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SubjectRe: [RFC] New Driver Model for 2.5
At 10:57 AM +0100 10/24/01, Alan Cox wrote:
> > the HW or not when getting a new request). In cases where a mid-layer
>> enters the scene, like SCSI, that wants to do timeouts, then well...
>> we can let it timeout (just stop processing requests), or we can
>> have the midlayer go to sleep as well :) That later solution
>> may cause some interesting ordering issues however...
>For scsi you have to complete the pending commands, you don't know what the
>transaction granularity is in some cases and half completing the sequence
>won't help you. In addition the upper layers have to queue additional scsi
>commands to do stuff like cd drawer locking and to ask the drive firmware
>to enter powerdown modes
>> For USB, for example, we can consider that when a device driver
>> (not a controller driver) suspend has been done, any URB it submits
>> can just be dropped (returned immediately with an error). We don't
>> need blocking here neither. Of course, that means we have the
>> framework to call devices' suspend/resume callbacks when the
> > controller is about to go to sleep.
>That will scramble large numbers of devices. Randomly erroring pending block
>writes is -not- civilised.

In our "extreme prejudice" suspend (this is in the context of masking
& recovering from a fault in a fault-tolerant machine) we have cases
in which completion of pending commands isn't possible. Our solution
is to issue a SCSI bus reset, and terminate all outstanding commands
with an appropriate (retryable) error. This is especially easy to
implement in drivers that use SCSI bus reset as a routine (though
last resort) error recovery mechanism, since the requisite logic is
already in place. Not pretty, I suppose, but effective.

One model we've considered (but haven't implemented yet) is to make
parents in the device tree responsible for suspending their children,
so the suspend propagates down the tree and each node "knows" how to
suspend its children, assuming any special action is required. So a
SCSI HBA, for example, would be asked by its bus parent to suspend,
and in turn would suspend its SCSI device children before suspending
itself. I'm not quite sure how virtual device layers like md would
fit into this scheme, since they can cut across device and power

At 11:54 AM +0100 10/24/01, Alan Cox wrote:
>So the scsi devices hang off sd, sr etc which in turn hang off scsi and
>the controllers hang off scsi (and or the bus layers)

Our first implementation was under Solaris 2.x (SPARC) in which the
parent->child relationship is bus->hba->sd. scsi isn't in the tree;
it's more of an interface layer between hba & sd. fwiw.
/Jonathan Lundell.
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