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