Date:	Thu, 5 Mar 2009 14:48:09 +0900
From:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>
To:	scameron@...rdog.cca.cpqcorp.net
Cc:	fujita.tomonori@....ntt.co.jp, linux-kernel@...r.kernel.org,
	mike.miller@...com, jens.axboe@...cle.com,
	akpm@...ux-foundation.org, linux-scsi@...r.kernel.org,
	coldwell@...hat.com, hare@...ell.com, iss_storagedev@...com
Subject: Re: [PATCH] hpsa: SCSI driver for HP Smart Array controllers

On Tue, 3 Mar 2009 10:28:21 -0600
scameron@...rdog.cca.cpqcorp.net wrote:

> On Tue, Mar 03, 2009 at 03:35:26PM +0900, FUJITA Tomonori wrote:
> > On Mon, 2 Mar 2009 08:56:50 -0600
> > scameron@...rdog.cca.cpqcorp.net wrote:
> > 
> > > [...]
> > > > > +     .this_id                = -1,
> > > > > +     .sg_tablesize           = MAXSGENTRIES,
> > > > 
> > > > MAXSGENTRIES (32) is the limitation of hardware? If not, it might be
> > > > better to enlarge this for better performance?
> > > 
> > > Yes, definitely, though this value varies from controller to controller,
> > > so this is just a default value that needs to be overridden, probably
> > > in hpsa_scsi_detect().
> > 
> > I see. If we override this in hpsa_scsi_detect(), we need a trick for
> > SG in CommandList_struct, I guess.
> 
> Yes.  There are some limits to what can be put into CommandList_struct
> directly, but there is also scatter gather chaining, in which we use
> the last element in the CommandList_struct to point to another buffer
> of SG entries.
> 
> If you have a system with a lot of controllers, having a large number of 
> scatter gathers can be a bit of a memory hog, and since this memory is all
> via pci_alloc_consistent, that can be a concern.  It would be nice if
> there was a way for the user to specify differing amounts of scatter
> gathers for different controller instances so for instance the controller
> which he's running his big oracle database, or webserver or whatever on
> gets lots, while the controller he's booted from that's mostly idle
> gets not so many.  I don't know what a good way for a user to identify
> what controller he's talking about in a module parameter would be 
> though.  Maybe by pci domain/bus/device/function?  Maybe something along
> the lines of:
> 
> 	modprobe hpsa dev1=0:0e:00.0 sg1=1000 dev2=0:0b:00.0 sg2=31
> 
> to say that one controller gets 1000 scatter gather elements, but
> another gets only 31.  But PCI busses can change if hardware 
> configuration changes, and this isn't exactly obvious, so seems less
> than ideal.  Any bright ideas on that front?

We have /sys/class/scsi_host/host*/sg_tablesize:

How about modifying this value on the fly?

fujita@...ver:/sys/class/scsi_host/host3$ echo 1000 > sg_tablesize


Well, this needs more changes (to both the block layer and the scsi
mid layer) but is it nice to change this value dynamically?

Anyway, I think that it's better to address this fancy feature later
on (after the mainline inclusion). Let's put hpsa driver into mainline
first.


> > > Hmm, this doesn't seem all that complicated to me, and this code snippet
> > > has been pretty stable for about 10 years. it's nearly identical to what's in
> > > cpqarray in the 2.2.13 kernel from 1999:
> > > 
> > >                 do {
> > >                         i = find_first_zero_bit(h->cmd_pool_bits, NR_CMDS);
> > >                         if (i == NR_CMDS)
> > >                                 return NULL;
> > >                 } while(test_and_set_bit(i%32, h->cmd_pool_bits+(i/32)) != 0)
> > > 
> > > It's fast, works well, and has needed very little maintenance over the 
> > > years.  Without knowing what you have in mind specifically, I don't see a
> > > big need to change this.
> > 
> > I see. Seems that some drivers want something similar. I might come
> > back later on with a patch to replace this with library
> > functions.
> 
> There was some other discussion about pushing this sort of thing to 
> upper layers, using a tag generated in the scsi layer as a means
> of allocating driver command buffers, since, presumably there's a
> one to one mapping.  (I didn't completely grok it all though.)

Oops, I meant that I might come back with a patch to convert hpsa to
use the the block layer tagging, which you and Mike Christie are
talking about (yeah, my first suggestion to use lists was wrong. using
the block layer tagging looks much better).


By the way, have you guys started to work on the review comments for
the next submission? The driver has some minor style issues that have
not been mentioned yet. For example, the comment style in the driver
is not preferred:

/* If this device a non-zero lun of a multi-lun device */
/* byte 4 of the 8-byte LUN addr will contain the logical */
/* unit no, zero otherise. */

The preferred style is:

/*
 * If this device a non-zero lun of a multi-lun device
 * byte 4 of the 8-byte LUN addr will contain the logical
 * unit no, zero otherise.
 */

Another example, I think that the SCSI-ml preferred style is (not
documented in CodingStyle though):

'if (!ptr)' rather than 'if (ptr == NULL)'
'if (!value)' rather than 'if (value == 0)'
'if (ptr)' rather than 'if (ptr != NULL)'
'if (value)' rather than 'if (value != 0)'


If you are already addressing the review comments, I just wait for the
next submission, then I'll send such minor patches. If you are not,
I'll send patches to address the review comments (including such minor
patches).
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