<HTML ><HEAD ><TITLE >Advice for boot loader authors</TITLE ><META NAME="GENERATOR" CONTENT="Modular DocBook HTML Stylesheet Version 1.7"><LINK REL="HOME" TITLE="Remote Serial Console HOWTO" HREF="index.html"><LINK REL="UP" TITLE="Gratuitous advice for developers" HREF="advice.html"><LINK REL="PREVIOUS" TITLE="Gratuitous advice for developers" HREF="advice.html"><LINK REL="NEXT" TITLE="Advice for BIOS authors" HREF="advice-bios.html"></HEAD ><BODY CLASS="SECTION" BGCOLOR="#FFFFFF" TEXT="#000000" LINK="#0000FF" VLINK="#840084" ALINK="#0000FF" ><DIV CLASS="NAVHEADER" ><TABLE SUMMARY="Header navigation table" WIDTH="100%" BORDER="0" CELLPADDING="0" CELLSPACING="0" ><TR ><TH COLSPAN="3" ALIGN="center" >Remote Serial Console HOWTO</TH ></TR ><TR ><TD WIDTH="10%" ALIGN="left" VALIGN="bottom" ><A HREF="advice.html" ACCESSKEY="P" >Prev</A ></TD ><TD WIDTH="80%" ALIGN="center" VALIGN="bottom" >Appendix F. Gratuitous advice for developers</TD ><TD WIDTH="10%" ALIGN="right" VALIGN="bottom" ><A HREF="advice-bios.html" ACCESSKEY="N" >Next</A ></TD ></TR ></TABLE ><HR ALIGN="LEFT" WIDTH="100%"></DIV ><DIV CLASS="SECTION" ><H1 CLASS="SECTION" ><A NAME="ADVICE-BOOTLOADER" ></A >F.1. Advice for boot loader authors</H1 ><P >Serial console support in a boot loader is very useful. Thank you for supporting it.</P ><P >The boot loader should support the <SPAN CLASS="PRODUCTNUMBER" >8250A</SPAN > <SPAN CLASS="ACRONYM" >UART</SPAN > and its programming-compatible <SPAN CLASS="PRODUCTNUMBER" >82510</SPAN >, <SPAN CLASS="PRODUCTNUMBER" >16450</SPAN >, <SPAN CLASS="PRODUCTNUMBER" >16550</SPAN > and <SPAN CLASS="PRODUCTNUMBER" >16750</SPAN > descendants. The serial chip used in the <SPAN CLASS="PRODUCTNAME" >IBM PC/XT</SPAN >, the <SPAN CLASS="PRODUCTNUMBER" >8250</SPAN > (no A), and its <SPAN CLASS="PRODUCTNUMBER" >8250B</SPAN > descendant need not be supported. The <SPAN CLASS="PRODUCTNUMBER" >8250A</SPAN > data sheet is <A HREF="http://www.intersil.com/data/FN/FN2/FN2958/FN2958.pdf" TARGET="_top" ><I CLASS="CITETITLE" ><SPAN CLASS="PRODUCTNUMBER" >82C50A</SPAN > <SPAN CLASS="PRODUCTNAME" >CMOS Asynchronous Communications Element</SPAN ></I ></A > and is updated by Intel's errata <A HREF="http://support.intel.com/support/controllers/peripheral/7513.htm" TARGET="_top" ><I CLASS="CITETITLE" ><SPAN CLASS="PRODUCTNUMBER" >82510</SPAN > PC Software Compatibility</I ></A >. The <SPAN CLASS="PRODUCTNUMBER" >16550</SPAN > data sheet is <A HREF="http://www.national.com/ds/PC/PC16550D.pdf" TARGET="_top" ><I CLASS="CITETITLE" ><SPAN CLASS="PRODUCTNUMBER" >PC16550D</SPAN > <SPAN CLASS="PRODUCTNAME" >Universal Asynchronous Receiver/Transmitter with FIFOs</SPAN ></I ></A >.</P ><P >To set the serial port and serial parameters, most <SPAN CLASS="ACRONYM" >Linux</SPAN > boot loaders use a syntax modeled upon the kernel's <TT CLASS="LITERAL" >console</TT > parameter. It would be nice to retain this consistency, since the user needs to learn the kernel syntax in any case.</P ><P >The default value should be 9600<SPAN CLASS="ABBREV" >bps</SPAN >, 8 data bits, no parity, 1 stop bit and <SPAN CLASS="ACRONYM" >CTS</SPAN >/<SPAN CLASS="ACRONYM" >RTS</SPAN > flow control. This gives the maximum interoperability with the other programs that use the serial console.