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PDC Stable Storage

We are dealing here with a part of the BootingSystem, namely the ProcessorDependentCode (PDC).


  • drivers/parisc/pdc_stable.c
  • arch/parisc/kernel/firmware.c
  • arch/parisc/kernel/drivers.c

Brief version history

This driver was first released in kernel 2.6.11-rc1-pa2. A major update (v0.22) was released in kernel 2.6.15-pa3. Code to access OS Dependent data was added in v0.30 released in kernel 2.6.17-rc3-pa3.

What is Stable Storage?

According to http://ftp.parisc-linux.org/docs/arch/pdc20-v1.0-Ch4-procs.pdf page 4-84, all PA-RISC architectures must implement a permanent storage area of at least 96 bytes, called Stable Storage. It is a system wide storage used to store some system variables such as Autoboot and Autosearch flags, Primary/Alternative Boot Paths, Console and Keyboard Paths, the amount of RAM to be tested during boot sequence, and other data.

What are we trying to do?

The main idea underlying this piece of code is to provide an interface between the kernel and that firmware storage area, in order to:

  • Get an interface to know what the current settings are.
  • Get an interface to be able to change the settings at will while Linux is running.

The "settings" being Autoboot, Autosearch, Paths, etc.

How do we do it?

Accessing the PDC

First, we had to implement the missing abstraction logic in firmware.c to access the Stable Storage. We had to code all the required interfaces to PDC_STABLE calls:

Interfaces (firmware.c)
Function Role
int pdc_stable_read(unsigned long staddr, void *memaddr, unsigned long count) This call reads count bytes from address staddr and writes the result in preallocated memaddr space
int pdc_stable_write(unsigned long staddr, void *memaddr, unsigned long count) This call writes count bytes at address staddr from address memaddr
int pdc_stable_get_size(unsigned long *size) This call gets the size of the Stable Storage area
int pdc_stable_verify_contents(void) This call checks the integrity of the contents of Stable Storage
int pdc_stable_initialize(void) This call initialize Stable Storage to zero

Now a few details: as it is noted in the above mentioned document, staddr and memaddr must be word-aligned. count must be multiple of 4. The Stable Storage address space starts at 0x0, and both read and write calls will fail if staddr + count > size.

Everything is now setup to get and write useful data from Stable Storage. Thanks to the documentation, we know where the various Paths and other variables are stored. There is a trick, though. Paths are BC Paths, also known as hardware paths in the PA-RISC terminology. That means that when reading a path, we have to convert it to the actual device and map it to a human readable device name, and when writing it, we will have to provide a human writeable way of designing a device, and then convert it back to its hardware path.

Converting Paths

The first thing that comes to mind is that we will need to convert hardware path to devices and devices back to their hardware path. Let see how we do that.

Anatomy Of Paths

The paths described by Stable Storage are made of two separate entities. First, they begin with a flag byte, which is only used in Primary Boot Path to store the values of Autoboot, Autosearch and the Timer. Then comes a regular BC path, made of 6 BC fields (bytes which values specify the fixed field of the bus converter port's HPA), and a MOD field (byte which is the fixed field of the specified module). Following these 8 bytes is an array of 6 longs (32 bits), called layers, used to describe the portion of the path to a device that is beyond the module and/or to contain device-dependent information. Typically, for a given SCSI device, layer[0] will be the SCSI ID, and layer[1] the SCSI LUN; or for a RS232 device, the layers will contains the serial port being used.

The Algorithms

These are implemented in arch/parisc/kernel/drivers.c.

Matching an existing hwpath back to its original device

That's what we want when reading the Stable Storage configuration.

When trying to convert a hardware path to the matching device, a first problem occured: we cannot use find_parisc_device(), for the actual device may not necessarily be a parisc_device (in particular, it's likely that it will be a pci_dev for a SCSI device).

What we did was first to write clones of create_parisc_device() and alloc_tree_node(), changing their logic a little. hwpath_to_device() deals with generic devices instead of parisc ones. parse_tree_node() no longer allocates anything, but instead check each device of a given level to match it against the current hardware path (depending on its type: parisc_device, pci_dev, etc), when a matching device is found, it's returned back to the caller (hwpath_to_device() in the present case) which will use it back into a recursive call. Thanks to MatthewWilcox who rewrote the parisc device layer to use generic devices, this code could be made much easier.

If it is a parisc_device (bus_type == &parisc_bus_type), then it tries to match the current device hw_path with the expected one.

If it is a pci_dev (bus_type == &pci_bus_type), then it parses the remaining tree to look either for a PCI bridge, or for a PCI device. In the case of a PCI device, we have to go down one level more in the hardware path, since PCI devices occupy the last BC field and the MOD field.

If it can't find a matching device, then hwpath_to_device() simply returns a NULL pointer.

Matching an existing device to its corresponding hardware path

We need that when writing the new configuration to Stable Storage.

Fortunately this is alot easier to do, the logic is already existing (get_node_path()). We just wrote a wrapper for that function, which we called device_to_hwpath() for obvious consistency reasons. That wrapper takes a struct device pointer and a preallocated struct hardware_path pointer as arguments, and will fill the later with the path corresponding to the former, doing appropriate stuff depending on the type of the device (be it a PCI or PA-RISC device, for instance).

Kernel Interface

Now that we have the logic in place, we have to think of a proper way for the kernel to expose the contents of Stable Storage. A first approach was to use the proc interface, but it quickly showed intrinsic limitations, and eventually SysFS seemed to be a far better choice.

