Hi, Kristian:

I only looked briefly at SBP-2, and at submit/callback paths it pulled,
because I do not understand most of the other issues.

Executive summary: please implement proper ORB cancellation. This is
how you verify that your locking model is worth anything: by interaction
of normal transactions with ->remove, ->suspend and SCSI aborts.
Currently, there's no telling if the code is sane or not.

I see that ORBs are always allocated with a call (like SKB) and not
embedded into drivers (like URBs). It's great, keep it up. Also,
never allow drivers to pass DMA-mapped buffers into fw_send_request
and friends. We made both of these mistakes in USB, and it hurts.

Now, about small things:

diff --git a/fw-sbp2.c b/fw-sbp2.c
index 8888f27..3a65095 100644
--- a/fw-sbp2.c
+++ b/fw-sbp2.c
@@ -303,7 +303,7 @@ complete_transaction(struct fw_card *card, int rcode,
 	unsigned long flags;
 
 	orb->rcode = rcode;
-	if (rcode != RCODE_COMPLETE) {
+	if (rcode != RCODE_COMPLETE) {		/* Huh? */
 		spin_lock_irqsave(&card->lock, flags);
 		list_del(&orb->link);
 		spin_unlock_irqrestore(&card->lock, flags);

This looks like an inverted test. Who does remove ORB from the list
if it's completed normally?

@@ -320,8 +320,7 @@ sbp2_send_orb(struct sbp2_orb *orb, struct fw_unit *unit,
 	unsigned long flags;
 
 	orb->pointer.high = 0;
-	orb->pointer.low = orb->request_bus;
-	fw_memcpy_to_be32(&orb->pointer, &orb->pointer, sizeof orb->pointer);
+	orb->pointer.low = cpu_to_be32(orb->request_bus);
 
 	spin_lock_irqsave(&device->card->lock, flags);
 	list_add_tail(&orb->link, &sd->orb_list);

I'll whine later about this

@@ -347,6 +346,7 @@ static void sbp2_cancel_orbs(struct fw_unit *unit)
 	list_splice_init(&sd->orb_list, &list);
 	spin_unlock_irqrestore(&device->card->lock, flags);
 
+	/* XXX Notify the hardware -- show-stopper. */
 	list_for_each_entry_safe(orb, next, &list, link) {
 		orb->rcode = RCODE_CANCELLED;
 		orb->callback(orb, NULL);
@@ -364,6 +364,9 @@ ...
[ message continues ]

Pete Zaitcev wrote:

Thanks Pete, I'm sure I will find some of these issues in mainline's
sbp2 too. :-)


Kristian just announced
http://gitweb.freedesktop.org/?p=users/krh/linux-2.6.git
git://people.freedesktop.org/~krh/linux-2.6
shortly before you posted.

(And time-lagged/ filtered versions: kernel.org's linux1394-2.6.git, and
non-canonical patchkits for 2.6.{18,19,20} on my homepage.)
-- 
Stefan Richter
-=====-=-=== ---= ==--=
http://arcgraph.de/sr/
-

[ message continues ]

Good catch, my ORB cancellation is a bit rough, and I believe there are a 
couple of races there, though I haven't been able to actually trigger these. 
When cancelling an ORB I need to cancel the ORB, the pending transaction and 
potentially the packet queued for DMA in the fw-ohci driver.  This is not 

Oh, the ORBs are SBP-2 specific data structures, struct fw_transaction is 
probably what corresponds to USB URBs.  This struct is defined in 
fw-transaction.h and is available for embedding into other structs, such as 
struct sbp2_orb in fw-sbp2.  Is that what you're suggesting against, and what 
are the problems with this approach?

As for passing DMA-mapped buffers, do you mean that fw_send_request (or code 
below that function) should map the payload as it currently does as opposed to 

An SBP-2 ORB transaction sequence consists of a number of FireWire 
transactions.  First the initiator prepares an SBP-2 ORB (object request 
block) in host memory and maps this for DMA.  Then the initiators sends its 
bus address and the physical address of the ORB to the SBP-2 device.  This is 
the request that complete_transaction deals with.  If we get rcode == 
RCODE_COMPLETE here, we successfully wrote the ORB address to the SBP-2 device 
and we leave the ORB in the list so the SBP-2 transaction can continue.  Any 
other rcode means that the write failed and we need to back out the 
transaction and leave it to upper layers to retry.

