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1da177e4 LT |
1 | README on the Compact Flash for Card Engines |
2 | ============================================ | |
3 | ||
4 | There are three challenges in supporting the CF interface of the Card | |
5 | Engines. First, every IO operation must be followed with IO to | |
6 | another memory region. Second, the slot is wired for one-to-one | |
7 | address mapping *and* it is wired for 16 bit access only. Second, the | |
8 | interrupt request line from the CF device isn't wired. | |
9 | ||
10 | The IOBARRIER issue is covered in README.IOBARRIER. This isn't an | |
11 | onerous problem. Enough said here. | |
12 | ||
13 | The addressing issue is solved in the | |
14 | arch/arm/mach-lh7a40x/ide-lpd7a40x.c file with some awkward | |
15 | work-arounds. We implement a special SELECT_DRIVE routine that is | |
16 | called before the IDE driver performs its own SELECT_DRIVE. Our code | |
17 | recognizes that the SELECT register cannot be modified without also | |
18 | writing a command. It send an IDLE_IMMEDIATE command on selecting a | |
19 | drive. The function also prevents drive select to the slave drive | |
20 | since there can be only one. The awkward part is that the IDE driver, | |
21 | even though we have a select procedure, also attempts to change the | |
22 | drive by writing directly the SELECT register. This attempt is | |
23 | explicitly blocked by the OUTB function--not pretty, but effective. | |
24 | ||
25 | The lack of interrupts is a more serious problem. Even though the CF | |
26 | card is fast when compared to a normal IDE device, we don't know that | |
27 | the CF is really flash. A user could use one of the very small hard | |
28 | drives being shipped with a CF interface. The IDE code includes a | |
29 | check for interfaces that lack an IRQ. In these cases, submitting a | |
30 | command to the IDE controller is followed by a call to poll for | |
31 | completion. If the device isn't immediately ready, it schedules a | |
32 | timer to poll again later. |