tc: make tc_bus_type const

Since commit d492cc2573a0 ("driver core: device.h: make struct
bus_type a const *"), the driver core can properly handle constant
struct bus_type, move the tc_bus_type variable to be a constant
structure as well, placing it into read-only memory which can not be
modified at runtime.

Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Suggested-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Ricardo B. Marliere <ricardo@marliere.net>
Acked-by: Maciej W. Rozycki <macro@orcam.me.uk>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>

TC: Set DMA masks for devices

Fix a TURBOchannel support regression with commit 205e1b7f51e4
("dma-mapping: warn when there is no coherent_dma_mask") that caused
coherent DMA allocations to produce a warning such as:

defxx: v1.11 2014/07/01 Lawrence V. Stefani and others
tc1: DEFTA at MMIO addr = 0x1e900000, IRQ = 20, Hardware addr = 08-00-2b-a3-a3-29
------------[ cut here ]------------
WARNING: CPU: 0 PID: 1 at ./include/linux/dma-mapping.h:516 dfx_dev_register+0x670/0x678
Modules linked in:
CPU: 0 PID: 1 Comm: swapper Not tainted 4.19.0-rc6 #2
Stack : ffffffff8009ffc0 fffffffffffffec0 0000000000000000 ffffffff80647650
0000000000000000 0000000000000000 ffffffff806f5f80 ffffffffffffffff
0000000000000000 0000000000000000 0000000000000001 ffffffff8065d4e8
98000000031b6300 ffffffff80563478 ffffffff805685b0 ffffffffffffffff
0000000000000000 ffffffff805d6720 0000000000000204 ffffffff80388df8
0000000000000000 0000000000000009 ffffffff8053efd0 ffffffff806657d0
0000000000000000 ffffffff803177f8 0000000000000000 ffffffff806d0000
9800000003078000 980000000307b9e0 000000001e900000 ffffffff80067940
0000000000000000 ffffffff805d6720 0000000000000204 ffffffff80388df8
ffffffff805176c0 ffffffff8004dc78 0000000000000000 ffffffff80067940
...
Call Trace:
[<ffffffff8004dc78>] show_stack+0xa0/0x130
[<ffffffff80067940>] __warn+0x128/0x170
---[ end trace b1d1e094f67f3bb2 ]---

This is because the TURBOchannel bus driver fails to set the coherent
DMA mask for devices enumerated.

Set the regular and coherent DMA masks for TURBOchannel devices then,
observing that the bus protocol supports a 34-bit (16GiB) DMA address
space, by interpreting the value presented in the address cycle across
the 32 `ad' lines as a 32-bit word rather than byte address[1]. The
architectural size of the TURBOchannel DMA address space exceeds the
maximum amount of RAM any actual TURBOchannel system in existence may
have, hence both masks are the same.

This removes the warning shown above.

References:

[1] "TURBOchannel Hardware Specification", EK-369AA-OD-007B, Digital
Equipment Corporation, January 1993, Section "DMA", pp. 1-15 -- 1-17

Signed-off-by: Maciej W. Rozycki <macro@linux-mips.org>
Signed-off-by: Paul Burton <paul.burton@mips.com>
Patchwork: https://patchwork.linux-mips.org/patch/20835/
Fixes: 205e1b7f51e4 ("dma-mapping: warn when there is no coherent_dma_mask")
Cc: stable@vger.kernel.org # 4.16+
Cc: Ralf Baechle <ralf@linux-mips.org>

[TC] MIPS: TURBOchannel update to the driver model

This is a set of changes to convert support for the TURBOchannel bus to the
driver model. It implements the usual set of calls similar to what other bus
drivers have: tc_register_driver(), tc_unregister_driver(), etc. All the
platform-specific bits have been removed and headers from asm-mips/dec/ have
been merged into linux/tc.h, which should be included by drivers.

Signed-off-by: Maciej W. Rozycki <macro@linux-mips.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>