[Bug 191265] New: [patch] uart(4) punishes UARTs for being too fast

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=191265

            Bug ID: 191265
           Summary: [patch] uart(4) punishes UARTs for being too fast
           Product: Base System
           Version: 11.0-CURRENT
          Hardware: Any
                OS: Any
            Status: Needs Triage
          Severity: Affects Only Me
          Priority: ---
         Component: kern
          Assignee: freebsd-bugs at FreeBSD.org
          Reporter: shurd at FreeBSD.org

Created attachment 144015
  --> https://bugs.freebsd.org/bugzilla/attachment.cgi?id=144015&action=edit
Fix for discarding data when internal buffer full.

For NS8250 family UARTs, svn 120146 -
http://svnweb.freebsd.org/base?view=revision&revision=120146 - made a change to
flush the UART if the kernel buffers are full.  The commit message suggests
this is because of how the read loop was structured.

However, this change mad it so that if the internal FreeBSD buffer is full, it
will drop all pending data... effectively punishing devices for having too much
data buffered.  At least one network exposed UART is capable of this.

The attached patch doesn't clear the interrupt if the rx buffer is full by
checking the buffer has space before reading LSR.

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