Root/Documentation/frv/features.txt

1             ===========================
2             FUJITSU FR-V LINUX FEATURES
3             ===========================
4
5This kernel port has a number of features of which the user should be aware:
6
7 (*) Linux and uClinux
8
9     The FR-V architecture port supports both normal MMU linux and uClinux out
10     of the same sources.
11
12
13 (*) CPU support
14
15     Support for the FR401, FR403, FR405, FR451 and FR555 CPUs should work with
16     the same uClinux kernel configuration.
17
18     In normal (MMU) Linux mode, only the FR451 CPU will work as that is the
19     only one with a suitably featured CPU.
20
21     The kernel is written and compiled with the assumption that only the
22     bottom 32 GR registers and no FR registers will be used by the kernel
23     itself, however all extra userspace registers will be saved on context
24     switch. Note that since most CPUs can't support lazy switching, no attempt
25     is made to do lazy register saving where that would be possible (FR555
26     only currently).
27
28
29 (*) Board support
30
31     The board on which the kernel will run can be configured on the "Processor
32     type and features" configuration tab.
33
34     Set the System to "MB93093-PDK" to boot from the MB93093 (FR403) PDK.
35
36     Set the System to "MB93091-VDK" to boot from the CB11, CB30, CB41, CB60,
37     CB70 or CB451 VDK boards. Set the Motherboard setting to "MB93090-MB00" to
38     boot with the standard ATA90590B VDK motherboard, and set it to "None" to
39     boot without any motherboard.
40
41
42 (*) Binary Formats
43
44     The only userspace binary format supported is FDPIC ELF. Normal ELF, FLAT
45     and AOUT binaries are not supported for this architecture.
46
47     FDPIC ELF supports shared library and program interpreter facilities.
48
49
50 (*) Scheduler Speed
51
52     The kernel scheduler runs at 100Hz irrespective of the clock speed on this
53     architecture. This value is set in asm/param.h (see the HZ macro defined
54     there).
55
56
57 (*) Normal (MMU) Linux Memory Layout.
58
59     See mmu-layout.txt in this directory for a description of the normal linux
60     memory layout
61
62     See include/asm-frv/mem-layout.h for constants pertaining to the memory
63     layout.
64
65     See include/asm-frv/mb-regs.h for the constants pertaining to the I/O bus
66     controller configuration.
67
68
69 (*) uClinux Memory Layout
70
71     The memory layout used by the uClinux kernel is as follows:
72
73    0x00000000 - 0x00000FFF Null pointer catch page
74    0x20000000 - 0x200FFFFF CS2# [PDK] FPGA
75    0xC0000000 - 0xCFFFFFFF SDRAM
76    0xC0000000 Base of Linux kernel image
77    0xE0000000 - 0xEFFFFFFF CS2# [VDK] SLBUS/PCI window
78    0xF0000000 - 0xF0FFFFFF CS5# MB93493 CSC area (DAV daughter board)
79    0xF1000000 - 0xF1FFFFFF CS7# [CB70/CB451] CPU-card PCMCIA port space
80    0xFC000000 - 0xFC0FFFFF CS1# [VDK] MB86943 config space
81    0xFC100000 - 0xFC1FFFFF CS6# [CB70/CB451] CPU-card DM9000 NIC space
82    0xFC100000 - 0xFC1FFFFF CS6# [PDK] AX88796 NIC space
83    0xFC200000 - 0xFC2FFFFF CS3# MB93493 CSR area (DAV daughter board)
84    0xFD000000 - 0xFDFFFFFF CS4# [CB70/CB451] CPU-card extra flash space
85    0xFE000000 - 0xFEFFFFFF Internal CPU peripherals
86    0xFF000000 - 0xFF1FFFFF CS0# Flash 1
87    0xFF200000 - 0xFF3FFFFF CS0# Flash 2
88    0xFFC00000 - 0xFFC0001F CS0# [VDK] FPGA
89
90     The kernel reads the size of the SDRAM from the memory bus controller
91     registers by default.
