proto

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]"

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

AUTHORS

The proto device driver and this manual page were written by Marcel Moolenaar <[email protected]>.

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.

August 7, 2015

Debian