[ECOS] What is a block device?

[Andrew Lunn <andrew at lunn dot ch>]

Hi Folks

Time for a discussion about block devices and there interface.

The cyg_io_bread() and cyg_io_block() functions are not very well
documented. None of the source code documents the exact meaning of the
paramters.

// Write data to a block device
Cyg_ErrNo cyg_io_bwrite(cyg_io_handle_t handle,
                       const void *buf,
                       cyg_uint32 *len,
                       cyg_uint32 pos);

// Read data from a block device
Cyg_ErrNo cyg_io_bread(cyg_io_handle_t handle,
                      void *buf,
                      cyg_uint32 *len,
                      cyg_uint32 pos);

Is len in bytes or blocks? is pos in bytes or blocks?

Currently we have two block devices, disk and flash. disk uses blocks
for both. Flash uses bytes for both. The blib code assumes blocks for
both. eCosCentric's documentation about the disks, which is not part
of anonymous CVS, says the len is in bytes.

My proposal is that we define the block device API to use blocks for
both len and pos. If this is agreed i will add comments to the code to
make this clear. 

If eCosCentric will contribute there Disk IO Package documentation i
will fix it and add it to anoncvs.

The harder question is what to do about the flash code. JFFS2 needs to
be able to write and read in less than blocks. In fact it really wants
a character interface. So we could in fact modify the current code and
turn it into a character device. A block device is possible, with one
block equaling one flash page, but is it useful? Could you put a fatfs
on top of this, when the block size could be anything from 128bytes to
64K?

For the moment i suggest we just put in comments that the flash block
device does not correctly implement the block interface and leave it
alone.

Comments please.

        Andrew




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