| README on the Compact Flash for Card Engines | |
| ============================================ | |
| There are three challenges in supporting the CF interface of the Card | |
| Engines. First, every IO operation must be followed with IO to | |
| another memory region. Second, the slot is wired for one-to-one | |
| address mapping *and* it is wired for 16 bit access only. Second, the | |
| interrupt request line from the CF device isn't wired. | |
| The IOBARRIER issue is covered in README.IOBARRIER. This isn't an | |
| onerous problem. Enough said here. | |
| The addressing issue is solved in the | |
| arch/arm/mach-lh7a40x/ide-lpd7a40x.c file with some awkward | |
| work-arounds. We implement a special SELECT_DRIVE routine that is | |
| called before the IDE driver performs its own SELECT_DRIVE. Our code | |
| recognizes that the SELECT register cannot be modified without also | |
| writing a command. It send an IDLE_IMMEDIATE command on selecting a | |
| drive. The function also prevents drive select to the slave drive | |
| since there can be only one. The awkward part is that the IDE driver, | |
| even though we have a select procedure, also attempts to change the | |
| drive by writing directly the SELECT register. This attempt is | |
| explicitly blocked by the OUTB function--not pretty, but effective. | |
| The lack of interrupts is a more serious problem. Even though the CF | |
| card is fast when compared to a normal IDE device, we don't know that | |
| the CF is really flash. A user could use one of the very small hard | |
| drives being shipped with a CF interface. The IDE code includes a | |
| check for interfaces that lack an IRQ. In these cases, submitting a | |
| command to the IDE controller is followed by a call to poll for | |
| completion. If the device isn't immediately ready, it schedules a | |
| timer to poll again later. |