# Direct access of SCSI/ATA drives from userland



## jacks (Sep 19, 2012)

Hi all,

I'm developing a userland utility for FreeBSD. The utility description follows.

The utility have options to select the hard drives and tape drives attached to the system.
Once the user selects a tape drive or hard drive, he/she can _read_, _write_, _reset_, _standby_(in case of ATA), _idl_e(in case of ATA), _start_, _stop_, issue identify device(in case of ATA), and _inquiry_ in case of SCSI device.

Now the point is, I need to make sure that this device _must not_ be mounted at all, so that user can't directly read/write to the device.

Another constraint is that there _must not be sorting/scheduling of disk/drive I/O requests_, not even at the kernel level.
e.g. If user read 10 LBAs beginning from LBA 5000 say, and the LBA 1000, and then LBA 6000, then the i/o requests must be passed in this same order to the disk/drive - ie there must not be reordering of disk i/o requests, and the kernel _must not cache/buffer the read/write data _.

Another constraint is that only the utility's process/thread must be able to access that drive, and _no other userland process_. 


I do not need filesystem access, just raw sector/byte read/write, for the selected drive.


Till now after reading docs and books related to FreeBSD, it seems that I need to write a kernel mode driver(in form of kernel module) for the purpose, that will communicate with userland  utility via a custom protocol. This driver will _attach_ itself to the drive that is selected by the user from utility. This driver need not concerned for other devices except the selected one, as kernel has already drivers/modules for this. 

This driver will issue appropriate commands to the device selected, and read/write the LBAs directly from/to userland utility buffer.

It seems I may need to write  a custom system call too. Am I right?

The point is, I need direct control of the device selected. The other devices can be read/write normally as they would be.
For e.g. say the utility reads LBA 5000 from the selected device, and somehow device failed to respond after a timeout, then the drive will issue reset command to the device.
The LBA read will be passed to userland process, and no disk i/o scheduling or sorting must be done on the i/o requests made by userland process.

In case of ATA disk drives, I also need to control that whether the data transfer will be via PIO or DMA transfers. 

Can someone enlighten me how could I begin with? I don't know at which I/O _layer_ such a driver should sit - _just above_ CAM peripheral layer, or _at_ CAM peripheral layer or CAM transport layer, or, at just above raw device layer, or at raw device layer itself, or something else.

Also  I want to know is there other way that goes w/o writing such a driver, so that only userland code will do the work effectively.

I'm targetting FreeBSD 9.0-RELEASE and onwards.

Thanks.


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## un_x (Nov 9, 2012)

I am interested to see if you get a worthwhile reply.  I have a somewhat similarly technical question on terminal buffers, and almost 20 years ago, I was trying to get an embedded systems project going with FreeBSD, and needed direct access to some non-standard devices, and I could never get any answers, so I had to drop FreeBSD.  Basically, you want to know how to write device drivers, as I did.  I sure wish there was a technical "programmers reference manual" to guide programmers into the lower-levels of the operating system, but when I expressed this at the time, the standard reply was that the system was changing so much that it was impossible to document these kind of things.  And people that do have the knowledge probably don't have time to lay things out in documentation.  The other common answer was to "read the source and figure it out myself" - which would be very time consuming, to say the least.


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## wblock@ (Nov 9, 2012)

This thread talks about Joseph Kong's device driver book.


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## un_x (Nov 9, 2012)

On another thread, there seems to be a new book out on this topic!  Yahoo.  Might be worthwhile getting - the thread discusses 2 books, one on RootKits, and another on Device Drivers ...

http://forums.freebsd.org/showthread.php?t=31610

Maybe that will help us.


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