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Daniel Vetter 39aead8373 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. `50145474f6` ("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 <tomi.valkeinen@ti.com> Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de> Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org> Acked-by: Sam Ravnborg <sam@ravnborg.org> Cc: Jiri Slaby <jirislaby@kernel.org> Cc: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Ben Skeggs <bskeggs@redhat.com> Cc: nouveau@lists.freedesktop.org Cc: Tomi Valkeinen <tomi.valkeinen@ti.com> Cc: Daniel Vetter <daniel.vetter@ffwll.ch> Cc: Jiri Slaby <jirislaby@kernel.org> Cc: "Gustavo A. R. Silva" <gustavoars@kernel.org> Cc: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp> Cc: Peilin Ye <yepeilin.cs@gmail.com> Cc: George Kennedy <george.kennedy@oracle.com> Cc: Nathan Chancellor <natechancellor@gmail.com> Cc: Peter Rosin <peda@axentia.se> Signed-off-by: Daniel Vetter <daniel.vetter@intel.com> Link: https://patchwork.freedesktop.org/patch/msgid/20201029132229.4068359-1-daniel.vetter@ffwll.ch		2020-11-17 11:00:29 +01:00
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README

Linux kernel
============

There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.

In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``.  The formatted documentation can also be read online at:

    https://www.kernel.org/doc/html/latest/

There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.

Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.