The SPU filesystem is used on PowerPC machines that implement the
Cell Broadband Engine Architecture in order to access Synergistic
Processor Units (SPUs).
The filesystem provides a name space similar to POSIX shared
memory or message queues. Users that have write permissions on
the filesystem can use spu_create(2) to establish SPU contexts
under the spufs root directory.
Every SPU context is represented by a directory containing a
predefined set of files. These files can be used for
manipulating the state of the logical SPU. Users can change
permissions on the files, but can't add or remove files.
Mount options
uid=<uid>
Set the user owning the mount point; the default is 0
(root).
gid=<gid>
Set the group owning the mount point; the default is 0
(root).
mode=<mode>
Set the mode of the top-level directory in spufs, as an
octal mode string. The default is 0775.
Files
The files in spufs mostly follow the standard behavior for
regular system calls like read(2) or write(2), but often support
only a subset of the operations supported on regular filesystems.
This list details the supported operations and the deviations
from the standard behavior described in the respective man pages.
All files that support the read(2) operation also support
readv(2) and all files that support the write(2) operation also
support writev(2). All files support the access(2) and stat(2)
family of operations, but for the latter call, the only fields of
the returned stat structure that contain reliable information are
st_mode, st_nlink, st_uid, and st_gid.
All files support the chmod(2)/fchmod(2) and chown(2)/fchown(2)
operations, but will not be able to grant permissions that
contradict the possible operations (e.g., read access on the wbox
file).
The current set of files is:
/capabilities
Contains a comma-delimited string representing the
capabilities of this SPU context. Possible capabilities
are:
sched This context may be scheduled.
step This context can be run in single-step mode, for
debugging.
New capabilities flags may be added in the future.
/mem the contents of the local storage memory of the SPU. This
can be accessed like a regular shared memory file and
contains both code and data in the address space of the
SPU. The possible operations on an open mem file are:
read(2), pread(2), write(2), pwrite(2), lseek(2)
These operate as usual, with the exception that
lseek(2), write(2), and pwrite(2) are not supported
beyond the end of the file. The file size is the
size of the local storage of the SPU, which is
normally 256 kilobytes.
mmap(2)
Mapping mem into the process address space provides
access to the SPU local storage within the process
address space. Only MAP_SHARED mappings are
allowed.
/regs Contains the saved general-purpose registers of the SPU
context. This file contains the 128-bit values of each
register, from register 0 to register 127, in order. This
allows the general-purpose registers to be inspected for
debugging.
Reading to or writing from this file requires that the
context is scheduled out, so use of this file is not
recommended in normal program operation.
The regs file is not present on contexts that have been
created with the SPU_CREATE_NOSCHED flag.
/mbox The first SPU-to-CPU communication mailbox. This file is
read-only and can be read in units of 4 bytes. The file
can be used only in nonblocking mode - even poll(2) cannot
be used to block on this file. The only possible
operation on an open mbox file is:
read(2)
If count is smaller than four, read(2) returns -1
and sets errno to EINVAL. If there is no data
available in the mailbox (i.e., the SPU has not
sent a mailbox message), the return value is set to
-1 and errno is set to EAGAIN. When data has been
read successfully, four bytes are placed in the
data buffer and the value four is returned.
/ibox The second SPU-to-CPU communication mailbox. This file is
similar to the first mailbox file, but can be read in
blocking I/O mode, thus calling read(2) on an open ibox
file will block until the SPU has written data to its
interrupt mailbox channel (unless the file has been opened
with O_NONBLOCK, see below). Also, poll(2) and similar
system calls can be used to monitor for the presence of
mailbox data.
The possible operations on an open ibox file are:
read(2)
If count is smaller than four, read(2) returns -1
and sets errno to EINVAL. If there is no data
available in the mailbox and the file descriptor
has been opened with O_NONBLOCK, the return value
is set to -1 and errno is set to EAGAIN.
If there is no data available in the mailbox and
the file descriptor has been opened without
O_NONBLOCK, the call will block until the SPU
writes to its interrupt mailbox channel. When data
has been read successfully, four bytes are placed
in the data buffer and the value four is returned.
poll(2)
Poll on the ibox file returns (POLLIN | POLLRDNORM)
whenever data is available for reading.
/wbox The CPU-to-SPU communication mailbox. It is write-only
and can be written in units of four bytes. If the mailbox
is full, write(2) will block, and poll(2) can be used to
block until the mailbox is available for writing again.
The possible operations on an open wbox file are:
write(2)
If count is smaller than four, write(2) returns -1
and sets errno to EINVAL. If there is no space
available in the mailbox and the file descriptor
has been opened with O_NONBLOCK, the return value
is set to -1 and errno is set to EAGAIN.