The rest of the SBP-2 transaction continues with the SBP-2 device issuing a 
FireWire read transaction to read out the ORB from host memory and then carry 
out the instructions in the ORB.  Once the device has completed the ORB it 
will do a status write to the status address specified in the ORB, at which 



I guess that should be in pci_probe in fw-ohci.c, but it isn't.  If the 
allocation is in physical memory above the 4G limit, will dma_map_single set 
up a trampoline buffer below 4G if the device DMA mask is 0xffffffff?  I have 
a similar ...
[ message continues ]

Fortunately we do not care about out-of-tree drivers, which are most
affected, you may even call it a feature ^_^. My main problem is,
we can't refcount URBs, so usbmon can't tap them and must copy.

I understand that fw_transaction points to an external buffer while
SKB does not, so any jujumon code won't be able to refcount data.

Yes, this is what I mean. It serves well for our networking stack,
which is pretty fast. For some reason someone decided that USB users
should be let mapping their buffers. This led to big trouble with
usbmon, extra fields in URB (although already fat...), tying up precious

You know, I wanted to use this picture for a long time:
 http://www.flickr.com/photos/zaitcev/369269557/
I hope it's all right.

I guess my problem is that I don't see just why the ORB transaction callback
is being delivered before the whole I/O was done.

Now that you drew my attention to sbp2_status_write(), this looks wrong:

        /* Lookup the orb corresponding to this status write. */
        spin_lock_irqsave(&card->lock, flags);
        list_for_each_entry(orb, &sd->orb_list, link) {
                if (status_get_orb_high(status) == 0 &&
                    status_get_orb_low(status) == orb->request_bus) {
                        list_del(&orb->link);
                        break;
                }
        }
        spin_unlock_irqrestore(&card->lock, flags);


If an IOMMU (or SWIOTLB) is not available, dma_map_single will fail.
If IOMMU is available, it will allocate a mapping so that DMA address
is constrained, failing if tables are full. At least this was my

Please ask James, I do not remember that either.

For block layer, Jens guarantees it for you (it's controlled by
blk_queue_bounce_limit), but I am not sure about SCSI. I don't expect


I challenge this claim. I am pretty sure that dma_unmap_single on a buffer
that wasn't mapped in the first place can cause havoc, primarily because

Well, on SPARC we lived for years with a panic when IOMMU ran ...
[ message continues ]

The fundamental thing about SBP-2 is that ORBs ( = SCSI command blocks
plus SBP-2 header) and data buffers all reside in the memory of the
initiator (or of a 3rd party on the FireWire bus). The target peeks and
pokes them when and how it sees fit. The initiator pushes only tiny
notifications about availability of new ORBs to the target. The target
eventually completes SCSI commands in-order or out-of-order and signals
so by pushing a status block per one or more completed commands.

(Juju's fw-sbp2 gives only one command at a time to the target.
Mainline's sbp2 can optionally give more commands in a row, but the
implementation is subtly broken in several ways and therefore disabled
by default until I fix it right after hell froze over.)

Another important thing to know in order to understand fw-sbp2 and sbp2
is that they currently rely on OHCI-1394's physical DMA feature, which
I'll not explain here. It means two things: 1. FireWire bus addresses of
ORBs and buffers are directly derived from the DMA mapped address.
(FireWire bus addresses are the addresses used in communication between
SBP-2 initiator and target.) 2. Almost all of the transfers done by the
target do not generate interrupts. (Just the status write generates an
interrupt.)


The target wrote an SBP-2 status block into our memory. The status block
contains the FireWire bus address of the ORB to which it belongs. Juju's
fw-sbp2 does the same as mainline's sbp2: Looking through the pile of
unfinished ORBs for one with the same FireWire bus address, which was
previously derived from the DMA mapped address. Since there aren't many
mapped ORBs per target, a linked list is a reasonable data structure to
search over. That said --- Kristian, doesn't fw-sbp2 have at most 1 ORB
in sd->orb_list?
-- 
Stefan Richter
-=====-=-=== ---= ==-=-
http://arcgraph.de/sr/
-

[ message continues ]

Another thing that probably makes my explanation a little confusing is that 
there are two types of transactions: FireWire transactions which consists of a 
  request followed by a response and are pretty much the smallest interaction 
you can have with a remote device.  Then there are SBP-2 transactions, which 
are a higer level sequence layered on top of FireWire transactions.  An SBP-2 
transaction consists of a sequence of FireWire transactions, the first of 
which is initiated by the initiator.  This is the FireWire transaction that 
complete_transaction handles.  When this first FireWire transaction finishes 
succesfully, we know that the SBP-2 transaction has been started and we sit 
back and wait for the target to do it's part.  If that initial FireWire 

But the status write actually does carry the address of the ORB it signals the 
completion of.  So in theory, we could just read out the ORB address from the 
status write packet and map that back to kernel virtual memory and do an 
appropriate container_of() call and we should have the struct sbp2_orb 
pointer.  The reason I still search through the list is of course that this is 
way to much trust to put into hardware as buggy as external storage devices. 
Blindly dereferencing a pointer returned by storage driver firmware is 
probably a very bad idea.