</P ><P >Please do not ignore the lower speeds, as remote serial console is at its most valuable when the computer is located three days walk up a mountain in the New Guinea highlands. It is difficult to get more than 75<SPAN CLASS="ABBREV" >bps</SPAN > from <SPAN CLASS="ACRONYM" >HF</SPAN > radio under adverse sky conditions.</P ><P >Be conservative in your use of the modem status lines. Even if you are ignoring incoming status (<SPAN CLASS="ACRONYM" >DSR</SPAN >, <SPAN CLASS="ACRONYM" >DCD</SPAN >) and handshaking lines (<SPAN CLASS="ACRONYM" >RTS</SPAN >) at least assert the outgoing status (<SPAN CLASS="ACRONYM" >DTR</SPAN >) and handshaking (<SPAN CLASS="ACRONYM" >CTS</SPAN >) lines. Correctly configured modems will not receive calls with <SPAN CLASS="ACRONYM" >DTR</SPAN > low, and dropping <SPAN CLASS="ACRONYM" >DTR</SPAN > will cause the modem to hang up.</P ><P >Consider that the <SPAN CLASS="ACRONYM" >BIOS</SPAN > may have already initialised the <SPAN CLASS="ACRONYM" >UART</SPAN > and provide a configuration option to allow the boot loader to be informed of that. When the boot loader initialises the <SPAN CLASS="ACRONYM" >UART</SPAN >, <SPAN CLASS="ACRONYM" >DTR</SPAN > will fall and the line will hang up. In some scenarios each hang up requires the satelite circuit to be re-booked before another call can be placed. </P ><P >Cater for line noise. Imagine the boot loader starting and then being sent nonsensical characters every few seconds. Although this is certainly wrong, a fault in a modem is difficult to remotely diagnose and correct if the machine is left stranded at the boot loader prompt. A solution is to boot the default image upon the expiry of a timer; the boot occurring even if the user (or line noise) has started to type. For example the boot loader configuration could say:</P ><DIV CLASS="INFORMALFIGURE" ><A NAME="AEN4174" ></A ><P ></P ><TABLE BORDER="0" BGCOLOR="#E0E0E0" WIDTH="100%" ><TR ><TD ><FONT COLOR="#000000" ><PRE CLASS="PROGRAMLISTING" ># Start the machine regardless after 30 minutes # 30 * 60 seconds per minute * units of tenths of seconds <B CLASS="COMMAND" >lifetime 18000</B ></PRE ></FONT ></TD ></TR ></TABLE ><P ></P ></DIV ><P >The default should be no life timer. The timer is also useful in high availability applications: when a machine is used in environments with an planned availability of 99.999% the lifetime value should be configured to three minutes or less.</P ><P >Check information read from the BIOS for reasonablness. For example, if the BIOS's Extended Data Area suggests 0x000 as the address for the serial port's registers then don't try to initialise the registers.</P ></DIV ><DIV CLASS="NAVFOOTER" ><HR ALIGN="LEFT" WIDTH="100%"><TABLE SUMMARY="Footer navigation table" WIDTH="100%" BORDER="0" CELLPADDING="0" CELLSPACING="0" ><TR ><TD WIDTH="33%" ALIGN="left" VALIGN="top" ><A HREF="advice.html" ACCESSKEY="P" >Prev</A ></TD ><TD WIDTH="34%" ALIGN="center" VALIGN="top" ><A HREF="index.html" ACCESSKEY="H" >Home</A ></TD ><TD WIDTH="33%" ALIGN="right" VALIGN="top" ><A HREF="advice-bios.html" ACCESSKEY="N" >Next</A ></TD ></TR ><TR ><TD WIDTH="33%" ALIGN="left" VALIGN="top" >Gratuitous advice for developers</TD ><TD WIDTH="34%" ALIGN="center" VALIGN="top" ><A HREF="advice.html" ACCESSKEY="U" >Up</A ></TD ><TD WIDTH="33%" ALIGN="right" VALIGN="top" >Advice for <SPAN CLASS="ACRONYM" >BIOS</SPAN > authors</TD ></TR ></TABLE ></DIV ></BODY ></HTML >
