From mboxrd@z Thu Jan 1 00:00:00 1970 From: Boaz Harrosh Subject: Re: [RFC 1/2] scsi core: alloc_cmnd Date: Tue, 23 Oct 2007 18:49:17 +0200 Message-ID: <471E260D.6030605@panasas.com> References: <20071019183331.GC30807@parisc-linux.org> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Return-path: Received: from gw-colo-pa.panasas.com ([66.238.117.130]:25047 "EHLO cassoulet.panasas.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1752403AbXJWQtY (ORCPT ); Tue, 23 Oct 2007 12:49:24 -0400 In-Reply-To: <20071019183331.GC30807@parisc-linux.org> Sender: linux-scsi-owner@vger.kernel.org List-Id: linux-scsi@vger.kernel.org To: Matthew Wilcox Cc: linux-scsi@vger.kernel.org On Fri, Oct 19 2007 at 20:33 +0200, Matthew Wilcox wrote: > This fairly naive patch introduces alloc_cmnd and destroy_cmnd. I'm not > exactly happy about passing down the gfp_mask -- I'd prefer to be able > to allocate scsi_cmnds before we grab the queue_lock (and hence get > rid of the possibility we might need to GFP_ATOMIC), but I don't see > anywhere to do that. Maybe there's a simple change we can make to the > block layer to allow it. > > Merely-an-RFC-by: Matthew Wilcox You know Matthew when you first talked about this, I envisioned something else. My idea was to have a new variable in scsi_host_template that states the host_cmnd_extra_bytes. The mid-layer allocates a mempool with sizeof(struct scsi_cmnd) + host_cmnd_extra_bytes. A utility function will return that space for drivers given a cmnd like: void *host_cmnd_priv(struct scsi_cmnd *cmd) { return cmd + 1; } This serves the drivers well because all they need to do is set host_cmnd_extra_bytes = size_of(my_cmnd_stuff) and start using it. much more simple than setting up cache pools. It also keeps the Q per host vs Q per driver. This also solves your problem with locks and allocation flags since nothing changes, and it is all private to the mid-layer. And it serves me, because when bidi comes I just do: sizeof(struct scsi_cmnd) + host_cmnd_extra_bytes + sizeof(bidi_stuff) for hosts that support bidi. And the rest of the work was done by you. Do you need that I scribble some tryout patch for above + example driver Thanks for doing this Boaz