[Etherboot-developers] IDE implementation ramblings....

[Eric W B. <ebi...@ln...>]

Starting with the IDE driver that has been in various etherboot
patches I am successfully booting from ide.  Hopefully I will check
this in the next couple of days.

IDE is a strange beast.  Reading and writing to a drive is easy.

Reliably finding a drive immediately after power up is hard.
Especially if you don't want to wait 31 seconds which is the worst
case drive spinup time.  The problem is that IDE drives don't respond
to any commands until after they have spunup.  And the maximum legal
spinup delay is 31 seconds so there is quite a wait before a command
can be said to have failed.

So I don't have to wait 31 seconds in every case I depend on the BSY
bit being set on the drive during spinup.  And then I just wait for
that bit to clear.  According to the IDE spec, IDE drives must set
their BSY bit during spinup, and on every board I have tested this
works the IDE controller relays this bit.  But I have reports this
technique does not work on every board.

Past that I weed out a lot of false positives, by writing 
multiple drive registers with different values, reading them back, and
seeing if any of the values are different.  This quickly weeds out
empty cables and the like.  So only in very rare circumstances do I
need to worry about waiting for a full command timeout to detect a
drive is actually missing from the system.

I don't plan to implement drive speed tuning.  For boot up that should
not be an issue.  I don't do DMA transfers either but that is just
because the PIO transfers are trivial to setup, and since I
performance isn't a major objective this shouldn't be an issue.

There are still a lot of pieces of the code that need little bits of
attention before it integrates in well with etherboot.  Roughly I need
to make IDE a layer like pci, or ISA_PNP.  And put my ide_disk code on
top of that, allowing for further developments like cdrom support.
Plus I need to implement code to actually find the image to boot on
the hard drive instead of just hard coding a fixed offset. 

But as I take the little problems one at a time they all seem to have
pretty decent solutions so I'm not to worried.

Sizewise the ide driver has about a 2k text section.  So it is about
the same size as a NIC driver.  So no extraordinary size impacts are
present.

Eric