>(WW) Open ACPI failed (/var/run/acpid.socket) (No such file or directory)
>(WW) "glx" will not be loaded unless you've specified it to be loaded elsewhere.
>(WW) "dri" will not be loaded unless you've specified it to be loaded elsewhere.
И куча подобных предупржедений. Надо ксорг.конфе это дело прописать:
Например так:
Section "Module"
Load "dri"
Load "drm"
Load "glx"
EndSection

Добавил - поменялось только про KMS:

localhost ~ # grep «KMS» /var/log/Xorg.0.log
(II) [KMS] drm report modesetting isn't supported.
(II) [KMS] drm report modesetting isn't supported.
(II) [KMS] drm report modesetting isn't supported.
(II) [KMS] drm report modesetting isn't supported.

localhost ~ # grep «Allocating» /var/log/Xorg.0.log
(II) RADEON(0): Cannot get VRAM scratch space. Allocating in main memory instead
(II) RADEON(1): Cannot get VRAM scratch space. Allocating in main memory instead
(II) RADEON(2): Cannot get VRAM scratch space. Allocating in main memory instead
(II) RADEON(3): Cannot get VRAM scratch space. Allocating in main memory instead

localhost ~ # grep «PCI BAR» /var/log/Xorg.0.log
(II) RADEON(0): Detected total video RAM=1048576K, accessible=262144K (PCI BAR=262144K)
(II) RADEON(1): Detected total video RAM=1048576K, accessible=262144K (PCI BAR=262144K)
(II) RADEON(2): Detected total video RAM=1048576K, accessible=262144K (PCI BAR=262144K)
(II) RADEON(3): Detected total video RAM=1048576K, accessible=262144K (PCI BAR=262144K)

localhost ~ # grep «MergedFB» /var/log/Xorg.0.log
(!!) RADEON(0): MergedFB support has been removed and replaced with xrandr 1.2 support
(!!) RADEON(1): MergedFB support has been removed and replaced with xrandr 1.2 support
(!!) RADEON(2): MergedFB support has been removed and replaced with xrandr 1.2 support
(!!) RADEON(3): MergedFB support has been removed and replaced with xrandr 1.2 support

localhost ~ # grep «Dynamic» /var/log/Xorg.0.log
(II) RADEON(0): Dynamic Power Management Disabled
(II) RADEON(1): Dynamic Power Management Disabled
(II) RADEON(2): Dynamic Power Management Disabled
(II) RADEON(3): Dynamic Power Management Disabled

localhost ~ # grep «(EE)» /var/log/Xorg.0.log
(WW) warning, (EE) error, (NI) not implemented, (??) unknown.
(EE) RADEON(0): Acceleration initialization failed
(EE) RADEON(1): Acceleration initialization failed
(EE) RADEON(2): Acceleration initialization failed
(EE) RADEON(3): Acceleration initialization failed

CONFIG_DRM_RADEON_KMS

for KMS you'll need
to enable staging drivers and set the radeon kms config option to y.

make sure:
zgrep CONFIG_DRM_RADEON_KMS= /proc/config.gz
zgrep CONFIG_FRAMEBUFFER_CONSOLE= /proc/config.gz
are set in your .config.

Symbol: DRM_RADEON_KMS [=n]
Prompt: Enable modesetting on radeon by default
Defined at drivers/gpu/drm/radeon/Kconfig

Choose this option if you want kernel modesetting enabled by default
and you have a new enough userspace to support this. Running old
userspaces with this enabled will cause pain.

When kernel modesetting is enabled the IOCTL of radeon/drm
driver are considered as invalid and an error message is printed
in the log and they return failure.

KMS enabled userspace will use new API to talk with the radeon/drm
driver. The new API provide functions to create/destroy/share/mmap
buffer object which are then managed by the kernel memory manager
(here TTM). In order to submit command to the GPU the userspace
provide a buffer holding the command stream, along this buffer
userspace have to provide a list of buffer object used by the
command stream. The kernel radeon driver will then place buffer
in GPU accessible memory and will update command stream to reflect
the position of the different buffers.

The kernel will also perform security check on command stream
provided by the user, we want to catch and forbid any illegal use
of the GPU such as DMA into random system memory or into memory
not owned by the process supplying the command stream. This part
of the code is still incomplete and this why we propose that patch
as a staging driver addition, future security might forbid current
experimental userspace to run.

This code support the following hardware : R1XX,R2XX,R3XX,R4XX,R5XX
(radeon up to X1950). Works is underway to provide support for R6XX,
R7XX and newer hardware (radeon from HD2XXX to HD4XXX).

KMS

Currently, most graphics modes are initialized during the X server startup. Kernel Modesetting (referred to as KMS hereafter) moves this process from the X server's DDX drivers to the kernel, and it enables several new features including:
* Improved Graphical Boot
* Faster fast user switching
* Seamless X server switching
* Graphical panic messages

With KMS, output setup services move from being the job of the 2D X driver, to being the job of a kernel driver. Historically, the X server was responsible for saving output state when it started up, and then restoring it when it switched back to text mode. Fast user switching was accomplished with a VT switch, so switching away from the first user's X server would blink once to go to text mode, then immediately blink again to go to the second user's session.

With KMS, a simple default policy is loaded into the kernel for initial output setup, which means connected displays should go to their native resolution as early as possible. Graphical display services, like the X server and the new Plymouth boot manager, simply reuse the existing display settings if they match what is desired. This allows for a seamless transition between bootup and login screen. Fast user switching can also take advantage of this; if both users have the same screen resolutions, then there's no need to blink to transition, and even if they're different, the kernel can make the switch exactly once (instead of twice like before).

Finally, since the kernel is aware of which regions of video memory are being displayed, it can print panic messages to the display, which will assist with troubleshooting.

KMS - kernel command line options

nomodeset - this will disable the kernel modesetting feature and drop the user back to the old X.org infrastructure and behaviour (nomodeset works for all drivers)

radeon.agpmode=x where x is -1 = Enable PCI mode on the GPU, disable all AGP. 1, 2, 4, 8 = Enable AGP speed. (radeon.agpmode is Radeon specific)

В общем, если я не добавляю параметр nomodeset к ядру - у меня x server завешивает компьютер при старте в момент включения графического режима. А если параметр добавляю - то работает как с самого начала работало.

libdrm тоже надо пересобрать c --enable-experimental-radeon-api. Как и xf86-radeon (сам найдет ядро с KMS).

Мне не понятно, куда и как этот ключ вписывать в генте и не нашел примера в интернете.

начиная с версии libdrm
/usr/portage/x11-libs/libdrm/libdrm-2.4.13.ebuild
там внутри такой ключ и так есть:

CONFIGURE_OPTIONS="--enable-udev --enable-nouveau-experimental-api --enable-radeon-experimental-api"

Вопрос - почему не работает если не указывать nomodeset?

https://bugs.freedesktop.org/show_bug.cgi?id=25862