Under what condition I can "write" but cannot "read" from a location?

Question

The purpose is to do DMA transfer betwee Processing System (PS) and Programmable Logic (PL) for a ZYNQ 7000 board. I am trying to use Userspace Input Outut (UIO) framework. I have a UIO device in my ZYNQ board. I am using mmap to access the UIO device. From my application I can write to the device but cannot read, what can cause such behaviour?

I am given below only the lines that is relavant, ommiting all other lines. I am not sure the other lines are relavant or not. Other lines are unchanged. In the first case, the write command return err=4 which is correct but in the second case, "read", nothing is returned?

fd_uio = open("/dev/uio0",O_RDWR);
mmap(NULL, size, PROT_READ | PROT_WRITE, MAP_SHARED, fd_uio, (off_t) 0);
err=write(fd_uio, (void *)&pending, sizeof(int));

fd_uio = open("/dev/uio0",O_RDWR);
mmap(NULL, size, PROT_READ | PROT_WRITE, MAP_SHARED, fd_uio, (off_t) 0);
err=read(fd_uio, (void *)&pending, sizeof(int))

The below is edited code in case we can use memcpy() instead of read?

fd_uio = open("/dev/uio0",O_RDWR);
fd_mem = open("/dev/mem",O_RDWR);
fromPtr=mmap(NULL, size, PROT_READ | PROT_WRITE, MAP_SHARED, fd_uio, (off_t) 0);
destPtr=mmap(NULL, size, PROT_READ | PROT_WRITE, MAP_SHARED, fd_mem, (off_t) 0);
memcpy( destPtr, fromPtr, size);

EDIT:

uint32_t Val       =89;
void *regs =   mmap(NULL, size, PROT_READ | PROT_WRITE, MAP_SHARED, fd_uio, (off_t) 0);
    *((volatile uint32_t *)(regs))=Val;
    uint32_t dma_status=*((volatile uint32_t *)(regs+4));

    printf("Value of errno: %d\n", errno);
    printf("dma_status %" PRIu32 "\n\r",dma_status);

In the above code I was expecting the dma_status to be 89 but I do not get 89. I get 0 if (regs+4) or I get 65563 if I do regs.

The following may help. I have tried to use reserve memory by adding the following lines in system-user.dtsi file:

    reserved-memory{
        #address-cells=<1>;
        #size-cells =<1>;
        ranges;
        dma_reserved:buffer@0{
            compatible="shared-dma-pool";
            no-map;
            reg=<0x30000000 0x1000000>;
        };
    };
    dma_proxy@0 {
        compatible ="xlnx,dma_proxy";
        dmas = <&axi_dma_0 1>;
        dma-names = "dma0";
        memory-region = <&dma_reserved>;
    };

But when I try to check if the reserved memory is there or not in the board, I get the following.

two:/home/petalinux# dmesg | grep eserve
OF: fdt: Reserved memory: failed to reserve memory for node 'buffer@0': base 0x00000005, size 0 MiB
OF: fdt: Reserved memory: failed to reserve memory for node 'buffer@0': base 0x00000000, size 512 MiB
cma: Reserved 16 MiB at 0x3f000000
Memory: 998020K/1048576K available (7168K kernel code, 243K rwdata, 1904K rodata, 1024K init, 118K bss, 34172K reserved, 16384K cma-reserved, 245760K highmem)
hw-breakpoint: found 5 (+1 reserved) breakpoint and 1 watchpoint registers.

Further edit:

The problem may be that when I type dmesg | grep eserve I get
0F: fdt: Reserved memory: failed to reserve memory for node 'buffer@0': base 0x00000005, size 0 Mib
and

0F: fdt: Reserved memory: failed to reserve memory for node 'buffer@0: base 0x00000000, size 512 Mib and

cma:Reserved 16 MiB at 0x3f000000 The output of cat /proc/iomem is below

three:~$ sudo cat /proc/iomem                                                                                       
00000000-2fffffff : System RAM
  00008000-00afffff : Kernel code
  00c00000-00c5c727 : Kernel data
31000000-3fffffff : System RAM
e0001000-e0001fff : xuartps
e000a000-e000afff : e000a000.gpio gpio@e000a000
e000b000-e000bfff : e000b000.ethernet ethernet@e000b000
e000d000-e000dfff : e000d000.spi spi@e000d000
e0100000-e0100fff : e0100000.mmc mmc@e0100000
f8003000-f8003fff : dma-controller@f8003000
  f8003000-f8003fff : f8003000.dma-controller dma-controller@f8003000
f8005000-f8005fff : f8005000.watchdog watchdog@f8005000
f8007000-f80070ff : f8007000.devcfg devcfg@f8007000
f8007100-f800711f : f8007100.adc adc@f8007100
f8801000-f8801fff : etb@f8801000
f8803000-f8803fff : tpiu@f8803000
f8804000-f8804fff : funnel@f8804000
f889c000-f889cfff : ptm@f889c000
f889d000-f889dfff : ptm@f889d000
fffc0000-fffcffff : fffc0000.sram sram@fffc0000