Re: [PATCH] megaraid_sas: Enable shared tag map

[James Bottomley <jbottomley@parallels.com>]

On Thu, 2014-10-02 at 08:51 +0200, Hannes Reinecke wrote:
> On 10/01/2014 11:10 PM, Christoph Hellwig wrote:
> > On Wed, Oct 01, 2014 at 02:51:31PM -0400, Webb Scales wrote:
> >> Hannes,
> >>
> >> In megasas_change_queue_type(), is it possible for sdev->queue_depth to be
> >> greater than 256?
> >>
> >> Unless I'm misunderstanding the SCSI code, we can request a
> >> queue-depth/tag-map-size which is greater than 256, but, since the
> >> scsi_cmnd::tag field is an unsigned char, depths greater than 256 may
> >> overflow the field when high-numbered tags are used...do I have that right?
> > 
> > Yes, we really need to increase the size of the tag field.  SAM allows
> > a transport specific limit of up to 64 _bytes_ for it, although I don't
> > know implementation that large.  Given that the block layer can generate
> > up to 32-bit tags both for the old blk-tag.c code and the new
> > blk-mq-tag.c version it would be good to use a u32 there.  Can you send
> > me a patch?
> > 
> Weeelll ...
> 
> I'm afraid it's not _that_ easy.
> SCSI-II tagged queueing has some specific tag values:
> 
> #define SIMPLE_QUEUE_TAG    0x20
> #define HEAD_OF_QUEUE_TAG   0x21
> #define ORDERED_QUEUE_TAG   0x22
> 
> drivers/scsi/vmw_pvscsi.c:
> 	e->tag = SIMPLE_QUEUE_TAG;
> 	if (sdev->tagged_supported &&
> 	    (cmd->tag == HEAD_OF_QUEUE_TAG ||
> 	     cmd->tag == ORDERED_QUEUE_TAG))
> 		e->tag = cmd->tag;

A SCSI-2 tag is a SPI two byte message.  The first byte is the message
type.  The values you have above identify the message type for simple,
ordered and head of queue tags.  The *second* byte is the tag value.

See page 55 and 56 of the SCSI-2 standard.  There's no connection (or
shouldn't be) between the message type and the tag value, so it looks
like a bug in the pvscsi driver.

James