Re: totally random "VFS: Cannot open root device"

[Jeff Garzik <jgarzik@pobox.com>]

Tejun Heo wrote:
> Jean-François Stenuit wrote:
> 
>> Hi Tejun,
>>
>> Thanks for taking the time to check.
>>
>> Output of your trace with ata_piix.override_PCS=0
>>  1st boot : success : combined=0 orig_mask=0x11
>>  2nd boot : success : combined=0 orig_mask=0x11
>>  3rd boot : failure : combined=0 orig_mask=0x0
>>  4th boot : failure : combined=0 orig_mask=0x0
>>  5th boot : failure : combined=0 orig_mask=0x0
>>  6th boot : failure : combined=0 orig_mask=0x0
>>  7th boot : success : combined=0 orig_mask=0x11
>> Output of your trace with ata_piix.override_PCS=1
>>  1st boot : success : combined=0 orig_mask=0x0
>>  2nd boot : success : combined=0 orig_mask=0x0
>>  3rd boot : success : combined=0 orig_mask=0x0
>>  4th boot : success : combined=0 orig_mask=0x0
>>  5th boot : success : combined=0 orig_mask=0x0
>>  6th boot : success : combined=0 orig_mask=0x0
>>  7th boot : success : combined=0 orig_mask=0x0
>>  8th boot : success : combined=0 orig_mask=0x0
>>
>> Looks like you have found a fix/workaround for this bug (but it still 
>> does not give the reason why it's failing).
>>
> 
> It probably is a BIOS issue.  The weird thing though is that the port 
> works fine with its corresponding ENABLED bit cleared.  Anyways, if it 
> works by ignoring the ENABLED bit, ignoring should just be fine.
> 
> 0602, can you verify this workaround works on your machine too?
> 
> Jeff (Hi!), if 0602 also confirms that this workaround works, I'll 
> submit a patch to make ata_piix ignore PCS values on ICH5's.  How does 
> that sound to you?

I am being dragged into this thread with little background info.  Here's 
some data points that may be relevant:

* until very recently, ata_piix's definitions for the ENABLED and 
PRESENT bits was reversed.

* the PRESENT bit reflects a device present status just like the SStatus 
phy register does.  This implies that one must wait, before assuming 
that the PRESENT bit's absence truly indicates absence of a device.

* the device may be in various power management states.  It may be wise 
to issue COMRESET by
	- clear ENABLED bit
	- set ENABLED bit
	- wait for device to appear
	- if no device appeared, clear ENABLED bit

In sum, think about the underlying SATA phy registers, and how they 
logically map to the PCS bits.

	Jeff