This information is probably suitable for the wiki.
We have implemented a number of configuration options in a file "config.txt" on boot partition (so /boot/config.txt from linux). These are read and acted on by GPU before the ARM boots. Some of these may become redundant in the future if we can control the options from kernel drivers.
The file is optional, and most users won't even need to create it. For a few hackers there may be some useful settings in here.
The format is "property=value". value is an integer. One option per line. # is comment character. E.g.
arm_freq : frequency of ARM in MHz. Default 700.
gpu_freq : Sets core_freq, h264_freq, isp_freq, v3d_freq together.
core_freq : frequency of GPU processor core in MHz. Default 250.
h264_freq: frequency of hardware video block in MHz. Default 250.
isp_freq: frequency of image sensor pipeline block in MHz. Default 250.
v3d_freq: frequency of 3D block in MHz. Default 250.
sdram_freq: frequency of SDRAM in MHz. Default 400.
over_voltage: ARM/GPU core voltage adjust. [-16,8] equates to [0.8V,1.4V]. Default 0 (1.2V)
over_voltage_sdram: Sets over_voltage_sdram_c, over_voltage_sdram_i, over_voltage_sdram_p together
over_voltage_sdram_c: SDRAM controller voltage adjust. [-16,8] equates to [0.8V,1.4V]. Default 0 (1.2V)
over_voltage_sdram_i: SDRAM I/O voltage adjust. [-16,8] equates to [0.8V,1.4V]. Default 0 (1.2V)
over_voltage_sdram_p: SDRAM phy voltage adjust. [-16,8] equates to [0.8V,1.4V]. Default 0 (1.2V)
sdtv_mode: composite tv mode. Default is 0 (NTSC)
sdtv_aspect: composite aspect ratio. Default is 1 (4:3)
hdmi_mode: hdmi mode. Default is negotiated with display.
overscan_left: number of pixels to skip on left
overscan_right: number of pixels to skip on right
overscan_top: number of pixels to skip on top
overscan_bottom: number of pixels to skip on bottom
framebuffer_width: console framebuffer width in pixels. Default matches display.
framebuffer_height: console framebuffer height in pixels. Default matches display.
test_mode: enable test sound/image during boot for manufacturing test.
enable_l2cache: enable arm access to GPU's L2 cache. Needs corresponding L2 enabled kernel. Default 0.
SDTV_MODE_NTSC = 0, /**<Normal NTSC */
SDTV_MODE_NTSC_J = 1, /**<Japanese version of NTSC - no pedestal.*/
SDTV_MODE_PAL = 2, /**<Normal PAL */
SDTV_MODE_PAL_M = 3, /**<Brazilian version of PAL - 525/60 rather than 625/50, different subcarrier */
SDTV_ASPECT_4_3 = 1, /**<4:3 */
SDTV_ASPECT_14_9 = 2, /**<14:9 */
SDTV_ASPECT_16_9 = 3 /**<16:9 */
HDMI_CEA_VGA = 1,
HDMI_CEA_480p60 = 2,
HDMI_CEA_480p60H = 3,
HDMI_CEA_720p60 = 4,
HDMI_CEA_1080i60 = 5,
HDMI_CEA_480i60 = 6,
HDMI_CEA_480i60H = 7,
HDMI_CEA_240p60 = 8,
HDMI_CEA_240p60H = 9,
HDMI_CEA_480i60_4x = 10,
HDMI_CEA_480i60_4xH = 11,
HDMI_CEA_240p60_4x = 12,
HDMI_CEA_240p60_4xH = 13,
HDMI_CEA_480p60_2x = 14,
HDMI_CEA_480p60_2xH = 15,
HDMI_CEA_1080p60 = 16,
HDMI_CEA_576p50 = 17,
HDMI_CEA_576p50H = 18,
HDMI_CEA_720p50 = 19,
HDMI_CEA_1080i50 = 20,
HDMI_CEA_576i50 = 21,
HDMI_CEA_576i50H = 22,
HDMI_CEA_288p50 = 23,
HDMI_CEA_288p50H = 24,
HDMI_CEA_576i50_4x = 25,
HDMI_CEA_576i50_4xH = 26,
HDMI_CEA_288p50_4x = 27,
HDMI_CEA_288p50_4xH = 28,
HDMI_CEA_576p50_2x = 29,
HDMI_CEA_576p50_2xH = 30,
HDMI_CEA_1080p50 = 31,
HDMI_CEA_1080p24 = 32,
HDMI_CEA_1080p25 = 33,
HDMI_CEA_1080p30 = 34,
HDMI_CEA_480p60_4x = 35,
HDMI_CEA_480p60_4xH = 36,
HDMI_CEA_576p50_4x = 37,
HDMI_CEA_576p50_4xH = 38,
HDMI_CEA_1080i50_rb = 39,
HDMI_CEA_1080i100 = 40,
HDMI_CEA_720p100 = 41,
HDMI_CEA_576p100 = 42,
HDMI_CEA_576p100H = 43,
HDMI_CEA_576i100 = 44,
HDMI_CEA_576i100H = 45,
HDMI_CEA_1080i120 = 46,
HDMI_CEA_720p120 = 47,
HDMI_CEA_480p120 = 48,
HDMI_CEA_480p120H = 49,
HDMI_CEA_480i120 = 50,
HDMI_CEA_480i120H = 51,
HDMI_CEA_576p200 = 52,
HDMI_CEA_576p200H = 53,
HDMI_CEA_576i200 = 54,
HDMI_CEA_576i200H = 55,
HDMI_CEA_480p240 = 56,
HDMI_CEA_480p240H = 57,
HDMI_CEA_480i240 = 58,
HDMI_CEA_480i240H = 59,