Memory Mapped Files

1
Memory Mapped Files

• Using the Linux mechanism for direct access to
  device data

2
Typical file access senario

• Use open(), lseek(), read(), write(), close()
• Each involves two privilege-transitions
• At most 4096 bytes transferred each time
• Inefficient for non-sequential data access

3
Alternative use mmap()

• Take advantage of the paging mechanism
• Associate virtual addresses with the data
• Similar to swapping or page-cacheing
• Simple standard C programming API
• mmap() creates the memory mapping
• munmap() deletes the memory mapping
• Example look at dump.cpp on website

4
Four easy steps

• 1) open the file
• 2) map the file
• 3) use the file
• 4) unmap the file and close the file

5
Device memory mapping

• Recall our vram.c character driver
• Allowed users to access display memory
• But lacks efficiency for serious graphics
• We implement driver mmap() method

6
mmap() driver-method

• Ideas from LDD textbook Chapter 13
• But also required lots of experimentation
• Four steps in the mmap() method
• 1) compute maps starting-point and length
• 2) check cannot map past end-of-memory
• 3) mark mapped area as non-swappable
• 4) request kernel to set up the page-tables

7
Information from vm_area_struct

• vm_start is starting address in user-space
• vm_end is ending address in user-space
• vm_pgoff is page-offset in device memory
• vm_page_prot is page-protection bitmap
• vm_flags is bitmap of requested attributes
• EAGAIN error-code tells kernel try again

8
Comparing execution-times

• We ccan use our tsc.c device-driver
• Step 1 read and save timestamp counter
• Step 2 perform our drawing operations
• Step 3 read and save timestamp counter
• Step 4 subtract timestamp counter values
• Step 5 report the number of CPU cycles