Re: returning scsi I/O to SCSI mid layer from Fast IRQ context

[James Smart <James.Smart@Emulex.Com>]

The midlayer is written to accept scsi i/o completion (via scsi_done())
from the context of an interrupt handler.  The scsi_done() path actually
just traverses up the stack a little, then queues the request on a done
queue, and scheduls a softirq thread for actual completion processing.

The belief is the best-performing drivers call the command done routine
from the interrupt handler (otherwise, you may have 2 context switches
before completion processing).

-- james s

Parav Pandit wrote:
> Let me simplify and ask in different way.
> 
> 1. Can I return a scsi I/O by invoking scsi_done()
> from the Fast IRQ context in the Low level SCSI HBA
> driver?
> 
> 2. Is the interrupt handler right place to call
> scsi_done() and invoke SCSI mid layer? or should I
> defer to the DPC thread context?
> 
> Parav
> 
> 
> 
> --- Parav Pandit <paravpandit@yahoo.com> wrote:
> 
>> Hi,
>>
>> A low level SCSI HBA driver can return the I/O back
>> to
>> the OS from various paths.
>>
>> Some of the common possible paths
>> 1, enqueue_command itself
>> 2. Timer handler (at the time of command timeout)
>> 3. Interrupt handler (when a response is received
>> from
>> the storage)
>> 4. DPC handler thread.
>>
>> Whenever an I/O is returned from the interrupt
>> handler
>> by invoking scsi_done callback function, it will
>> invoke the SCSI mid layer.
>>
>> 1. Is it good practice to return call scsi_done
>> callback function from Interrupt handler? 
>> If no, may I know the implications of doing that?
>> (other then blocking the interrupt context for a
>> longer time)
>>
>> 2. if multiple I/Os are returned in a single
>> interrupt, how many  commands should be returned by
>> calling scsi_done before shifting to dpc thread?
>>
>> Regards,
>> Parav Pandit
>>
>>
>>
>>        
>>
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