Here is the tree we are presenting in SysFS:


Roughly put, all global data (such as the boot flags, the amount of RAM checked during boot, etc) is shown in the various files placed at the root of the stable directory, while we are creating in the paths/ subfolder 4 more - whenever possible - sub directories (one per boot path: Primary, Alternative, Console and Keyboard), each containing 2 files and (usually) one symlink:

  • device is the symlink to the actual device folder in SysFS.
  • layer contains the hardware layer, in the form x y z, where x,y,z are unsigned decimals.
  • hwpath contains the hardware path, in the form x/y/z, where x,y,z are unsigned decimals.

In order for the kernel to change the boot paths values, layer and hwpath can be modified by writing to them (as super user) data in the following form:

  • x/y/z for hwpath, where x,y,z are unsigned decimals, to change the hardware path.
  • x.y.z for layer, where x,y,z are unsigned decimals, to change layer[0]->layer[n], n < 6.

For instance, one can do the following:

# echo -n "0/0/2/1" > /sys/firmware/stable/paths/alternative/hwpath
PDC Stable Storage: changed "alternative" path to "0/0/2/1"
# echo -n "15" > /sys/firmware/stable/paths/alternative/layer
PDC Stable Storage: changed "alternative" layers to "15"

The "`PDC Stable Storage:`" messages are printed to INFO loglevel upon successful completion. If the user attempts to set an unmappable hwpath, an error message is sent to the WARNING loglevel.

In fact, minimal integrity checks are performed (see bellow for more information):

  • For hwpath, the code makes sure it can map the provided path to an existing device, and fails otherwise.
  • For layer, the code simply checks arguments are integers.

Finally, one can toggle Autoboot and Autosearch flags by writing to the autoboot and autosearch files, as follows:

# echo 1 > /sys/firmware/stable/autoboot
PDC Stable Storage: changed "autoboot" to "On"
# echo 0 > /sys/firmware/stable/autosearch
PDC Stable Storage: changed "autosearch" to "Off"

It pretty much speaks for itself. There again, messages are sent to INFO loglevel upon successful completion, and to WARNING on failure. The result can be checked by looking at the content of these files afterwards:

# cat /sys/firmware/stable/autoboot
# cat /sys/firmware/stable/autosearch

The content of the diagnostic file are pretty cryptic. It's not documented, so the file reflects the raw hex value.

OS-Dependent Data

Two files are of particular interest: osdep1 and osdep2. These two files are read/write accessible, as they contain OS-Dependent data, that is, data that can be modified by the host's operating system. The first file should always be available, according to the PDC specs. The second one will always be there, but may not necessary contain anything, as the size of the second OS-Dependent area is HVERSION-dependent. Most of the time it's 32 bytes. If there's no data available, reading will return -ENODATA, and writing to the file will return -ENOSYS.

The read/write access is pretty basic:

* Reading the file will display raw hex values, 32 bits per line.
* Writing is made on a byte-by-byte fashion, 0-padded. Here's an example to illustrate:
# cat /sys/firmware/stable/osdep1
# echo -n "123" > /sys/firmware/stable/osdep1
# cat /sys/firmware/stable/osdep1

0x31 is ascii "1", 0x32 is ascii "2", and 0x33 is ascii "3". Another example:

# echo -e "\0001\0002\0003" > /sys/firmware/stable/osdep1
# cat /sys/firmware/stable/osdep1

0x0a is ascii "\n".

If you try to write more than the available storage size, you will get -EMSGSIZE.

Note: reading/writing from/to osdep2 is much more expensive than from/to osdep1 (see the source for details).

Of course, the key interest of these files is the fact that their content is preserved accross reboots (it's Stable Storage, remember? ;) )

File Permissions

The general policy toward file access shall respect the following rules:

  1. Write permission granted to root only
  2. Read permission to files that trigger PDC calls granted to root only
  3. Read permission to other files granted to everyone

In particular, the following files are root accessible only:



Design Choices & Known Features

Currently the code is unable to deal properly with some kinds of devices, especially SuckyIO ones and EISA ones. In particular, it is impossible for the underlying mechanism to map a path pointing to a SuckyIO or EISA device to the corresponding generic `device` pointer. This has two main consequences:

  • When such a boot path is encountered, no symlink is created in the corresponding SysFS folder.
  • It is impossible to set any hwpath to such a device, the code will complaint that it can't map the path provided to an existing device.

On the other hand, that kind of check is barely enough: a PCI bridge, or even worse, a CPU, would yield a valid device and thus nothing prevents anyone to setup their CPU as primary boot path. Keep in mind that this driver aims at being a facility, providing a syntax to be used. Thorough checking is left to userspace. What's more, only the super user can change the settings...

Another thing to bear in mind is the high dependency of the code on the PDC calls reliability. Truth told, should one of this call fail (especially the write call), the system would most likely end-up in a rather bad shape upon next reboot. The same applies in case of a sudden power failure while the content of the Stable Storage is being modified, but heh, it's not supposed to happen anyway, right? ;)

In fact, we do not care that much about that issue since should a Bad Thing™ occur, first, you'll be warned (error message sent to ERR loglevel), and it'll be easily fixable through the PDC boot console when the system powers up. In other word, there is no way that code would leave the system in such a bad shape it'd be impossible to fix and would need some HP techie to repair it! Relieved, heh? :)


Something that would be nice to have is a userland tool that would help setting the path in an easy way, providing some user-friendly glue between the mechanism in /sys and the way a normal user would designate a device (eg: /dev/sda4, /dev/ttyS0, etc).

The hooks for such a tool to work are mostly there (in particular since the code can be passed a struct device pointer, it'll be able to deal with it fine), so my guess is that it's a matter of finding the proper glue (that is probably not trivial) and eventually add a sysctl, if one wants not to use the SysFS interface.

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