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Implement ec_putc() (emergency kernel [syscons] console putc()) and use

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it in emergency in sc_cnputc().

Locking fixes in sc_cnputc() previously turned off normal output in
near-deadlock conditions and added deferred output which might never
be completed.  Emergency output goes to the frame buffer using
sufficiently atomic non-blocking writes if the console is in text
mode (in graphics mode, nothing is done, modulo races setting the
graphics mode bit).  Screen updates overwrite the emergency output
if the emergency condition clears enough to reach them.

ec_putc() also works for "early" console output in normal x86 text
mode as soon as this mode is initialized (if ever).  This uses a
hard-coded x86 frame buffer address before cninit() and a hopefully
MI address after cninit().  But non-x86 is more likely to not support
text mode, when ec_putc() will be null.  ec_putc() has no dependencies
of syscons before cninit(), and only has them later to track syscons'
mode changes.  This commit doesn't attach ec_putc() for early use.

To test emergency use, put a breakpoint in central syscons output code
like sc_puts() and do some user output.  The system used to race or
deadlock in ddb output soon after entry to ddb.  The locking fixes
deferred the output until after leaving ddb, so ddb was unusable and
you had to try typing c[ontinue] blindly until it exited, or better use
a serial console in parallel.  Now the output goes to a window in the
middle 2/3 of the screen.  Scrolling is circular and there is no cursor,
but otherwise ec_putc() provides full dumb terminal functionality and
very fast output that hides artificates from dumb overwrites.
This commit is contained in:
bde 2017-03-04 08:47:31 +00:00
parent 21fddf2294
commit 1e2149857a
1 changed files with 64 additions and 2 deletions
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66
sys/dev/syscons/syscons.c
View File

@ -266,6 +266,65 @@ static struct cdevsw consolectl_devsw = {
.d_name = "consolectl",
};
/* ec -- emergency console. */
static u_int ec_scroffset;
/*
* Fake enough of main_console for ec_putc() to work very early on x86 if
* the kernel starts in normal color text mode. On non-x86, scribbling
* to the x86 normal color text frame buffer's addresses is unsafe, so
* set (likely non-fake) graphics mode to get a null initial ec_putc().
*/
static scr_stat fake_main_console = {
.scr.vtb_buffer = 0xb8000,
.xsize = 80,
.ysize = 25,
#if !defined(__amd64__) && !defined(__i386__)
.status = GRAPHICS_MODE,
#endif
};
#define main_console (sc_console == NULL ? fake_main_console : main_console)
static void
ec_putc(int c)
{
u_short *scrptr;
u_int ind;
int attr, column, mysize, width, xsize, yborder, ysize;
if (main_console.status & GRAPHICS_MODE ||
c < 0 || c > 0xff || c == '\a')
return;
xsize = main_console.xsize;
ysize = main_console.ysize;
yborder = ysize / 5;
scrptr = (u_short *)main_console.scr.vtb_buffer + xsize * yborder;
mysize = xsize * (ysize - 2 * yborder);
do {
ind = ec_scroffset;
column = ind % xsize;
width = (c == '\b' ? -1 : c == '\t' ? (column + 8) & ~7 :
c == '\r' ? -column : c == '\n' ? xsize - column : 1);
if (width == 0 || (width < 0 && ind < -width))
return;
} while (atomic_cmpset_rel_int(&ec_scroffset, ind, ind + width) == 0);
if (c == '\b' || c == '\r')
return;
if (c == '\n')
ind += xsize; /* XXX clearing from new pos is not atomic */
attr = sc_kattr();
if (c == '\t' || c == '\n')
c = ' ';
do
scrptr[ind++ % mysize] = (attr << 8) | c;
while (--width != 0);
}
#undef main_console
int
sc_probe_unit(int unit, int flags)
{
@ -1861,10 +1920,13 @@ sc_cnputc(struct consdev *cd, int c)
sc_cnputc_log[head % sizeof(sc_cnputc_log)] = c;
/*
* If we couldn't open, return to defer output.
* If we couldn't open, do special reentrant output and return to defer
* normal output.
*/
if (!st.scr_opened)
if (!st.scr_opened) {
ec_putc(c);
return;
}
#ifndef SC_NO_HISTORY
if (scp == scp->sc->cur_scp && scp->status & SLKED) {