Then to help you we need the output of this command, typed as root:As it's not really this thread's topic it would have been better to open a new thread for your issue, but let's continue with this one now.

Ok, I don't have it installed right now, using Arch Linux right now, but I'll install
it again and type that command and let you know. Btw just to see if my cards work with FreeBSD and they do, seems they should work with slackware as it claims it's bsd unix-like but hey, there is differences between unix and linux albeit not big differences.

You can type that command from Arch Linux, you should get exactly the same results. I assume that you have the same display issue in Arch, right?

I have no display issues with arch, everything installs just fine, anyway I'm just in the process of installing slackware right now, will type that command at the virtual terminal when it boots since it hangs at the a actual terminal.

Then comparing the results from Slackware vs Arch will help, as that can help us to know what goes wrong with Slackware and possibly use in it the same settings and drivers as those that work well with Arch.

So please provide output of the command in both cases, and tell us the kernel parameters included in the boot command line in both cases as well.

Too late for Arch, already deleted it, this also installs fine on ubuntu, here is the output from that:

01:00.0 VGA compatible controller [0300]: Advanced Micro Devices, Inc. [AMD/ATI] Curacao PRO [Radeon R9 270] [1002:6811]
Subsystem: ASUSTeK Computer Inc. Device [1043:048d]
Kernel driver in use: radeon
01:00.1 Audio device [0403]: Advanced Micro Devices, Inc. [AMD/ATI] Cape Verde/Pitcairn HDMI Audio [Radeon HD 7700/7800 Series] [1002:aab0]
--
02:00.0 VGA compatible controller [0300]: Advanced Micro Devices, Inc. [AMD/ATI] Curacao PRO [Radeon R9 270] [1002:6811]
Subsystem: ASUSTeK Computer Inc. Device [1043:048d]
Kernel driver in use: radeon
02:00.1 Audio device [0403]: Advanced Micro Devices, Inc. [AMD/ATI] Cape Verde/Pitcairn HDMI Audio [Radeon HD 7700/7800 Series] [1002:aab0]

Ok here is my output from slackware: root@slackware:~# lspci -knn|grep -iA3 VGA
01:00.0 VGA compatible controller [0300]: AMD/ATI [Advanced Micro Devices, Inc.] Pitcairn [1002:6811]
Subsystem: ASUSTeK Computer Inc. Device [1043:048d]
Kernel driver in use: radeon
Kernel modules: radeon
--
02:00.0 VGA compatible controller [0300]: AMD/ATI [Advanced Micro Devices, Inc.] Pitcairn [1002:6811]
Subsystem: ASUSTeK Computer Inc. Device [1043:048d]
Kernel driver in use: radeon
Kernel modules: radeon

Also if this pertains, I'm connected to a 46 inch Samsung TV at 1920x1080p.

There are no extra kernel parameters included in the boot command line.

Are you sure? Please you provide the GRUB command line from Ubuntu (in grub.conf, I guess)

Also I checked that in Slackware 14.1 I see in /usr/share/hwdata/pci.ids "Pitcairn" for your vendor:device id [1002:6811] with a list of PCI IDs dated 2013-04-19 14:54:09 (as expected).

But after having updated the list with "update-pciids" (then dated 2015-06-21 03:15:01) for the same PCI ID I get "Curacao PRO [Radeon R7 370 / R9 270/370 OEM]" which is similar to your output on Ubuntu.

So _maybe_ some more recent software in case of Ubuntu (even more probable in case of Arch, of course) can explain the difference. Could you tell us the kernel version of your Ubuntu system, please? and confirm what is the kernel version in your Slackware?

I am also slightly puzzled not to see a "kernel modules" line in case of Ubuntu. Can really the radeon driver be built-in the kernel?

Ubuntu kernel version initrd.img-3.19.0-21-generic

Ubuntu grub.cfg consists of this:

#
# DO NOT EDIT THIS FILE
#
# It is automatically generated by grub-mkconfig using templates
# from /etc/grub.d and settings from /etc/default/grub
#

