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53f40ddc8e
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. |
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