Discussion
Many machines come with a builtin software or hardware terminal for GRUB to use instead of implementing its own! So today I'm studying the simpler of GRUB's machine-specific terminal drivers.
To output a char on Xen-emulated machines until there's space in a ringbuffer it spinlocks (via a helper) on Xen's event-channel before writing enqueueing it, saves some sort of timestamp, & unsets a flag. Memory barriers surround the ringbuffer manipulations.
Chars are read at an incremented index.
1/?
On UBoot machines GRUB reads a input char via the TSTC & & GETC system calls, whilst it outputs using the PUTC system call.
Both the Xen & UBoot drivers are subclasses of the terminfo driver, even if for some reason the UBoot drivers includes some trivial wrappers the other subclasses don't.
There's a helper initializer for PCI serial terminals, by iterating over PCI devices with a callback that retrieves/validates each device's data & inits a corresponding NS8250 serial driver.
2/?
On CoreBoot Machines GRUB offers a "cbmemc" command which carefully outputs data from the firmware whilst the global pointer to it is temporarily unset. As part of initializing this driver it extracts that firmware table into said global.
To output a char it carefully inserts it into the firmware's array at an incremented address whilst saving the new index into firmware memory bitpacked with bitflags, possibly adding an OVERFLOW flag.
3/?
There's an initializer for serial I/O on Arc machines which iterates over Arc's bus with callback that validates its got a serial device & inits a corresponding serial driver.
The config method for which is a noop, though both input & output methods may use a firmware routine to reopen the serial terminal for any updated config before updating flags. If valid both input & output defers to the corresponding firmware.
And there's Arc's internal terminal!
4/5!
Arc's internal terminal is yet another terminfo subclass, whose initializer also checks Arc's envvars & display-status to get the window size in chars. The envvar (checked via a couple shared helpers) is there to see if its hooked up to a serial terminal.
Output's handled via the write() firmware routine, whilst input's handled by the get_read_status() & read() firmware routines.
5/5 Fin for today! Tomorrow: EFI! And maybe others.
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