### BEGIN /etc/grub.d/00_header ###
if [ -s $prefix/grubenv ]; then
set have_grubenv=true
load_env
fi
if [ "${next_entry}" ] ; then
set default="${next_entry}"
set next_entry=
save_env next_entry
set boot_once=true
else
set default="0"
fi

if [ x"${feature_menuentry_id}" = xy ]; then
menuentry_id_option="--id"
else
menuentry_id_option=""
fi

export menuentry_id_option

if [ "${prev_saved_entry}" ]; then
set saved_entry="${prev_saved_entry}"
save_env saved_entry
set prev_saved_entry=
save_env prev_saved_entry
set boot_once=true
fi

function savedefault {
if [ -z "${boot_once}" ]; then
saved_entry="${chosen}"
save_env saved_entry
fi
}
function recordfail {
set recordfail=1
if [ -n "${have_grubenv}" ]; then if [ -z "${boot_once}" ]; then save_env recordfail; fi; fi
}
function load_video {
if [ x$feature_all_video_module = xy ]; then
insmod all_video
else
insmod efi_gop
insmod efi_uga
insmod ieee1275_fb
insmod vbe
insmod vga
insmod video_bochs
insmod video_cirrus
fi
}

if [ x$feature_default_font_path = xy ] ; then
font=unicode
else
insmod part_msdos
insmod ext2
set root='hd2,msdos2'
if [ x$feature_platform_search_hint = xy ]; then
search --no-floppy --fs-uuid --set=root --hint-bios=hd2,msdos2 --hint-efi=hd2,msdos2 --hint-baremetal=ahci2,msdos2 1de7763e-4845-46fb-a8b8-bc66c54d7bc6
else
search --no-floppy --fs-uuid --set=root 1de7763e-4845-46fb-a8b8-bc66c54d7bc6
fi
font="/usr/share/grub/unicode.pf2"
fi

if loadfont $font ; then
set gfxmode=auto
load_video
insmod gfxterm
set locale_dir=$prefix/locale
set lang=en_CA
insmod gettext
fi
terminal_output gfxterm
if [ "${recordfail}" = 1 ] ; then
set timeout=-1
else
if [ x$feature_timeout_style = xy ] ; then
set timeout_style=menu
set timeout=10
# Fallback normal timeout code in case the timeout_style feature is
# unavailable.
else
set timeout=10
fi
fi
### END /etc/grub.d/00_header ###

### BEGIN /etc/grub.d/05_debian_theme ###
set menu_color_normal=white/black
set menu_color_highlight=black/light-gray
if background_color 45,51,53; then
clear
fi
### END /etc/grub.d/05_debian_theme ###