92
93     The kernel initialisation code (1) adjusts the SDRAM base addresses to
94     move the SDRAM to desired address, (2) moves the kernel image down to the
95     bottom of SDRAM, (3) adjusts the bus controller registers to move I/O
96     windows, and (4) rearranges the protection registers to protect all of
97     this.
98
99     The reasons for doing this are: (1) the page at address 0 should be
100     inaccessible so that NULL pointer errors can be caught; and (2) the bottom
101     three quarters are left unoccupied so that an FR-V CPU with an MMU can use
102     it for virtual userspace mappings.
103
104     See include/asm-frv/mem-layout.h for constants pertaining to the memory
105     layout.
106
107     See include/asm-frv/mb-regs.h for the constants pertaining to the I/O bus
108     controller configuration.
109
110
111 (*) uClinux Memory Protection
112
113     A DAMPR register is used to cover the entire region used for I/O
114     (0xE0000000 - 0xFFFFFFFF). This permits the kernel to make uncached
115     accesses to this region. Userspace is not permitted to access it.
116
117     The DAMPR/IAMPR protection registers not in use for any other purpose are
118     tiled over the top of the SDRAM such that:
119
120    (1) The core kernel image is covered by as small a tile as possible
121            granting only the kernel access to the underlying data, whilst
122            making sure no SDRAM is actually made unavailable by this approach.
123
124    (2) All other tiles are arranged to permit userspace access to the rest
125            of the SDRAM.
126
127     Barring point (1), there is nothing to protect kernel data against
128     userspace damage - but this is uClinux.
129
130
131 (*) Exceptions and Fixups
132
133     Since the FR40x and FR55x CPUs that do not have full MMUs generate
134     imprecise data error exceptions, there are currently no automatic fixup
135     services available in uClinux. This includes misaligned memory access
136     fixups.
137
138     Userspace EFAULT errors can be trapped by issuing a MEMBAR instruction and
139     forcing the fault to happen there.
140
141     On the FR451, however, data exceptions are mostly precise, and so
142     exception fixup handling is implemented as normal.
143
144
145 (*) Userspace Breakpoints
146
147     The ptrace() system call supports the following userspace debugging
148     features:
149
150    (1) Hardware assisted single step.
151
152    (2) Breakpoint via the FR-V "BREAK" instruction.
153
154    (3) Breakpoint via the FR-V "TIRA GR0, #1" instruction.
155
156    (4) Syscall entry/exit trap.
157
158     Each of the above generates a SIGTRAP.
159
160
161 (*) On-Chip Serial Ports
162
163     The FR-V on-chip serial ports are made available as ttyS0 and ttyS1. Note
164     that if the GDB stub is compiled in, ttyS1 will not actually be available
165     as it will be being used for the GDB stub.
166
167     These ports can be made by:
168
169    mknod /dev/ttyS0 c 4 64
170    mknod /dev/ttyS1 c 4 65
171
172
173 (*) Maskable Interrupts
174
175     Level 15 (Non-maskable) interrupts are dealt with by the GDB stub if
176     present, and cause a panic if not. If the GDB stub is present, ttyS1's
177     interrupts are rated at level 15.
178
179     All other interrupts are distributed over the set of available priorities
180     so that no IRQs are shared where possible. The arch interrupt handling
181     routines attempt to disentangle the various sources available through the
182     CPU's own multiplexor, and those on off-CPU peripherals.
183
184
185 (*) Accessing PCI Devices
186
187     Where PCI is available, care must be taken when dealing with drivers that
188     access PCI devices. PCI devices present their data in little-endian form,
189     but the CPU sees it in big-endian form. The macros in asm/io.h try to get
190     this right, but may not under all circumstances...
191
192
193 (*) Ax88796 Ethernet Driver
194
195     The MB93093 PDK board has an Ax88796 ethernet chipset (an NE2000 clone). A
196     driver has been written to deal specifically with this. The driver
197     provides MII services for the card.
