linux-kernel - Re: [GIT PULL] SCSI queuecommand API change for 2.6.37-rc1

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Message-ID: <AANLkTik_eXv1_ZaqD01CoT6igzjBZtmECD0e+QUkMhxP@mail.gmail.com>
Date:	Fri, 12 Nov 2010 20:42:50 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Matthew Wilcox <matthew@....cx>
Cc:	James Bottomley <James.Bottomley@...e.de>,
	"Nicholas A. Bellinger" <nab@...ux-iscsi.org>,
	Jeff Garzik <jeff@...zik.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-scsi <linux-scsi@...r.kernel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [GIT PULL] SCSI queuecommand API change for 2.6.37-rc1

On Fri, Nov 12, 2010 at 8:28 PM, Matthew Wilcox <matthew@....cx> wrote:
> Actually, it'd be even better if we took away an argument.
>
> int queuecommand(struct scsi_cmnd *cmd);
>
> Every single driver sets cmd->scsi_done = done;

.. and i that's all they really ever do with it, I guess that that
would indeed be an improvement.

I still do think that passing in shost is actually a good idea, since
you'll normally be needing it for locking and other things anyway
(looking at the libata-scsi.c one, for example, the very first thing
it wants to do is to look up the port from the shost pointer).

Having to inevitably do that silly double dereference
("cmd->device->host") seems bogus. Passing in the host you want to
queue something on seems to be natural, and avoids that nasty chain of
dereferences, so it really should improve the code.

I don't think there are any drivers that don't want that host pointer
pretty much immediately, even if they can elide the locking. But I
admit I only looked at two (qla4xxx driver does the exact same thing:
the very first thing in the queuecommand function is
"to_qla_host(cmd->device->host)", but it wants that hostdata thing
too, so whatever.)

So the shost pointer is certainly a lot more useful than the 'done'
function pointer.  So yeah, I'd switch them around - pass the 'done'
thing indirectly, and the shost directly.

                        Linus
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