spufs(7) Miscellaneous Information Manual spufs(7)
NAME
spufs - SPU filesystem
DESCRIPTION
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 sys-
tem 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 de-
scribed 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) opera-
tions, 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 debug-
ging.
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 op-
erations 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 ac-
cess 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 nor-
mal 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 mes-
sage), 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 re-
turned.
/ibox The second SPU-to-CPU communication mailbox. This file is simi-
lar 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 (un-
less 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) when-
ever 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 re-
turn a big-endian binary integer number. The only possible op-
eration 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) re-
quire 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 descrip-
tor 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 de-
scriptor overwrite the previous setting.
Except for the npc file, these files are not present on
contexts that have been created with the SPU_CRE-
ATE_NOSCHED flag.
/fpcr This file provides access to the Floating Point Status and Con-
trol Register (fcpr) as a binary, four-byte file. The opera-
tions 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 noti-
fication 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 con-
tents of the signal1_type or signal2_type files respec-
tively.
/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 hard-
ware replaces the contents of the signal channel with the data
that is written to it. In mode 1 (logical OR), the hardware ac-
cumulates 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 descrip-
tor 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 de-
scriptor 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 mail-
boxes and DMA queues. This allows the SPU status to be in-
spected, 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 mes-
sages, which have been sent to the SPU. With current CBEA ma-
chines, 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 un-
defined.
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 de-
scribes 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. Read-
ing from the file returns the contents of the SPU's MFC Tag Sta-
tus 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_NON-
BLOCK), then the read will block until a DMA tag (as per-
formed by a previous write) is complete. In nonblocking
mode, the MFC tag status register will be returned with-
out 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 Synchroniza-
tion (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 sup-
ported.
/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, applica-
tions can set the object ID of the SPU context. Any pre-
vious 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.
EXAMPLES
To automatically mount(8) the SPU filesystem when booting, at the loca-
tion /spu chosen by the user, put this line into the fstab(5) configu-
ration file:
none /spu spufs gid=spu 0 0
SEE ALSO
close(2), spu_create(2), spu_run(2), capabilities(7)
The Cell Broadband Engine Architecture (CBEA) specification
Linux man-pages 6.7 2023-10-31 spufs(7)
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