Memory Mapped Files

Using the Linux mechanism for direct access to device data

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

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

Four easy steps

• 1) open the file

• 2) map the file

• 3) use the file

• 4) unmap the file and close the file

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

‘mmap()’ driver-method

• Ideas from LDD textbook: Chapter 13

• But also required lots of experimentation

• Four steps in the ‘mmap()’ method

1) compute map’s 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

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’

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