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NAME
proto — Generic prototyping and diagnostics driver
SYNOPSIS
To compile this driver into the kernel, place the following line in your kernel configuration file:
device proto
Alternatively, to load the driver as a module at boot time, place the following line in loader.conf(5):
proto_load="YES"
To have the driver attach to a device instead of its regular driver, mention it in the list of devices
assigned to the following loader variable:
hw.proto.attach="desc[,desc]"
DESCRIPTION
The proto device driver attaches to PCI or ISA devices when no other device drivers are present for those
devices and it creates device special files for all resources associated with the device. The driver
itself has no knowledge of the device it attaches to. Programs can open these device special files and
perform register-level reads and writes. As such, the proto device driver is nothing but a conduit or
gateway between user space programs and the hardware device.
Examples for why this is useful include hardware diagnostics and prototyping. In both these use cases,
it is far more convenient to develop and run the logic in user space. Especially hardware diagnostics
requires a somewhat user-friendly interface and adequate reporting. Neither is done easily as kernel
code.
I/O port resources
Device special files created for I/O port resources allow lseek(2), read(2), write(2) and ioctl(2)
operations to be performed on them. The read(2) and write(2) system calls are used to perform input and
output (resp.) on the port. The amount of data that can be read or written at any single time is either
1, 2 or 4 bytes. While the proto driver does not prevent reading or writing 8 bytes at a time for some
architectures, it should not be assumed that such actually produces correct results. The lseek(2) system
call is used to select the port number, relative to the I/O port region being represented by the device
special file. If, for example, the device special file corresponds to an I/O port region from 0x3f8 to
0x3ff inclusive, then an offset of 4 given to lseek with a whence value of SEEK_SET will target port
0x3fc on the next read or write operation. The ioctl(2) system call can be used for the PROTO_IOC_REGION
request. This ioctl request returns the extend of the resource covered by this device special file. The
extend is returned in the following structure:
struct proto_ioc_region {
unsigned long address;
unsigned long size;
};
Memory mapped I/O resources
The device special files created for memory mapped I/O resources behave in the same way as those created
for I/O port resources. Additionally, device special files for memory mapped I/O resources allow the
memory to be mapped into the process' address space using mmap(2). Reads and writes to the memory
address returned by mmap(2) go directly to the hardware. As such the use of read(2) and write(2) can be
avoided, reducing the access overhead significantly. Alignment and access width constraints put forth by
the underlying device apply. Also, make sure the compiler does not optimize memory accesses away or has
them coalesced into bigger accesses.
DMA pseudo resource
A device special file named busdma is created for the purpose of doing DMA. It only supports ioctl(2)
and only for the PROTO_IOC_BUSDMA request. This device special file does not support read(2) nor
write(2). The PROTO_IOC_BUSDMA request has an argument that is both in and out and is defined as
follows:
struct proto_ioc_busdma {
unsigned int request;
unsigned long key;
union {
struct {
unsigned long align;
unsigned long bndry;
unsigned long maxaddr;
unsigned long maxsz;
unsigned long maxsegsz;
unsigned int nsegs;
unsigned int datarate;
unsigned int flags;
} tag;
struct {
unsigned long tag;
unsigned int flags;
unsigned long virt_addr;
unsigned long virt_size;
unsigned int phys_nsegs;
unsigned long phys_addr;
unsigned long bus_addr;
unsigned int bus_nsegs;
} md;
struct {
unsigned int op;
unsigned long base;
unsigned long size;
} sync;
} u;
unsigned long result;
};
The request field is used to specify which DMA operation is to be performed. The key field is used to
specify which object the operation applies to. An object is either a tag or a memory descriptor (md).
The following DMA operations are defined:
PROTO_IOC_BUSDMA_TAG_CREATE
Create a root tag. The result field is set on output with the key of the DMA tag. The tag is
created with the constraints given by the tag sub-structure. These constraints correspond roughly
to those that can be given to the bus_dma_tag_create(9) function.
PROTO_IOC_BUSDMA_TAG_DERIVE
Create a derived tag. The key field is used to identify the parent tag from which to derive the
new tag. The key of the derived tag is returned in the result field. The derived tag combines the
constraints of the parent tag with those given by the tag sub-structure. The combined constraints
are written back to the tag sub-structure on return.
PROTO_IOC_BUSDMA_TAG_DESTROY
Destroy a root or derived tag previously created. The key field specifies the tag to destroy. A
tag can only be destroyed when not referenced anymore. This means that derived tags that have this
tag as a parent and memory descriptors created from this tag must be destroyed first.
PROTO_IOC_BUSDMA_MEM_ALLOC
Allocate memory that satisfies the constraints put forth by the tag given in the tag field of the
md sub-structure. The key of the memory descriptor for this memory is returned in the result
field. The md sub-structure is filled on return with details of the allocation. The kernel
virtual address and the size of the allocated memory are returned in the virt_addr and virt_size
fields. The number of contigous physical memory segments and the address of the first segment are
returned in the phys_nsegs and phys_addr fields. Allocated memory is automatically loaded and thus
mapped into bus space. The number of bus segments and the address of the first segment are
returned in the bus_nsegs and bus_addr fields. The behaviour of this operation banks heavily on
how bus_dmamem_alloc(9) is implemented, which means that memory is currently always allocated as a
single contigous region of physical memory. In practice this also tends to give a single contigous
region in bus space. This may change over time.
