.

A little bit of patching will make the jsm driver run any exar
17[CD]15[248] based card, like the ones we use on the routers I work on.
The digi cards use a rebranded exar chip with a different PCI id.  Much
better than using the 8250 driver to run the exar's in PCI 16550A mode,
although it does work.

They have PCIe cards too although I have no idea if they use the same
interface or not (it would make sense to do so on the part of exar
though).

> It seems like some companies (notably *cough* Intel) are
> on a little jihad to get rid of serial ports...
> 
> I'd think that in a controlled environment (fixed set of
> USB connections) USB should be able to meet fairly chosen
> "real time" latency ceilings.  The stack probably needs a
> few semantic updates to make it happen -- e.g. URB completions
> are issued in_irq() -- but it shouldn't be insurmountable.

-- 
Len Sorensen
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