xdma0_user lseek fails

[KOVI89alipes]

Hi,

I have the following setup

• Dell workstation
• Alveo U50 card
• XDMA IP Core 4.1 (reference design, configured through JTAG cable)
• Ubuntu 18.04
• Xilinx DMA IP drivers from 1fe2bd0

The problem - can't change the offset for the xdma0_user device. I tried to open this device in 3 different modes from C++ - as a stream, file and char device, all 3 ways are failing. All of 3 ways work fine for xdma0_[h2c][c2]_[0..3] devices.

See the full error reproducing code here
https://paste.ofcode.org/Q5iP3cVfzH9mevswKLXUXm

Errors from stdout for xdma0_user.

STREAM:	Seekg Before -1, after  -1
CHAR:  	Leek return code  0xFFFFFFFFFFFFFFFF (d-1) ERROR 
POSIX: 	Fseek return code  -1 

Two aspects of this problem

1. When I call it for

    FILE *pFile = nullptr;
    pFile = fopen( dev.c_str(), "w+");

    if (pFile){
        fprintf(stdout, "POSIX: \t");
        auto rc  =  fseek ( pFile , offset , SEEK_CUR );
        fprintf(stdout, "Fseek return code  %d \n",rc);   

it leads to the following error in kernel log

[112106.318541] xdma:char_ctrl_ioctl: cmd 0x5401, xdev 0x00000000753b09a1, pdev 0x00000000ad1dd365.
[112106.318544] xdma:char_ctrl_ioctl: cmd 21505, bad magic 0x54/0x78

From

dma_ip_drivers/XDMA/linux-kernel/xdma/cdev_ctrl.c

Line 146 in 1fe2bd0

if (_IOC_TYPE(cmd) != XDMA_IOC_MAGIC) {

2. When I open xdma0_user device as a stream, I can't read .tellg() value

        std::fstream is ( dev );
        if (is) {
            fprintf(stdout, "STREAM:\t");
            int length_b = is.tellg(); // -1 here for xdma0_user

http://www.cplusplus.com/reference/istream/istream/tellg/
So something is wrong even before offset changing

[jpiat]

If you look at the reg_rw tool you'll notice that the way to use the xdma0_user is by mmap'ing it. After doing the mmap you can access your memory through the mmap pointer.

[JamieLuo]

The xdma0_user device does not implement the lseek, it was using the mmap to access the bar space.

[Prandr]

The answers above are not quite correct, because it is not the only and IMO not the best way to access xdmaN_user. Upstream XDMA already implements read/write functions, which can be utilised with Linux POSIX pread/pwrite system functions.

I've had the same problem with lseek and I created #299, which adds lseek to all interfaces. Please have a look and check.

I would like to add that <stdio.h> with fseek shouldn't be used with XDMA as it has those side effects with ioctl. Only pure Linux POSIX file operation functions (including lseek) must be used.