198
199     The driver can be configured by running make xconfig, and going to:
200
201    (*) Network device support
202        - turn on "Network device support"
203        (*) Ethernet (10 or 100Mbit)
204        - turn on "Ethernet (10 or 100Mbit)"
205        - turn on "AX88796 NE2000 compatible chipset"
206
207     The driver can be found in:
208
209    drivers/net/ax88796.c
210    include/asm/ax88796.h
211
212
213 (*) WorkRAM Driver
214
215     This driver provides a character device that permits access to the WorkRAM
216     that can be found on the FR451 CPU. Each page is accessible through a
217     separate minor number, thereby permitting each page to have its own
218     filesystem permissions set on the device file.
219
220     The device files should be:
221
222    mknod /dev/frv/workram0 c 240 0
223    mknod /dev/frv/workram1 c 240 1
224    mknod /dev/frv/workram2 c 240 2
225    ...
226
227     The driver will not permit the opening of any device file that does not
228     correspond to at least a partial page of WorkRAM. So the first device file
229     is the only one available on the FR451. If any other CPU is detected, none
230     of the devices will be openable.
231
232     The devices can be accessed with read, write and llseek, and can also be
233     mmapped. If they're mmapped, they will only map at the appropriate
234     0x7e8nnnnn address on linux and at the 0xfe8nnnnn address on uClinux. If
235     MAP_FIXED is not specified, the appropriate address will be chosen anyway.
236
237     The mappings must be MAP_SHARED not MAP_PRIVATE, and must not be
238     PROT_EXEC. They must also start at file offset 0, and must not be longer
239     than one page in size.
240
241     This driver can be configured by running make xconfig, and going to:
242
243    (*) Character devices
244        - turn on "Fujitsu FR-V CPU WorkRAM support"
245
246
247 (*) Dynamic data cache write mode changing
248
249     It is possible to view and to change the data cache's write mode through
250     the /proc/sys/frv/cache-mode file while the kernel is running. There are
251     two modes available:
252
253    NAME MEANING
254    ===== ==========================================
255    wthru Data cache is in Write-Through mode
256    wback Data cache is in Write-Back/Copy-Back mode
257
258     To read the cache mode:
259
260    # cat /proc/sys/frv/cache-mode
261    wthru
262
263     To change the cache mode:
264
265    # echo wback >/proc/sys/frv/cache-mode
266    # cat /proc/sys/frv/cache-mode
267    wback
268
269
270 (*) MMU Context IDs and Pinning
271
272     On MMU Linux the CPU supports the concept of a context ID in its MMU to
273     make it more efficient (TLB entries are labelled with a context ID to link
274     them to specific tasks).
275
276     Normally once a context ID is allocated, it will remain affixed to a task
277     or CLONE_VM'd group of tasks for as long as it exists. However, since the
278     kernel is capable of supporting more tasks than there are possible ID
279     numbers, the kernel will pass context IDs from one task to another if
280     there are insufficient available.
281
282     The context ID currently in use by a task can be viewed in /proc:
283
284    # grep CXNR /proc/1/status
285    CXNR: 1
286
287     Note that kernel threads do not have a userspace context, and so will not
288     show a CXNR entry in that file.
289
290     Under some circumstances, however, it is desirable to pin a context ID on
291     a process such that the kernel won't pass it on. This can be done by
292     writing the process ID of the target process to a special file:
293
294    # echo 17 >/proc/sys/frv/pin-cxnr
295
296     Reading from the file will then show the context ID pinned.
297
298    # cat /proc/sys/frv/pin-cxnr
299    4
300
301     The context ID will remain pinned as long as any process is using that
302     context, i.e.: when the all the subscribing processes have exited or
303     exec'd; or when an unpinning request happens:
304
305    # echo 0 >/proc/sys/frv/pin-cxnr
306
307     When there isn't a pinned context, the file shows -1:
308
309    # cat /proc/sys/frv/pin-cxnr
310    -1
311

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