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I am looking into the fpga driver code which will write some value to FPGA device at low level. At top level in user space value is being written to /dev/fpga, now I guess this is the logic how driver gets its value from user-space and exposed file in user space is "/dev/fpga". But now how actually this value from fpga is reached to device , there must some callback maintained.

But I really could not figure out how it actually happens,Is there any standard way?

Anybody can help me find out this userspace to kernel space link.

Amit Singh Tomar
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  • I don't understand your question. Perhaps some diagrams or source-code might help? – marko Sep 19 '13 at 11:38
  • Few days back I asked same kind of question where how callbacks for sysfs is mainted,http://stackoverflow.com/questions/18405700/how-callback-is-maintained-from-userspace-to-kernel-space – Amit Singh Tomar Sep 19 '13 at 11:48

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It's probably a character device. You can create one in your kernel module, and your callback functions will be called in the kernel when it is opened, something is written to it etc. See:

http://linux.die.net/lkmpg/x569.html

for an explanation how it works and sample code.

ldx
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  • Thanks ld for your response ,Only Doubt I have is in userspace when we we call write(x,y,z) with three arguments ,how at low level fpga driver code it is called with four arguments fpga_rf_write(x,y,z,n)?? – Amit Singh Tomar Sep 19 '13 at 12:03
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    @AmitSinghTomar, fpga_rf_write() might be called from write() of that char driver. check `file_operations struct` which will be registered probably in init_module. – Jeyaram Sep 19 '13 at 12:21
  • This point I understood but how system call is linked to the driver code,When we call write in userspace it shoud go to sys_write in kernel space with same number of arguments and then from there how it links to driver code with one more extra argument?? – Amit Singh Tomar Sep 19 '13 at 14:51
  • Driver `write()` operation supports multiple userspace functions. For example, `pwrite()` writes data at a certain offset into a file. So for a regular `write()` userspace call, the kernel tracks the current write position of the file descriptor, and calls the driver `write()` operation at that file offset. – Peter Sep 19 '13 at 16:46
  • Thanks Peter for your comment ,Its almost same happening in my driver code.Could you please provide some good docs on same ,how driver functions are linked to user-space function and do these driver function are standard?? – Amit Singh Tomar Sep 24 '13 at 17:36