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test if port does exist via using scratch register

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The SCR, scratch register was not present on the 8250 and 8250B UART, so we
can use to test if we actually do have serial port.

We need this test because some systems will get long delays while attempting
to write to non-existing port and this will slow down the console IO
to extreme.

MFC after:	1 week
This commit is contained in:
Toomas Soome 2020-03-12 06:45:08 +00:00
parent 68983a2bc7
commit 7f505e7f79
Notes: svn2git 2020-12-20 02:59:44 +00:00 
svn path=/head/; revision=358906
1 changed files with 10 additions and 0 deletions
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10
stand/i386/libi386/comconsole.c
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@ -330,6 +330,16 @@ comc_setup(int speed, int port)
if ((comconsole.c_flags & (C_ACTIVEIN | C_ACTIVEOUT)) == 0)
return;
#define COMC_TEST 0xbb
/*
* Write byte to scratch register and read it out.
*/
outb(comc_port + com_scr, COMC_TEST);
if (inb(comc_port + com_scr) != COMC_TEST) {
comconsole.c_flags &= ~(C_PRESENTIN | C_PRESENTOUT);
return;
}
outb(comc_port + com_cfcr, CFCR_DLAB | COMC_FMT);
outb(comc_port + com_dlbl, COMC_BPS(speed) & 0xff);
outb(comc_port + com_dlbh, COMC_BPS(speed) >> 8);