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    <p>Realistically, if you want this level of detail control you <em>are</em> going to end up having to write your own low-level drivers.</p> <p>Your constraint about avoiding I/O buffering and scheduling may be particularly challenging - you might avoid DMA, but a modern processor has its I/O rather decoupled from internal operations for performance reasons. Perhaps if you can fully disable all interrupts you might at least be able to timestamp when you do things. You will probably want the drive on it's own interface adapter, certainly not sharing with a running filesystem. </p> <p>Doing things from userspace would probably require working by means of proxies in the kernel - you'll need to do your timing critical things on the kernel side though.</p> <p>A far simpler solution, if it would meet your needs, would be to use a USB-to-SATA or PATA adapter. You can tell the existing kernel drivers to ignore the VID/PID using the quirks mode of modprobe, then talk to the device via libusb from userspace. However, there will certainly be latency there.</p> <p>For the finest level of control, you probably need to connect the drive to an embedded processor which has no I/O buffering, or perhaps even an FPGA. This was not particularly hard to do for low data rates with PATA, SATA probably requires specialized transceivers but may not be out of the realm of possibility (or perhaps you can work through one of the adapters). You would probably end up connecting this custom peripheral to a PC via USB or even a serial port, and using that to issue tasks and obtain results (would be convenient if you set it up so the PC can download the device's firmware/bitstream, so you have flexibility).</p>
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    1. POBypassing I/O scheduling and linux kernel page buffering
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    1. COThanks for throwing light on alternative approaches. But, as I mentioned, I need to access the ATA Status and ATA Error registers of Hard Drive to report actual ATA status and error to my application, and with USB or any other interface except PATA/SATA interface, it wouldn't be possible, as far as I know. Also, to implement custom FPGA and Embedded processor is too costly for me. The data transfer(reads from hard drive) would also be quite difficult for me, as I have to implement full ATA protocol inside that FPGA or embedded processor.
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    2. COAt this point I understand that I need to write a low-level driver, but what I don't know is at what level this driver should reside in linux kernel and whether it should communicate with libata directly or with PATA/SATA HBA driver directly. And one more thing, how should my application talks to this driver bypassing VFS(if possible), kernel buffer and I/O scheduler. I have never written a linux driver in my life till now!
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    3. COIf the project is even possible on PC-type hardware, you should write it at the lowest possible level, ie the one which talks directly to the processor bus-to-PATA or SATA bridge device.
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