Was there ever a Linux kernel driver for accessing disks via BIOS?

Question

If there is some piece of PC compatible hardware where there are no (publicly) existing drivers for Linux, the only option is to access the disk with BIOS calls. I'm aware that this imposes restrictions on the maximum size of disk and/or partition to be used.

Has some old Linux Kernel featured such a driver? Searching online yields a lot of irrelevant hits about more modern systems and their particular booting challenges.

My goal is to be able to run Linux from the emulated disk on an old AS/400 IPCS card. OS drivers are available only for Windows NT 4 and 2k.

Answer 1

To my knowledge, no such driver has ever been written.

Since the very earliest versions, Linux has been a pure 32-bit protected-mode kernel that drove most devices (including disk controllers) directly, without going through the BIOS. The kernel has never had any facility to invoke 16-bit code from kernel mode during normal operation (after early boot); even the APM driver only ever supported the 32-bit protected-mode entry point. While support for 16-bit protected mode and virtual 8086 mode has been added, it was only ever driven by userspace, i.e. by DOSEMU, Wine and (userspace) VBE video drivers. The closest Linux got to a mechanism to invoke real-mode code from kernel mode is in a patch for the vesafb driver in Linux 2.6.20 that was never merged into the mainline kernel; the patch’s successor, uvesafb, likewise invoked the video BIOS from userspace.

In particular, there has never been a BIOS-based kernel disk driver. Apparently some kind of driver of this sort was planned at one point: in Linux 1.3.22, the first release to contain the devices.txt documentation file containing device number assignments, major block device numbers 12 and 14 are reserved for ‘MSCDEX CD-ROM callback support’ and ‘BIOS harddrive callback support’ respectively. The reservations have never been claimed by an actual driver, however, or even added to the linux/major.h header file, and they have been removed from devices.txt in Linux 2.6.30. (Thanks to @Joshua for letting me know this reservation existed, prompting me to investigate further.)

Such a driver does exist in ELKS (a fork of Linux focusing on embedded systems), but I assume that ELKS and mainline Linux have already diverged enough to make adapting this driver for the latter a highly non-trivial task. (This driver does not use the above-mentioned reservations.)

---

Your best bet is probably to write a driver for this device yourself: either via reverse-engineering or by creating a virtual 8086 mode monitor/emulator to run the BIOS-based driver in, like userspace VBE drivers do. In the general case, I expect the latter approach to be very fragile, as the ROM BIOS code may assume that it has the entirety of the hardware at its disposal, make all sorts of assumptions about its state, and attempt to perform operations that are difficult to emulate, especially from userspace code. In particular it may try to:

• raise a System Management Interrupt (the BIOS on a laptop I am writing this on does that),
• turn off interrupts to guarantee atomicity,
• reconfigure the Programmable Interrupt Controller and other hardware,
• perform DMA transfers,

And many other things. Most of these considerations usually do not apply to video BIOSes, as those usually confine themselves to operating on the video hardware itself. As such, they only require access to I/O ports and memory, and those are rather easy to provide.

Also note that the BIOS interrupt calls were not designed to be reentrant or execute under the supervision of a multitasking operating system (they were designed as drivers for DOS, after all). Given that, environments which do provide BIOS-based disk drivers are either single-tasking systems like DOS anyway or take some pains to ensure that BIOS calls have exclusive access to all the hardware (including the CPU) and do not interfere with anything else:

• GNU GRUB 2, a boot loader containing both BIOS and ‘bare-metal’ disk drivers, is a single-tasking, pure ring-0 environment that contains some logic to ensure firmware-based and native drivers are not used at the same time;
• The real-mode mapper in Windows 9x (i.e. its DOS/BIOS file system driver) executes real-mode code in ring 0 and guards it by the critical section, which is essentially a global kernel lock (cf. Linux removing the Big Kernel Lock entirely in version 2.6.39)

All of the above told, things are not hopeless: if you stick to a narrow goal of writing a driver that works with a specific BIOS whose behaviour is known so that you can apply workarounds specific to your firmware (as opposed to creating a fully general solution), there is a good chance it might actually work quite reliably. Especially if the BIOS comes from an option ROM on an extension card, as those are much less free to assume things about other hardware that may be present.

Answer 2

It's perfectly possible for both userspace and the kernel to access the BIOS. In fact, the kernel offers a vm86 syscall, which is an emulation of real mode.

This syscall was used for a long time in the vesa driver for X (before it got replaced with the vesafb kernel driver; see e.g. here for some code), there are projects like Linux Real-Mode Interface which use it to provide a DPMI-like interface to real mode BIOS, etc.

However, the problem with using the BIOS calls for disk access is that those depend on in-memory tables that are not preserved when Linux boots – in particular because different BIOSes do this differently. Therefore it was much easier and cleaner for Linux to just provide its own drivers for disk access than to try to deal with this mess.

That's why the kernel never featured such a driver.

So for your particular case, you'd need to write the drivers in one way or other. You can either go through the trouble to figure out where how the BIOS of the IPCS stores the tables, or you can figure out how your existing drivers work. Both would presumably have some way to communicate with the AS/400 host to transfer blocks from and to the emulated disk. So reverse engineering is needed anyhow, and then you can write the Linux drivers for it.

This looks like a fun project, but it probably can be quite time consuming.