remap_page_range an caching

[thomas joseph]

Hi All,

 I am using remap_page_range to map a memory region
 to user space. (This is a device memory mapped in 
 to kernel space by ioremap_nocache.)

 I am setting the page protection flags as
  _PAGE_PCD | _PAGE_PWT to disable caching as 
  specified in the  documentation. 

  But when I try to read these device registers in the
  user space it shows different values from
  those of driver. I declared the necessary
  members as volatile. But it still does not help.

  When I try to read the register twice I seem to be
getting the correct value. 

  Like,
   /* ulBWStatus is defined as volatile. */
   status = pRpRegs->ulBStatus;
   status = pRpRegs->ulBStatus;
This gives me the updated values. Could you
just help me if I am missing anything here.

 D0 I need to insert a memory barrier in the user
space
 before accessing these registers ?
 If I do so then it looks like giving correct values
too.

 Could somebody help to understand on what is
happening here.

Thanks in advance.

best regards,

 --thomas

 

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