Re: [PATCH 0/10] megaraid_sas: Updates for scsi-misc

[Hannes Reinecke <hare@suse.de>]

On 10/10/2011 02:56 AM, Douglas Gilbert wrote:
> On 11-10-09 10:10 AM, James Bottomley wrote:
>> On Sun, 2011-10-09 at 09:56 -0400, Douglas Gilbert wrote:
>>> On 11-10-08 09:14 PM, adam radford wrote:
>>>> James/Linux-scsi,
>>>>
>>>> The following patch series for megaraid_sas brings the driver up
>>>> to v6.12-rc1:
>>>>
>>>> 1. Continue booting immediately if FW in FAULT at driver load time.
>>>> 2. Increase default cmds per lun to 256.
>>>> 3. Fix mismatch in megasas_reset_fusion() mutex lock-unlock.
>>>> 4. Remove some un-necessary code.
>>>> 5. Clear state change interrupts for Fusion/Invader.
>>>> 6. Clear FUSION_IN_RESET before enabling interrupts.
>>>> 7. Add support for MegaRAID 9360/9380 12GB/s controllers.
>>>> 8. Add multiple MSI-X vector/multiple reply queue support.
>>>> 9. Add driver workaround for PERC5/1068 kdump kernel panic.
>>>> 10. Version and Changelog update.
>>>
>>> I haven't checked all SAS HBAs in Linux but of those that I
>>> have checked only one doesn't support the SMP pass-through
>>> in the bsg driver. And that HBA driver is megaraid_sas **.
>>
>> Technically the megaraid_sas isn't really a SAS HBA, it's a RAID one
>> that just happens to have SAS/SATA disk attachments. None of the RAID
>> HBA's (including the IBM power raid) expose a BSG interface or even
>> attach properly to the in-kernel SAS interfaces.
>
> In JBOD mode MegaRaid SAS controllers are SAS HBAs
> (marketing BS aside). Their firmware has the same
> SMP pass-through as LSI's MPT SAS Fusion controllers.
>
> There is no reason why a MegaRaid controller user
> couldn't place a SAS-2 expander in front of an array
> of disks and make some disks available to a MegaRaid
> volume. Other array disks could be used by other
> machines and the different sets could be isolated
> from each other with SAS-2 zoning. The problem then
> would be that via a MegaRaid SAS controller a user
> could not monitor or control that zoning. Seems a
> bit wasteful to add a "real" SAS HBA just to get
> that capability.
>
Yes, true. But that's the megaraid SAS design.
It's actually the same with every other RAID controller.
Everyone (well, every reasonably recent one) supports SAS, but
virtually no-one exposes them as such.

Problem here is that the RAID controller have invented
their own command set to talk to the RAID firmware.
Most RAID firmware have support for SCSI passthrough
commands (that's how we managed to get rid of cciss :-)
but virtually none of those have support for transport-specific
passthrough commands.

So yes, they are using the same SAS cores as 'normal'
SAS HBAs do. But no, there is no way how you could get
through to them. Or, at least, no documented one.

Would be cool to have them, but you could _severely_ screw up 
operations by using them.

Cheers,

Hannes
-- 
Dr. Hannes Reinecke		      zSeries & Storage
hare@suse.de			      +49 911 74053 688
SUSE LINUX Products GmbH, Maxfeldstr. 5, 90409 Nürnberg
GF: J. Hawn, J. Guild, F. Imendörffer, HRB 16746 (AG Nürnberg)
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