The Broadcom SiByte BCM1250, BCM1125H and BCM1125 SOCs have an onchip
32-bit PCI host bridge, and the two former SOCs also have an onchip HT
host bridge.  The HT host bridge, where present, appears in the PCI
configuration space as if it was a device on the 32-bit PCI bus behind
the PCI host bridge, however at the hardware level its signals are
routed separately, so these two devices are actually peer host bridges.

As documented[1] and observed in reality the 32-bit PCI host bridge does
not support 64-bit addressing as it does not support the Dual Address
Cycle (DAC) PCI command, and naturally, being 32-bit only, it has no
means to carry the high 32 address bits otherwise.  However the DRAM
controller also included in the SOC supports memory amounts of up to
16GiB, and due to how the address decoder has been wired in the SOC any
memory beyond 1GiB is actually mapped starting from 4GiB physical up,
that is beyond the 32-bit addressable limit.  Consequently if the
maximum amount of memory has been installed, then it will span up to
19GiB.

Contrariwise, the HT host bridge does support full 40-bit addressing
defined by the HyperTransport (formerly LDT) specification the bridge
adheres to, depending on the peripherals revision of the SOC[2] either
revision 0.17[3] or revision 1.03[4].  This allows addressing any and
all memory installed, and well beyond.

Set the bus mask then to limit DMA addressing to 32 bits for all the
devices down the 32-bit PCI host bridge, excluding however any devices
that are down the HT host bridge.

References:

[1] "BCM1250/BCM1125/BCM1125H User Manual", Revision 1250_1125-UM100-R,
    Broadcom Corporation, 21 Oct 2002, Section 8: "PCI Bus and
    HyperTransport Fabric", "Introduction", p. 190

[2] same, Table 140: "HyperTransport Configuration Header (Type 1)", p.
    245

[3] "Lightning Data Transport IO Specification", Revision 0.17, Advanced
    Micro Devices, 21 Jan 2000, Section 3.2.1.2 "Command Packet", p. 8

[4] "HyperTransport I/O Link Specification", Revision 1.03,
    HyperTransport Technology Consortium, 10 Oct 2001, Section 3.2.1.2
    "Request Packet", pp. 27-28
Signed-off-by: Maciej W. Rozycki <macro@linux-mips.org>
Signed-off-by: Paul Burton <paul.burton@mips.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Patchwork: https://patchwork.linux-mips.org/patch/21106/
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org