PROTO_IOC_BUSDMA_MEM_FREE
Free previously allocated memory and destroy the memory desciptor. The proto driver is not in a
position to track whether the memory has been mapped in the process' address space, so the
application is responsible for unmapping the memory before it is freed. The proto driver also
cannot protect against the hardware writing to or reading from the memory, even after it has been
freed. When the memory is reused for other purposes it can be corrupted or cause the hardware to
behave in unpredictable ways when DMA has not stopped completely before freeing.
PROTO_IOC_BUSDMA_MD_CREATE
Create an empty memory descriptor with the tag specified in the tag field of the md sub-structure.
The key of the memory descriptor is returned in the result field.
PROTO_IOC_BUSDMA_MD_DESTROY
Destroy the previously created memory descriptor specified by the key field. When the memory
descriptor is still loaded, it is unloaded first.
PROTO_IOC_BUSDMA_MD_LOAD
Load a contigous region of memory in the memory descriptor specified by the key field. The size
and address in the process' virtual address space are specified by the virt_size and virt_addr
fields. On return, the md sub-structure contains the result of the operation. The number of
physical segments and the address of the first segment is returned in the phys_nsegs and phys_addr
fields. The number of bus space segments and the address of the first segment in bus space is
returned in the bus_nsegs and bus_addr fields.
PROTO_IOC_BUSDMA_MD_UNLOAD
Unload the memory descriptor specified by the key field.
PROTO_IOC_BUSDMA_SYNC
Guarantee that all hardware components have a coherent view of the memory tracked by the memory
descriptor, specified by the key field. A sub-section of the memory can be targeted by specifying
the relative offset and size of the memory to make coherent. The offset and size are given by the
base and size fields of the sync sub-structure. The op field holds the sync operation to be
performed. This is similar to the bus_dmamap_sync(9) function.
PCI configuration space
Access to PCI configuration space is possible through the pcicfg device special file. The device special
file supports lseek(2), read(2) and write(2). Usage is the asme as for I/O port resources.
FILES
All device special files corresponding to a PCI device are located under /dev/proto/pci<d>:<b>:<s>:<f>
with pci<d>:<b>:<s>:<f> representing the location of the PCI device in the PCI hierarchy. A PCI location
includes:
<d> The PCI domain number
<b> The PCI bus number
<s> The PCI slot or device number
<f> The PCI function number
Every PCI device has a device special file called pcicfg. This device special file gives access to the
PCI configuration space. A device special file called busdma is also created. This device special file
provides the interfaces needed for doing DMA. For each valid base address register (BAR), a device
special file is created that contains the BAR offset and the resource type. A resource type can be
either io or mem representing I/O port or memory mapped I/O space (resp.)
ISA devices do not have a location. Instead, they are identified by the first I/O port address or first
memory mapped I/O address. Consequently, all device special files corresponding to an ISA device are
located under /dev/proto/isa:<addr> with addr the address in hexadecimal notation. For each I/O port or
memory mapped I/O address, a device special file is created that contains the resource identification
used by the kernel and the resource type. The resource type can be either io or mem representing I/O
port or memory mapped I/O space (resp.) When the device has a DMA channel assigned to it, a device
special file with the name busdma is created as well. This device special file provides the interfaces
needed for doing DMA.
If the ISA device is not a Plug-and-Play device nor present in the ACPI device tree, it must have the
appropriate hints so that the kernel can reserve the resources for it.
EXAMPLES
A single function PCI device in domain 0, on bus 1, in slot 2 and having a single memory mapped I/O
region will have the following device special files:
/dev/proto/pci0:1:2:0/10.mem
/dev/proto/pci0:1:2:0/pcicfg
A legacy floppy controller will have the following device files:
/dev/proto/isa:0x3f0/00.io
/dev/proto/isa:0x3f0/01.io
/dev/proto/isa:0x3f0/busdma
SEE ALSO
ioctl(2), lseek(2), mmap(2), read(2), write(2), bus_dma_tag_create(9), bus_dmamap_sync(9),
bus_dmamem_alloc(9)
AUTHORS
The proto device driver and this manual page were written by Marcel Moolenaar <marcel@xcllnt.net>.
SECURITY CONSIDERATIONS
Because programs have direct access to the hardware, the proto driver is inherently insecure. It is not
advisable to use this driver on a production machine.
MISSING FUNCTIONALITY
The proto driver does not fully support memory descriptors that need multiple physical memory segments or
multiple bus space segments. At the very least, an operation is needed on the DMA pseudo resource for
the application to obtain all segments.
The proto driver does not yet support interrupts. Since interrupts cannot be handled by the driver
itself, they must be converted into signals and delivered to the program that has registered for
interrupts. A satisfactory mechanism for keeping the interrupt masked during the signal handling is
still being worked out.
DMA support for devices other than busmaster devices is not present yet. The details of how a program is
to interact with the DMA controller still need to be fleshed out.
Debian August 7, 2015 PROTO(4)