Long radeon stalls on recent kernels

[Andy Lutomirski]

I have a Caicos card, like this:

[    3.077260] [drm] radeon kernel modesetting enabled.
[    3.077338] checking generic (e0000000 600000) vs hw (e0000000 10000000)
[    3.077339] fb: switching to radeondrmfb from EFI VGA
[    3.077377] Console: switching to colour dummy device 80x25
[    3.078881] [drm] initializing kernel modesetting (CAICOS
0x1002:0x6779 0x174B:0xE164).
[    3.078903] [drm] register mmio base: 0xF4A20000
[    3.078904] [drm] register mmio size: 131072
[    3.078982] ATOM BIOS: C26401
[    3.079572] radeon 0000:09:00.0: VRAM: 1024M 0x0000000000000000 -
0x000000003FFFFFFF (1024M used)
[    3.079574] radeon 0000:09:00.0: GTT: 1024M 0x0000000040000000 -
0x000000007FFFFFFF
[    3.079576] [drm] Detected VRAM RAM=1024M, BAR=256M
[    3.079577] [drm] RAM width 64bits DDR
[    3.079755] [TTM] Zone  kernel: Available graphics memory: 8186568 kiB
[    3.079757] [TTM] Zone   dma32: Available graphics memory: 2097152 kiB
[    3.079757] [TTM] Initializing pool allocator
[    3.079773] [TTM] Initializing DMA pool allocator
[    3.080011] [drm] radeon: 1024M of VRAM memory ready
[    3.080012] [drm] radeon: 1024M of GTT memory ready.
[    3.080049] [drm] Loading CAICOS Microcode
[    3.080330] [drm] Internal thermal controller without fan control
[    3.081425] [drm] radeon: power management initialized
[    3.081551] [drm] GART: num cpu pages 262144, num gpu pages 262144
[    3.082589] [drm] enabling PCIE gen 2 link speeds, disable with
radeon.pcie_gen2=0
[    3.085030] [drm] PCIE GART of 1024M enabled (table at 0x0000000000274000).
[    3.085221] radeon 0000:09:00.0: WB enabled
[    3.085224] radeon 0000:09:00.0: fence driver on ring 0 use gpu
addr 0x0000000040000c00 and cpu addr 0xffff88043d914c00
[    3.085225] radeon 0000:09:00.0: fence driver on ring 3 use gpu
addr 0x0000000040000c0c and cpu addr 0xffff88043d914c0c
[    3.097438] radeon 0000:09:00.0: fence driver on ring 5 use gpu
addr 0x0000000000072118 and cpu addr 0xffffc900128b2118
[    3.097441] [drm] Supports vblank timestamp caching Rev 2 (21.10.2013).
[    3.097442] [drm] Driver supports precise vblank timestamp query.
[    3.097514] radeon 0000:09:00.0: irq 56 for MSI/MSI-X
[    3.097544] radeon 0000:09:00.0: radeon: using MSI.
[    3.097614] [drm] radeon: irq initialized.

On recent kernels (3.16 through 3.18-rc4, perhaps), doing anything
graphics intensive seems to cause my system to become unusable for
tens of seconds.  Pointing Firefox at Google Maps is a big offender --
it can take several minutes for me to move my mouse far enough to
close the tab and get my computer back.

On bootup, I get this warning:
[drm:btc_dpm_set_power_state] *ERROR*
rv770_restrict_performance_levels_before_switch failed

Setting radeon.dpm=0 seems to work around this problem at the cost of
giving my rather slow graphics.

Are there known issues here?

Thanks,
Andy