If there is no space available in the mailbox and
the file descriptor has been opened without
O_NONBLOCK, the call will block until the SPU reads
from its PPE (PowerPC Processing Element) mailbox
channel. When data has been written successfully,
the system call returns four as its function
result.
poll(2)
A poll on the wbox file returns (POLLOUT |
POLLWRNORM) whenever space is available for
writing.
/mbox_stat, /ibox_stat, /wbox_stat
These are read-only files that contain the length of the
current queue of each mailbox—that is, how many words can
be read from mbox or ibox or how many words can be written
to wbox without blocking. The files can be read only in
four-byte units and return a big-endian binary integer
number. The only possible operation on an open *box_stat
file is:
read(2)
If count is smaller than four, read(2) returns -1
and sets errno to EINVAL. Otherwise, a four-byte
value is placed in the data buffer. This value is
the number of elements that can be read from (for
mbox_stat and ibox_stat) or written to (for
wbox_stat) the respective mailbox without blocking
or returning an EAGAIN error.
/npc, /decr, /decr_status, /spu_tag_mask, /event_mask,
/event_status, /srr0, /lslr
Internal registers of the SPU. These files contain an
ASCII string representing the hex value of the specified
register. Reads and writes on these files (except for
npc, see below) require that the SPU context be scheduled
out, so frequent access to these files is not recommended
for normal program operation.
The contents of these files are:
npc Next Program Counter - valid only when the SPU is
in a stopped state.
decr SPU Decrementer
decr_status
Decrementer Status
spu_tag_mask
MFC tag mask for SPU DMA
event_mask
Event mask for SPU interrupts
event_status
Number of SPU events pending (read-only)
srr0 Interrupt Return address register
lslr Local Store Limit Register
The possible operations on these files are:
read(2)
Reads the current register value. If the register
value is larger than the buffer passed to the
read(2) system call, subsequent reads will continue
reading from the same buffer, until the end of the
buffer is reached.
When a complete string has been read, all
subsequent read operations will return zero bytes
and a new file descriptor needs to be opened to
read a new value.
write(2)
A write(2) operation on the file sets the register
to the value given in the string. The string is
parsed from the beginning until the first
nonnumeric character or the end of the buffer.
Subsequent writes to the same file descriptor
overwrite the previous setting.
Except for the npc file, these files are not
present on contexts that have been created with the
SPU_CREATE_NOSCHED flag.
/fpcr This file provides access to the Floating Point Status and
Control Register (fcpr) as a binary, four-byte file. The
operations on the fpcr file are:
read(2)
If count is smaller than four, read(2) returns -1
and sets errno to EINVAL. Otherwise, a four-byte
value is placed in the data buffer; this is the
current value of the fpcr register.
write(2)
If count is smaller than four, write(2) returns -1
and sets errno to EINVAL. Otherwise, a four-byte
value is copied from the data buffer, updating the
value of the fpcr register.
/signal1, /signal2
The files provide access to the two signal notification
channels of an SPU. These are read-write files that
operate on four-byte words. Writing to one of these files
triggers an interrupt on the SPU. The value written to
the signal files can be read from the SPU through a
channel read or from host user space through the file.
After the value has been read by the SPU, it is reset to
zero. The possible operations on an open signal1 or
signal2 file are:
read(2)
If count is smaller than four, read(2) returns -1
and sets errno to EINVAL. Otherwise, a four-byte
value is placed in the data buffer; this is the
current value of the specified signal notification
register.
write(2)
If count is smaller than four, write(2) returns -1
and sets errno to EINVAL. Otherwise, a four-byte
value is copied from the data buffer, updating the
value of the specified signal notification
register. The signal notification register will
either be replaced with the input data or will be
updated to the bitwise OR operation of the old
value and the input data, depending on the contents
of the signal1_type or signal2_type files
respectively.
/signal1_type, /signal2_type
These two files change the behavior of the signal1 and
signal2 notification files. They contain a numeric ASCII
string which is read as either "1" or "0". In mode 0
(overwrite), the hardware replaces the contents of the
signal channel with the data that is written to it. In
mode 1 (logical OR), the hardware accumulates the bits
that are subsequently written to it. The possible
operations on an open signal1_type or signal2_type file
are:
read(2)
When the count supplied to the read(2) call is
shorter than the required length for the digit
(plus a newline character), subsequent reads from
the same file descriptor will complete the string.
When a complete string has been read, all
subsequent read operations will return zero bytes
and a new file descriptor needs to be opened to
read the value again.
write(2)
A write(2) operation on the file sets the register
to the value given in the string. The string is
parsed from the beginning until the first
nonnumeric character or the end of the buffer.
Subsequent writes to the same file descriptor
overwrite the previous setting.