### BEGIN /etc/grub.d/10_linux ###
function gfxmode {
set gfxpayload="${1}"
if [ "${1}" = "keep" ]; then
set vt_handoff=vt.handoff=7
else
set vt_handoff=
fi
}
if [ "${recordfail}" != 1 ]; then
if [ -e ${prefix}/gfxblacklist.txt ]; then
if hwmatch ${prefix}/gfxblacklist.txt 3; then
if [ ${match} = 0 ]; then
set linux_gfx_mode=keep
else
set linux_gfx_mode=text
fi
else
set linux_gfx_mode=text
fi
else
set linux_gfx_mode=keep
fi
else
set linux_gfx_mode=text
fi
export linux_gfx_mode
menuentry 'Ubuntu' --class ubuntu --class gnu-linux --class gnu --class os $menuentry_id_option 'gnulinux-simple-1de7763e-4845-46fb-a8b8-bc66c54d7bc6' {
recordfail
load_video
gfxmode $linux_gfx_mode
insmod gzio
if [ x$grub_platform = xxen ]; then insmod xzio; insmod lzopio; fi
insmod part_msdos
insmod ext2
set root='hd2,msdos2'
if [ x$feature_platform_search_hint = xy ]; then
search --no-floppy --fs-uuid --set=root --hint-bios=hd2,msdos2 --hint-efi=hd2,msdos2 --hint-baremetal=ahci2,msdos2 1de7763e-4845-46fb-a8b8-bc66c54d7bc6
else
search --no-floppy --fs-uuid --set=root 1de7763e-4845-46fb-a8b8-bc66c54d7bc6
fi
linux /boot/vmlinuz-3.19.0-21-generic root=UUID=1de7763e-4845-46fb-a8b8-bc66c54d7bc6 ro quiet splash $vt_handoff
initrd /boot/initrd.img-3.19.0-21-generic
}
submenu 'Advanced options for Ubuntu' $menuentry_id_option 'gnulinux-advanced-1de7763e-4845-46fb-a8b8-bc66c54d7bc6' {
menuentry 'Ubuntu, with Linux 3.19.0-21-generic' --class ubuntu --class gnu-linux --class gnu --class os $menuentry_id_option 'gnulinux-3.19.0-21-generic-advanced-1de7763e-4845-46fb-a8b8-bc66c54d7bc6' {
recordfail
load_video
gfxmode $linux_gfx_mode
insmod gzio
if [ x$grub_platform = xxen ]; then insmod xzio; insmod lzopio; fi
insmod part_msdos
insmod ext2
set root='hd2,msdos2'
if [ x$feature_platform_search_hint = xy ]; then
search --no-floppy --fs-uuid --set=root --hint-bios=hd2,msdos2 --hint-efi=hd2,msdos2 --hint-baremetal=ahci2,msdos2 1de7763e-4845-46fb-a8b8-bc66c54d7bc6
else
search --no-floppy --fs-uuid --set=root 1de7763e-4845-46fb-a8b8-bc66c54d7bc6
fi
echo 'Loading Linux 3.19.0-21-generic ...'
linux /boot/vmlinuz-3.19.0-21-generic root=UUID=1de7763e-4845-46fb-a8b8-bc66c54d7bc6 ro quiet splash $vt_handoff
echo 'Loading initial ramdisk ...'
initrd /boot/initrd.img-3.19.0-21-generic
}
menuentry 'Ubuntu, with Linux 3.19.0-21-generic (recovery mode)' --class ubuntu --class gnu-linux --class gnu --class os $menuentry_id_option 'gnulinux-3.19.0-21-generic-recovery-1de7763e-4845-46fb-a8b8-bc66c54d7bc6' {
recordfail
load_video
insmod gzio
if [ x$grub_platform = xxen ]; then insmod xzio; insmod lzopio; fi
insmod part_msdos
insmod ext2
set root='hd2,msdos2'
if [ x$feature_platform_search_hint = xy ]; then
search --no-floppy --fs-uuid --set=root --hint-bios=hd2,msdos2 --hint-efi=hd2,msdos2 --hint-baremetal=ahci2,msdos2 1de7763e-4845-46fb-a8b8-bc66c54d7bc6
else
search --no-floppy --fs-uuid --set=root 1de7763e-4845-46fb-a8b8-bc66c54d7bc6
fi
echo 'Loading Linux 3.19.0-21-generic ...'
linux /boot/vmlinuz-3.19.0-21-generic root=UUID=1de7763e-4845-46fb-a8b8-bc66c54d7bc6 ro recovery nomodeset
echo 'Loading initial ramdisk ...'
initrd /boot/initrd.img-3.19.0-21-generic
}
menuentry 'Ubuntu, with Linux 3.19.0-15-generic' --class ubuntu --class gnu-linux --class gnu --class os $menuentry_id_option 'gnulinux-3.19.0-15-generic-advanced-1de7763e-4845-46fb-a8b8-bc66c54d7bc6' {
recordfail
load_video
gfxmode $linux_gfx_mode
insmod gzio
if [ x$grub_platform = xxen ]; then insmod xzio; insmod lzopio; fi
insmod part_msdos
insmod ext2
set root='hd2,msdos2'
if [ x$feature_platform_search_hint = xy ]; then
search --no-floppy --fs-uuid --set=root --hint-bios=hd2,msdos2 --hint-efi=hd2,msdos2 --hint-baremetal=ahci2,msdos2 1de7763e-4845-46fb-a8b8-bc66c54d7bc6
else
search --no-floppy --fs-uuid --set=root 1de7763e-4845-46fb-a8b8-bc66c54d7bc6
fi
echo 'Loading Linux 3.19.0-15-generic ...'
linux /boot/vmlinuz-3.19.0-15-generic root=UUID=1de7763e-4845-46fb-a8b8-bc66c54d7bc6 ro quiet splash $vt_handoff
echo 'Loading initial ramdisk ...'
initrd /boot/initrd.img-3.19.0-15-generic
}
menuentry 'Ubuntu, with Linux 3.19.0-15-generic (recovery mode)' --class ubuntu --class gnu-linux --class gnu --class os $menuentry_id_option 'gnulinux-3.19.0-15-generic-recovery-1de7763e-4845-46fb-a8b8-bc66c54d7bc6' {
recordfail
load_video
insmod gzio
if [ x$grub_platform = xxen ]; then insmod xzio; insmod lzopio; fi
insmod part_msdos
insmod ext2
set root='hd2,msdos2'
if [ x$feature_platform_search_hint = xy ]; then
search --no-floppy --fs-uuid --set=root --hint-bios=hd2,msdos2 --hint-efi=hd2,msdos2 --hint-baremetal=ahci2,msdos2 1de7763e-4845-46fb-a8b8-bc66c54d7bc6
else
search --no-floppy --fs-uuid --set=root 1de7763e-4845-46fb-a8b8-bc66c54d7bc6
fi
echo 'Loading Linux 3.19.0-15-generic ...'
linux /boot/vmlinuz-3.19.0-15-generic root=UUID=1de7763e-4845-46fb-a8b8-bc66c54d7bc6 ro recovery nomodeset
echo 'Loading initial ramdisk ...'
initrd /boot/initrd.img-3.19.0-15-generic
}
}

