On Mon, May 17, 2021 at 3:07 PM Daniel Vetter <daniel@xxxxxxxx> wrote: > > On Fri, May 14, 2021 at 10:33 PM Linus Torvalds > <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote: > > > > On Fri, May 14, 2021 at 1:25 PM Maciej W. Rozycki <macro@xxxxxxxxxxx> wrote: > > > > > > Overall I think it does make sense to resize the text console at any > > > time, even if the visible console (VT) chosen is in the graphics mode, > > > > It might make sense, but only if we call the function to update the > > low-level data. > > > > Not calling it, and then starting to randomly use the (wrong) > > geometry, and just limiting it so that it's all within the buffer - > > THAT does not make sense. > > > > So I think your patch is fundamentally wrong. It basically says "let's > > use random stale incorrect data, but just make sure that the end > > result is still within the allocated buffer". > > > > My patch is at least conceptually sane. > > > > An alternative would be to just remove the "vcmode != KD_GRAPHICS" > > check entirely, and always call con_resize() to update the low-level > > data, but honestly, that seems very likelty to break something very > > fundamentally, since it's not how any of fbcon has ever been tested, > > Just an aside: I think with fbdev drivers this would go boom, because > you'd have fbcon interferring with a direct /dev/fb/* user. Boom here means a bit of screen corruption, because fbcon overdraws your X sessions. Fixed by the next redraw of X. > But if your fbdev driver is actually a drm modeset driver, then we > have additional limitations: If the userspace accesses the display > through /dev/dri/card0, then the kernel blocks all access through > /dev/fb/* (including fbcon) to the actual display (it only goes into > the buffer used for fbdev emulation). And everything would be fine. > > Also generally you'd get away with this even in problematic cases, > since usually you resize your console when looking at it, not when X > or something else is using your fbdev direct access. > > The one thing that's left out here a bit in the cold is userspace > modeset drivers in X. Those would get hosed. But also, we stopped > supporting those in at least i915/amd/radeon/nouveau drivers, > automatically falling back to the fbdev stuff in most cases (with or > without the drm drivers underneath that), and no one screamed. So > probably not many users left. This one could lead to incosistent hw state, which would be worse. > So I /think/ we could wager this, if it's the least intrusive fix from > the kernel pov. But it has some risks that we need to revert again if > we break some of the really old use-cases here. Cheers, Daniel > > Another alternative would be to just delay the resize to when vcmode > > is put back to text mode again. That sounds somewhat reasonable to me, > > but it's a pretty big thing. > > > > But no, your patch to just "knowingly use entirely wrong values, then > > add a limit check because we know the values are possibly garbage and > > not consistent with reality" is simply not acceptable. > > > > Linus > > > > -- > Daniel Vetter > Software Engineer, Intel Corporation > http://blog.ffwll.ch -- Daniel Vetter Software Engineer, Intel Corporation http://blog.ffwll.ch
