Synopsis:

How to address an intermittent problem observed on some GK104-based products.

Description:

A diagnostic setting was inadvertently left enabled in the production VBIOS
builds on some NVIDIA GK104-based graphics cards.  This setting can result in a
red screen being displayed in some rare cases, for example in response to
certain mode-set operations.

The setting in question is used as part of the product engineering process to
make display engine underflows more easily human-detectable.  This is a scenario
in which the bandwidth provided by the GPU's video memory interface, in the
current performance state (at relevant clock frequencies defined by this state),
is insufficient to satisfy the current isochronous streaming requirements of the
display engine.

The NVIDIA Accelerated Linux Graphics Driver disables this setting when it
initializes the GPU.  However, it is NVIDIA's suggestion that independently
developed drivers also disable it.

Workaround:

The underflow detection setting can be disabled by means of the per-head
register NV_PDISP_RG_UNDERFLOW(i), accessible via the GPUs' BAR0 (where 'i' is
the head index):

  NV_PDISP_RG_UNDERFLOW(i) (0x00616308 + (i) * 2048) 

Since the affected GPUs implement four heads, four such registers are
available. To disable the underflow detection, the registers need to be
programmed with the value 0. 

Scope:

The problem described above affects GK104-based products with VBIOS versions
below 80.04.63.00.00.  This includes (but may not be limited to) the following
products:

  PCI ID  Product Name
  ------- -----------------
  0x11a0  GeForce GTX 680M
  0x11a1  GeForce GTX 670MX
  0x11a2  GeForce GTX 675MX
  0x11a3  GeForce GTX 680MX
  0x11bc  Quadro K5000M
  0x11bd  Quadro K4000M
  0x11be  Quadro K3000M

Register Description:

This register is used for detecting underflows and also determines what action
the hw should take when an underflow occurs.

ENABLE Turns on underflow reporting. Note that this has no effect on the
underflow mode. The underflow mode (what to do when an underflow occurs, REPEAT
or RED, is always active).

UNDERFLOWED Indicates whether an underflow has occurred. This is a "sticky" bit.
To clear it, software writes a '1' to this bit.  UNDERFLOWED is active
only when the above ENABLE bit is set.

UNDERFLOW_MODE Indicates what to do when an underflow occurs. If REPEAT
is set, then the raster generator will repeat the last pixel, when a new
pixel is not available. If RED is set, the raster generator will display
bright red for any missing pixels.

 31            24 23           16 15            8 7             0
.-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-. 
|                             |0 0 0 0 0 0 0|                   | RG_UNDERFLOW
`-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-'

#define NV_PDISP_RG_UNDERFLOW(i)             (0x00616308+(i)*2048) /* RW-4A */
#define NV_PDISP_RG_UNDERFLOW__SIZE_1                            4 /*       */
#define NV_PDISP_RG_UNDERFLOW_ENABLE                           0:0 /* RWIVF */
#define NV_PDISP_RG_UNDERFLOW_ENABLE_INIT               0x00000000 /* RWI-V */
#define NV_PDISP_RG_UNDERFLOW_ENABLE_ENABLE             0x00000001 /* RW--V */
#define NV_PDISP_RG_UNDERFLOW_ENABLE_DISABLE            0x00000000 /* RW--V */
#define NV_PDISP_RG_UNDERFLOW_UNDERFLOWED                      4:4 /* RWIVF */
#define NV_PDISP_RG_UNDERFLOW_UNDERFLOWED_INIT          0x00000000 /* RWI-V */
#define NV_PDISP_RG_UNDERFLOW_UNDERFLOWED_CLR           0x00000001 /* -W--V */
#define NV_PDISP_RG_UNDERFLOW_UNDERFLOWED_YES           0x00000001 /* R---V */
#define NV_PDISP_RG_UNDERFLOW_UNDERFLOWED_NO            0x00000000 /* R---V */
#define NV_PDISP_RG_UNDERFLOW_MODE                             8:8 /* RWIVF */
#define NV_PDISP_RG_UNDERFLOW_MODE_INIT                 0x00000000 /* RWI-V */
#define NV_PDISP_RG_UNDERFLOW_MODE_REPEAT               0x00000000 /* RW--V */
#define NV_PDISP_RG_UNDERFLOW_MODE_RED                  0x00000001 /* RW--V */