Re: [PATCH] megaraid_sas: Enable shared tag map

Hannes Reinecke
2014-10-02T12:14:21Z

On 10/02/2014 02:05 PM, James Bottomley wrote:
> 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.
>
It is. I've already sent a patch.

But that just proves the scmd->tag is essentially a duplicate
and we should be removing it.

Cheers,

Hannes
--
Dr. Hannes Reinecke zSeries & Storage
hare@suse.de +49 911 74053 688
SUSE LINUX Products GmbH, Maxfeldstr. 5, 90409 Nürnberg
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