Re: [PATCH v9 06/10] ata: zpodd: check zero power ready status

[Aaron Lu <aaron.lu@intel.com>]

On 11/28/2012 09:39 AM, Tejun Heo wrote:
> Hey, Rafael.
> 
> On Wed, Nov 28, 2012 at 01:51:00AM +0100, Rafael J. Wysocki wrote:
>> Having considered that a bit more I'm now thinking that in fact the power state
>> the device is in at the moment doesn't really matter, so the polling code need
>> not really know what PM is doing.  What it needs to know is that the device
>> will generate a hardware event when something interesting happens, so it is not
>> necessary to poll it.
>>
>> In this particular case it is related to power management (apparently, hardware
>> events will only be generated after the device has been put into ACPI D3cold,
>> or so Aaron seems to be claiming), but it need not be in general, at least in
>> principle.
>>
>> It looks like we need an "event driven" flag that the can be set in the lower
>> layers and read by the upper layers.  I suppose this means it would need to be
>> in struct device, but not necessarily in the PM-specific part of it.
> 
> We already have that.  That's what gendisk->async_events is for (as
> opposed to gendisk->events).  If all events are async_events, block
> won't poll for events, but I'm not sure that's the golden bullet.
> 
> * None implements async_events yet and an API is missing -
>   disk_check_events() - which is trivial to add, but it's the same
>   story.  We'll need a mechanism to shoot up notification from libata
>   to block layer.  It's admittedly easier to justify routing through

I don't see a way to do this, since libata has no chance of accessing
the gendisk pointer. Or we can add a new field to struct device,
something like no_poll, but I don't think it is the right thing to do,
as not all devices are block ones.

Any other suggestions/ideas please?

Thanks,
Aaron

>   SCSI tho.  So, we're mostly shifting the problem.  Given that async
>   events is nice to have, so it isn't a bad idea.
> 
> * Still dunno much about zpodd but IIUC the notification from
>   zero-power is via ACPI.  To advertise that the device doesn't need
>   polling, it should also be able to do async notification while
>   powered up, which isn't covered by zpodd but ATA async notification.
>   So, ummm... that's another obstacle.  If zpodd requires the device
>   to support ATA async notification, it might not be too bad tho.
> 
> Thanks.
>