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- 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.
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