From 39aead8373b3c20bb5965c024dfb51a94e526151 Mon Sep 17 00:00:00 2001 From: Daniel Vetter Date: Thu, 29 Oct 2020 14:22:29 +0100 Subject: fbcon: Disable accelerated scrolling So ever since syzbot discovered fbcon, we have solid proof that it's full of bugs. And often the solution is to just delete code and remove features, e.g. 50145474f6ef ("fbcon: remove soft scrollback code"). Now the problem is that most modern-ish drivers really only treat fbcon as an dumb kernel console until userspace takes over, and Oops printer for some emergencies. Looking at drm drivers and the basic vesa/efi fbdev drivers shows that only 3 drivers support any kind of acceleration: - nouveau, seems to be enabled by default - omapdrm, when a DMM remapper exists using remapper rewriting for y/xpanning - gma500, but that is getting deleted now for the GTT remapper trick, and the accelerated copyarea never set the FBINFO_HWACCEL_COPYAREA flag, so unused (and could be deleted already I think). No other driver supportes accelerated fbcon. And fbcon is the only user of this accel code (it's not exposed as uapi through ioctls), which means we could garbage collect fairly enormous amounts of code if we kill this. Plus because syzbot only runs on virtual hardware, and none of the drivers for that have acceleration, we'd remove a huge gap in testing. And there's no other even remotely comprehensive testing aside from syzbot. This patch here just disables the acceleration code by always redrawing when scrolling. The plan is that once this has been merged for well over a year in released kernels, we can start to go around and delete a lot of code. v2: - Drop a few more unused local variables, somehow I missed the compiler warnings (Sam) - Fix typo in comment (Jiri) - add a todo entry for the cleanup (Thomas) v3: Remove more unused variables (0day) Reviewed-by: Tomi Valkeinen Reviewed-by: Thomas Zimmermann Reviewed-by: Greg Kroah-Hartman Acked-by: Sam Ravnborg Cc: Jiri Slaby Cc: Bartlomiej Zolnierkiewicz Cc: Greg Kroah-Hartman Cc: Linus Torvalds Cc: Ben Skeggs Cc: nouveau@lists.freedesktop.org Cc: Tomi Valkeinen Cc: Daniel Vetter Cc: Jiri Slaby Cc: "Gustavo A. R. Silva" Cc: Tetsuo Handa Cc: Peilin Ye Cc: George Kennedy Cc: Nathan Chancellor Cc: Peter Rosin Signed-off-by: Daniel Vetter Link: https://patchwork.freedesktop.org/patch/msgid/20201029132229.4068359-1-daniel.vetter@ffwll.ch --- Documentation/gpu/todo.rst | 18 ++++++++++++++++++ 1 file changed, 18 insertions(+) (limited to 'Documentation/gpu') diff --git a/Documentation/gpu/todo.rst b/Documentation/gpu/todo.rst index acca232b025b..94fc76f0ecd4 100644 --- a/Documentation/gpu/todo.rst +++ b/Documentation/gpu/todo.rst @@ -292,6 +292,24 @@ Contact: Daniel Vetter, Noralf Tronnes Level: Advanced +Garbage collect fbdev scrolling acceleration +-------------------------------------------- + +Scroll acceleration is disabled in fbcon by hard-wiring p->scrollmode = +SCROLL_REDRAW. There's a ton of code this will allow us to remove: +- lots of code in fbcon.c +- a bunch of the hooks in fbcon_ops, maybe the remaining hooks could be called + directly instead of the function table (with a switch on p->rotate) +- fb_copyarea is unused after this, and can be deleted from all drivers + +Note that not all acceleration code can be deleted, since clearing and cursor +support is still accelerated, which might be good candidates for further +deletion projects. + +Contact: Daniel Vetter + +Level: Intermediate + idr_init_base() --------------- -- cgit v1.2.3