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SubjectRE: n_tty.c driver patch (semantic and performance correction) (a ll recent versions)
On Sat, June 15, 2002 at 9:01 PM, Robert White wrote:
> Kernel Versions: 2.2, 2.4, 2.5 (all of them since 1996 really 8-)
> The n_tty line discipline module contains a "semantic error" that
> limits its speed and usefulness in many uses. The attached patch
> directly addresses serial performance in a completely backwards-
> compatible way.
> In particular, the current handling of VMIN hugely limits,
> complicates, and/or slows down optimal serial use. The most
> obvious example is that if you call read(2) with a buffer size less
> than the current value of VMIN, the line discipline will insist
> that the read call wait for characters that can not be returned to
> that call. The POSIX standard is silent on the subject of whether
> this is right or wrong. Common sense says it is wrong.


IIRC, the way VMIN>0,VTIME=0 is supposed to work is to make characters
available to the top level queue to be read when the low level input
queue contains VMIN or more characters. Until that moment, there are
no characters available to a read of any buffer size regardless of how
many characters have been received at the low level. This is why a
single character read blocks when at least one character has been
received but not yet VMIN characters. Only data in the top level queue
can be read. If the line discipline has not yet released data to the
top level queue because of VMIN, then no data can be read, but this is
not an error.

Many have been tempted to change the behavior of this part of the
system. IMHO, it is not worth tossing away application portability.

Standards compliance can feel a bit like vertigo while instrument
flying. Sometimes one has to just stare at the artificial horizon and
say "I believe it" to one's self until the gut is convinced.

Best regards,

Ed Vance
Macrolink, Inc. 1500 N. Kellogg Dr Anaheim, CA 92807
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