/mbox_info, /ibox_info, /wbox_info, /dma_into, /proxydma_info
Read-only files that contain the saved state of the SPU
mailboxes and DMA queues. This allows the SPU status to
be inspected, mainly for debugging. The mbox_info and
ibox_info files each contain the four-byte mailbox message
that has been written by the SPU. If no message has been
written to these mailboxes, then contents of these files
is undefined. The mbox_stat, ibox_stat, and wbox_stat
files contain the available message count.
The wbox_info file contains an array of four-byte mailbox
messages, which have been sent to the SPU. With current
CBEA machines, the array is four items in length, so up to
4 * 4 = 16 bytes can be read from this file. If any
mailbox queue entry is empty, then the bytes read at the
corresponding location are undefined.
The dma_info file contains the contents of the SPU MFC DMA
queue, represented as the following structure:
struct spu_dma_info {
uint64_t dma_info_type;
uint64_t dma_info_mask;
uint64_t dma_info_status;
uint64_t dma_info_stall_and_notify;
uint64_t dma_info_atomic_command_status;
struct mfc_cq_sr dma_info_command_data[16];
};
The last member of this data structure is the actual DMA
queue, containing 16 entries. The mfc_cq_sr structure is
defined as:
struct mfc_cq_sr {
uint64_t mfc_cq_data0_RW;
uint64_t mfc_cq_data1_RW;
uint64_t mfc_cq_data2_RW;
uint64_t mfc_cq_data3_RW;
};
The proxydma_info file contains similar information, but
describes the proxy DMA queue (i.e., DMAs initiated by
entities outside the SPU) instead. The file is in the
following format:
struct spu_proxydma_info {
uint64_t proxydma_info_type;
uint64_t proxydma_info_mask;
uint64_t proxydma_info_status;
struct mfc_cq_sr proxydma_info_command_data[8];
};
Accessing these files requires that the SPU context is
scheduled out - frequent use can be inefficient. These
files should not be used for normal program operation.
These files are not present on contexts that have been
created with the SPU_CREATE_NOSCHED flag.
/cntl This file provides access to the SPU Run Control and SPU
status registers, as an ASCII string. The following
operations are supported:
read(2)
Reads from the cntl file will return an ASCII
string with the hex value of the SPU Status
register.
write(2)
Writes to the cntl file will set the context's SPU
Run Control register.
/mfc Provides access to the Memory Flow Controller of the SPU.
Reading from the file returns the contents of the SPU's
MFC Tag Status register, and writing to the file initiates
a DMA from the MFC. The following operations are
supported:
write(2)
Writes to this file need to be in the format of a
MFC DMA command, defined as follows:
struct mfc_dma_command {
int32_t pad; /* reserved */
uint32_t lsa; /* local storage address */
uint64_t ea; /* effective address */
uint16_t size; /* transfer size */
uint16_t tag; /* command tag */
uint16_t class; /* class ID */
uint16_t cmd; /* command opcode */
};
Writes are required to be exactly sizeof(struct
mfc_dma_command) bytes in size. The command will
be sent to the SPU's MFC proxy queue, and the tag
stored in the kernel (see below).
read(2)
Reads the contents of the tag status register. If
the file is opened in blocking mode (i.e., without
O_NONBLOCK), then the read will block until a DMA
tag (as performed by a previous write) is complete.
In nonblocking mode, the MFC tag status register
will be returned without waiting.
poll(2)
Calling poll(2) on the mfc file will block until a
new DMA can be started (by checking for POLLOUT) or
until a previously started DMA (by checking for
POLLIN) has been completed.
/mss Provides access to the MFC MultiSource
Synchronization (MSS) facility. By mmap(2)-ing
this file, processes can access the MSS area of the
SPU.
The following operations are supported:
mmap(2)
Mapping mss into the process address space gives
access to the SPU MSS area within the process
address space. Only MAP_SHARED mappings are
allowed.
/psmap Provides access to the whole problem-state mapping of the
SPU. Applications can use this area to interface to the
SPU, rather than writing to individual register files in
spufs.
The following operations are supported:
mmap(2)
Mapping psmap gives a process a direct map of the
SPU problem state area. Only MAP_SHARED mappings
are supported.
/phys-id
Read-only file containing the physical SPU number that the
SPU context is running on. When the context is not
running, this file contains the string "-1".
The physical SPU number is given by an ASCII hex string.
/object-id
Allows applications to store (or retrieve) a single 64-bit
ID into the context. This ID is later used by profiling
tools to uniquely identify the context.
write(2)
By writing an ASCII hex value into this file,
applications can set the object ID of the SPU
context. Any previous value of the object ID is
overwritten.
read(2)
Reading this file gives an ASCII hex string
representing the object ID for this SPU context.
