Some chips appear to have the 2D engine hang during screen redraw,
typically in a sequence of copyarea operations. This appear to be
solved by adding a flush of the engine destination pixel cache
and waiting for the engine to be idle before issuing the accel
operation. The performance impact seems to be fairly small.

Here is a trace on an RV370 (PCI device ID 0x5b64), it records the
RBBM_STATUS register, then the source x/y, destination x/y, and
width/height used for the copy:

----------------------------------------
radeonfb_prim_copyarea: STATUS[00000140] src[210:70] dst[210:60] wh[a0:10]
radeonfb_prim_copyarea: STATUS[00000140] src[2b8:70] dst[2b8:60] wh[88:10]
radeonfb_prim_copyarea: STATUS[00000140] src[348:70] dst[348:60] wh[40:10]
radeonfb_prim_copyarea: STATUS[80020140] src[390:70] dst[390:60] wh[88:10]
radeonfb_prim_copyarea: STATUS[8002613f] src[40:80] dst[40:70] wh[28:10]
radeonfb_prim_copyarea: STATUS[80026139] src[a8:80] dst[a8:70] wh[38:10]
radeonfb_prim_copyarea: STATUS[80026133] src[e8:80] dst[e8:70] wh[80:10]
radeonfb_prim_copyarea: STATUS[8002612d] src[170:80] dst[170:70] wh[30:10]
radeonfb_prim_copyarea: STATUS[80026127] src[1a8:80] dst[1a8:70] wh[8:10]
radeonfb_prim_copyarea: STATUS[80026121] src[1b8:80] dst[1b8:70] wh[88:10]
radeonfb_prim_copyarea: STATUS[8002611b] src[248:80] dst[248:70] wh[68:10]
----------------------------------------

When things are going fine the copies complete before the next ROP is
even issued, but all of a sudden the 2D unit becomes active (bit 17 in
RBBM_STATUS) and the FIFO retry (bit 13) and FIFO pipeline busy (bit
14) are set as well.  The FIFO begins to backup until it becomes full.

What happens next is the radeon_fifo_wait() times out, and we access
the chip illegally leading to a bus error which usually wedges the
box.  None of this makes it to the console screen, of course :-)
radeon_fifo_wait() should be modified to reset the accelerator when
this timeout happens instead of programming the chip anyways.

----------------------------------------
radeonfb: FIFO Timeout !
ERROR(0): Cheetah error trap taken afsr[0010080005000000] afar[000007f900800e40] TL1(0)
ERROR(0): TPC[595114] TNPC[595118] O7[459788] TSTATE[11009601]
ERROR(0): TPC<radeonfb_copyarea+0xfc/0x248>
ERROR(0): M_SYND(0),  E_SYND(0), Privileged
ERROR(0): Highest priority error (0000080000000000) "Bus error response from system bus"
ERROR(0): D-cache idx[0] tag[0000000000000000] utag[0000000000000000] stag[0000000000000000]
ERROR(0): D-cache data0[0000000000000000] data1[0000000000000000] data2[0000000000000000] data3[0000000000000000]
ERROR(0): I-cache idx[0] tag[0000000000000000] utag[0000000000000000] stag[0000000000000000] u[0000000000000000] l[00\

ERROR(0): I-cache INSN0[0000000000000000] INSN1[0000000000000000] INSN2[0000000000000000] INSN3[0000000000000000]
ERROR(0): I-cache INSN4[0000000000000000] INSN5[0000000000000000] INSN6[0000000000000000] INSN7[0000000000000000]
ERROR(0): E-cache idx[800e40] tag[000000000e049f4c]
ERROR(0): E-cache data0[fffff8127d300180] data1[00000000004b5384] data2[0000000000000000] data3[0000000000000000]
Ker:xnel panic - not syncing: Irrecoverable deferred error trap.
----------------------------------------

Another quirk is that these copyarea calls will not happen until the
first drivers/char/vt.c:redraw_screen() occurs.  This will only happen
if you 1) VC switch or 2) run "consolechars" or 3) unblank the screen.

This seems to happen because until a redraw_screen() the screen scrolling
method used by fbcon is not finalized yet.  I've seen this with other fb
drivers too.

So if all you do is boot straight into X you will never see this bug on
the relevant chips.
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: <stable@kernel.org>		[2.6.25.x, 2.6.26.x]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>