### END /etc/grub.d/10_linux ###

### BEGIN /etc/grub.d/20_linux_xen ###

### END /etc/grub.d/20_linux_xen ###

### BEGIN /etc/grub.d/20_memtest86+ ###
menuentry 'Memory test (memtest86+)' {
insmod part_msdos
insmod ext2
set root='hd2,msdos2'
if [ x$feature_platform_search_hint = xy ]; then
search --no-floppy --fs-uuid --set=root --hint-bios=hd2,msdos2 --hint-efi=hd2,msdos2 --hint-baremetal=ahci2,msdos2 1de7763e-4845-46fb-a8b8-bc66c54d7bc6
else
search --no-floppy --fs-uuid --set=root 1de7763e-4845-46fb-a8b8-bc66c54d7bc6
fi
knetbsd /boot/memtest86+.elf
}
menuentry 'Memory test (memtest86+, serial console 115200)' {
insmod part_msdos
insmod ext2
set root='hd2,msdos2'
if [ x$feature_platform_search_hint = xy ]; then
search --no-floppy --fs-uuid --set=root --hint-bios=hd2,msdos2 --hint-efi=hd2,msdos2 --hint-baremetal=ahci2,msdos2 1de7763e-4845-46fb-a8b8-bc66c54d7bc6
else
search --no-floppy --fs-uuid --set=root 1de7763e-4845-46fb-a8b8-bc66c54d7bc6
fi
linux16 /boot/memtest86+.bin console=ttyS0,115200n8
}
### END /etc/grub.d/20_memtest86+ ###

### BEGIN /etc/grub.d/30_os-prober ###
menuentry 'Windows Recovery Environment (loader) (on /dev/sdd1)' --class windows --class os $menuentry_id_option 'osprober-chain-62DC1B16DC1AE3D7' {
insmod part_msdos
insmod ntfs
set root='hd3,msdos1'
if [ x$feature_platform_search_hint = xy ]; then
search --no-floppy --fs-uuid --set=root --hint-bios=hd3,msdos1 --hint-efi=hd3,msdos1 --hint-baremetal=ahci3,msdos1 62DC1B16DC1AE3D7
else
search --no-floppy --fs-uuid --set=root 62DC1B16DC1AE3D7
fi
parttool ${root} hidden-
drivemap -s (hd0) ${root}
chainloader +1
}
set timeout_style=menu
if [ "${timeout}" = 0 ]; then
set timeout=10
fi
### END /etc/grub.d/30_os-prober ###

### BEGIN /etc/grub.d/30_uefi-firmware ###
### END /etc/grub.d/30_uefi-firmware ###

### BEGIN /etc/grub.d/40_custom ###
# This file provides an easy way to add custom menu entries. Simply type the
# menu entries you want to add after this comment. Be careful not to change
# the 'exec tail' line above.
### END /etc/grub.d/40_custom ###

### BEGIN /etc/grub.d/41_custom ###
if [ -f ${config_directory}/custom.cfg ]; then
source ${config_directory}/custom.cfg
elif [ -z "${config_directory}" -a -f $prefix/custom.cfg ]; then
source $prefix/custom.cfg;
fi
### END /etc/grub.d/41_custom ###

Hmm... So yes the kernel you use in Ubuntu is way more recent than the one shipped in Slackware 14.1.

Kind of a shot in the dark: you could try to upgrade your kernel to 4.0.5 using the kernel source from kernel.org and one of the config files provided here. But that's only assuming that you are familiar with building a Linux kernel from source and I certainly can't guarantee that this will solve your issue.

Oops sorry forgot to give you slackware version, its 3.18.11 14.1-current

Then I will just ask: did you try with Slackware 14.1 (stable)?

Your link "building a Linux kernel from source" is bare, says "This topic does not exist yet". Would it be the same way as building from "linux from scratch", cause I've done that before?

---------- Post added 06-21-15 at 06:45 PM ----------

No but I can, ok will try.