On 03.09.2014 16:57, Alexey Dokuchaev wrote: >> The VGA vt(4) issues all stem from the relatively complex and arcane >> VGA hardware and a somewhat limited vt_vga implementation. > > Can you elaborate a bit on the technical side of things? Particularly, it > looks strange that syscons(4) was able to work just fine on "relatively > complex and arcane VGA hardware", while more modern vt(4) is fighting with > problems.
VGA requires that you write 8 pixels at a time (8 pixels stored in one byte). To update one pixel, the previous version of vt_vga would read one byte so that the 8 pixels are loaded in special registers (called "latches"), then would write a new byte to the video memory. The VGA hardware would process the new byte and the content of the latches to compute the final data. That's how one pixel out of eight could be modified. Unfortunately, reading from the video memory is very expensive. The new version of vt_vga never reads from the video memory. Instead, it uses the console history to know what those 8 pixels should look like and write one byte which doesn't need further processing. One bug in vt(4) was that the mouse cursor position, even if it was invisible before moused(8) starts, was always considered "dirty" and required a redraw. The default position being [0;0], each new character written would trigger a full refresh of the screen from [0;0] to the position of this character. Those two problems combined explain the slownness of vt(4), especially with discrete GPU and virtual machines; i915 users were mostly spared. Regarding the incorrect refresh when vt-switching, it was caused by a race between the redraw thread and the switch. The redraw thread was not stopped during a switch. Therefore, if the thread ran while the switch was in progress, it could mark the screen as "up-to-date" even though it displayed the wrong data. One change did reduce the vt-switch time specifically: in vt_vga, a switch triggers a clear of the video memory. The loop did read from the video memory, then wrote 8 black pixels. The useless read was removed. -- Jean-Sébastien Pédron
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