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verified publisher icon Xilinx

mmap of a bar fails with EINVAL on kernel 6.6

Open michkrom opened this issue 8 months ago • 5 comments

Hi,

I am porting a yokto system from 5.5 kernel to 6.6 so I had to upgrade the XDMA driver from 2019.2.51 to latest. Now the mmap() to resource0 (bar0 but really any bar) fails with EINVAL.

The mmap() to bar0 is using /sys/devices/.../resource0. It works normally on old os/xdma but not on the new OS giving EINVAL. This is only after loading the xdma module - it is fine if the module is not loaded. There is no log output from the driver for mmap() call (added kprintf into bridge_mmap and build it with DEBUG). mmap() via /dev/xdma_.... works and produces log output. This is also reproducible with a standard desktop linux OS (used Manjaro). For testing I used the tools/reg_rw .

Also, I have hacked the old driver version to build with new kernel, manually apply all API changes, but same result so it's likely not the new vs old xdma driver but rather the kernel version itself.

I think it is some change in kernel or maybe driver that prevents normal handling of mmap syscall? I suspect the EINVAL is possibly because kernel may have linked the sysfs entry to a char device that does not implement mmap? (but I do not know for sure).

Any comments or ideas? Did anything change?

Regards, Michal

michkrom avatar Mar 25 '25 20:03 michkrom

Further information:

  • repeatable on Arch/Manjaro (tried 6.6 and 6.13 kernels)
  • not repeatable on Ubuntu (tried 6.8 and 6.11 kernels)

So likely kernel config options?

michkrom avatar Apr 01 '25 01:04 michkrom

Indeed, it was CONFIG_STRICT_DEVMEM. Apparently, Arch kernel is compiled with it but Ubuntu is not.

michkrom avatar Apr 04 '25 05:04 michkrom

did you find a way to get it to be compatible with CONFIG_STRICT_DEVMEM

hmaarrfk avatar Apr 04 '25 15:04 hmaarrfk

Well, yes. In my yokto I had to disable CONFIG_STRICT_DEVMEM as it was on by default. For Arch, you'd have rebuild the kernel yourself. Alternatively, instead of mmaping via sysfs, you can map via /dev/h2c etc created by the xdma driver.

michkrom avatar Apr 04 '25 16:04 michkrom

Alternatively, instead of mmaping via sysfs, you can map via /dev/h2c etc created by the xdma driver.

I see this was the kind of "pointer" i was looking for.

Thanks for sharing!

  • Your friend in this lack of support from Xilinx!

hmaarrfk avatar Apr 04 '25 16:04 hmaarrfk