Re: FTL and flash drivers for ISA flash cards and DiskOnChip devices.

Philip Gladstone (
Tue, 09 Sep 1997 17:43:40 -0400

David Woodhouse wrote:
> I'm putting together a driver for the above devices. What I think I ought to
> do is pretend that it's a PCMCIA device, and use the existing ftl_cs drivers
> to handle the filesystem on it.
> Could anyone offer any advice on the feasibility of getting a standard ISA
> device to offer itself through CardServices, and of getting all the PCMCIA
> drivers to compile statically into the kernel, as this device needs to be able
> to hold the root filesystem?

For an internal project, I wrote an emulator for card services that
supports enough
to be able to load and run memory_cs.o and ftl_cs.o and have them talk
to the
embedded flash. I had the advantge that my system was not (also) a
so I didn't have to worry about compatibility.

For the general case, I'd be inclined to use macros to change the name
of all
the external entry points in memory_cs and ftl_cs and build the same
of stub that I did (for whatever type of flash you have). Then you
should be
able to link it straight into the kernel.

My stub is around 1100 lines and supports the following operations:

case GetCardServicesInfo:
case RegisterClient:
case DeregisterClient:
case RegisterEraseQueue:
case DeregisterEraseQueue:
case GetFirstRegion:
case GetNextRegion:
case OpenMemory:
case ReadMemory:
case WriteMemory:
case CloseMemory:
case CheckEraseQueue:
case ReportError:

I also took lots of shortcuts - for example, I only support erasing a
block per request. It turns out that that is all that is needed. [My
strategy was to implement functions as required. Once done, a quick code
inspection of the memory_cs and ftl_cs revealed that no more operations

As a side note, the ftl_cs code reveals a significant problem with the
layer -- there is no way for the filesystem to signal that a block is
unused. This
is important because once a flash disk is full, deleting files does not
the number of (flash) allocated blocks. This has an impact on the write


Philip Gladstone                           +1 617 487 7700
Raptor Systems, Waltham, MA