Unable to read PCI BAR from kernel mode

I have written some code to probe registers of an AMD Vega from userspace using a mmap of the /sys resource. This is working and I am able to read the register values without any problems. However when I implement the kernel mode version of this, all reads return 0xffffffff. I feel like I am missing something simple as the userspace code is returning expected values, where the kernel code fails to read the registers.

Here is the working userspace code:

#include <fcntl.h>
#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/mman.h>
#include <stdint.h>

#define RES_PATH "/sys/devices/pci0000:40/0000:40:01.3/0000:42:00.0/0000:43:00.0/0000:44:00.0/resource5"
#define RES_SIZE (512*1024)

#define MP0_BASE                     0x16000
#define mmMP0_SMN_C2PMSG_33          ((MP0_BASE + 0x61) * 4)
#define mmMP0_SMN_C2PMSG_64          ((MP0_BASE + 0x80) * 4)
#define mmMP0_SMN_C2PMSG_81          ((MP0_BASE + 0x91) * 4)

void * map;

uint32_t _preadl(const char * name, uint32_t reg)
{
  uint32_t val = *(uint32_t*)(map + reg);
  printf("readl %20s (0x%08x / 0x%08x) = 0x%08x\n", name, reg / 4, reg, val);
  return val;
}

#define preadl(x) _preadl(#x, x)

int main(int argc, char * argv[])
{
  int fd = open(RES_PATH, O_RDWR | O_SYNC, 0);
  map = mmap(NULL, RES_SIZE, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
  printf("mmap = %p\n", map);

  preadl(mmMP0_SMN_C2PMSG_33);
  preadl(mmMP0_SMN_C2PMSG_64);
  preadl(mmMP0_SMN_C2PMSG_81);

  return 0;
}

And here is the non working kernel code:

static int reset_amdgpu_vega(struct pci_dev *dev, int probe)
{
#define MP0_BASE                        0x16000
#define mmMP0_SMN_C2PMSG_33             ((MP0_BASE + 0x61) * 4)
#define mmMP0_SMN_C2PMSG_64             ((MP0_BASE + 0x80) * 4)
#define mmMP0_SMN_C2PMSG_81             ((MP0_BASE + 0x91) * 4)

        resource_size_t mmio_base, mmio_size;
        void __iomem *mmio;
        int ret;
        int i;
        uint32_t val;

        if (probe)
                return 0;

        mmio_base = pci_resource_start(dev, 5);
        mmio_size = pci_resource_len(dev, 5);
        mmio = pci_iomap(dev, 5, 0);
        if (mmio == NULL) {
                printk(KERN_ERR
                       "[reset_amdgpu_vega] failed to map the resource\n");
                ret = -ENOMEM;
                goto out;
        }

        printk(KERN_INFO "mmio: %llx %llx %lx\n",
               mmio_base,
               mmio_size,
               (uintptr_t)mmio);

        printk(KERN_INFO "regs: %08x %08x %08x\n",
                mmMP0_SMN_C2PMSG_33,
                mmMP0_SMN_C2PMSG_64,
                mmMP0_SMN_C2PMSG_81);

        printk(KERN_INFO "vals: %08x %08x %08x\n",
               ioread32(mmio + mmMP0_SMN_C2PMSG_33),
               ioread32(mmio + mmMP0_SMN_C2PMSG_64),
               ioread32(mmio + mmMP0_SMN_C2PMSG_81));

        pci_iounmap(dev, mmio);
        ret = 0;
out:
        return ret;
}

The problem was that before the pci reset quirk is called, the kernel turns off the PCI_COMMAND_MEMORY flag on the COMMAND register. By doing so the device is no longer readable. The solution is to re-enable the flag first:

 pci_read_config_word(dev, PCI_COMMAND, &cfg);
 cfg |= PCI_COMMAND_MEMORY;
 pci_write_config_word(dev, PCI_COMMAND, cfg);