One thing I want to do (though very low priority) is to allocate the ORBs out 
of a preallocated circular buffer.  We can then check that the ORB pointer 
returned in the status write points into this buffer and that it's a multiple 
of the ORB size, at which point it should be safe to dereference it.


Yes, there is only ever one pending ORB in the list, so looking through the 
list is not exactly a time sink :)

Kristian

-

[ message continues ]

[...]

Exactly. One SBP-2 transaction is one SCSI task's request and completion
and contains
  1. one 1394 write transaction, requested by initiator node,
  2. one 1394 read transaction, requested by the target node,
  3. zero to many 1394 read or write transactions, requested by the
     target node,
  4. one 1394 write transaction, requested by the target node
(if everything goes well and if we don't have dynamically appended ORB
lists). So it might be less confusing if we only speak of "1394
transactions" and "SCSI tasks".

BTW, SBP-3 allows to wrap step 2 into step 1. I read that some SBP-2
targets support this SBP-3 feature.


"Map back to virtual memory" is exactly what we do already, although in
a rather dumb fashion. It would be easier if there was a portable
bus_to_virt() which would do the job for us. This is much more of an
issue if we want to work without OHCI-1394 physical DMA: Then we have to
do this inverse DMA mapping also for arbitrary pieces of all scatter
gather elements of the SCSI data buffer.


Incidentally, one thing I want to do (though at low priority) is to use
a circular ORB buffer... Well, linux1394 has a number of more serious
issues pending at bugzilla.kernel.org, and meanwhile people are eager to
run things like FreeBob or Kino on Juju --- thus it remains to be seen
who of us gets to it first. :-)
-- 
Stefan Richter
-=====-=-=== ---= ==-=-
http://arcgraph.de/sr/
-

[ message continues ]

I see. SRP has a more flexible tag which can be used to look up
the just completed command more effectively. But if we only submit

Righto. I'm used to having thousands of oustanding commands in arrays.

-- Pete
-

[ message continues ]

And indeed, in the SBP-2 realm the ORB's address is actually the tag of

If I ever get to fix sbp2's handling of dynamically appended ORB lists,
the next step would be to measure which queue depth is optimal for
different hardware and different workloads. (Certainly much less than
thousands for typical SBP-2 target(s)+initiator(s) setups.)
-- 
Stefan Richter
-=====-=-=== ---= ==-=-
http://arcgraph.de/sr/
-

[ message continues ]

urbs are reference counted, it's just that not all drivers who create
them use them that way :(

Perhaps you can inforce this in the new codebase...

thanks,

greg k-h
-

[ message continues ]

It's a small change to make the fw_transaction struct opaque and ref-counted, 
and it's definitely still doable.  But the nice thing about embedding the 
struct is that you have one memory allocation failure path less to worry 
about.  And I haven't yet, and don't expect to see a use case that will need 
ref-counted struct fw_transaction, the ownership is always clearly defined. 
But I can go either way on this and if there is a good reason to ref count 
them it's a pretty small change.

cheers,
Kristian
-

[ message continues ]

This seems to have disappeared. Was it moved or dropped?

-- Pete
-

[ message continues ]

No, it's still there, and I just did a git clone on it.  How does it fail for 
you?  In any case, you're probably better of pulling from Stefans kernel.org tree:

   git://git.kernel.org/pub/scm/linux/kernel/git/ieee1394/linux1394-2.6.git

since that's where new work is going to show up.  I'm still trying to figure 
out a good workflow for this, but if all patches are going to through 
linux1394-devel, there's not much point in me publishing a branch in the 
freedesktop.org repo, if the patches are going to be edited/tweaked and the 
committed to Stefans repo.

Kristian

-

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Oh never mind, sorry. It said "Already up-to-date", but since HTTP

I see, thanks.

-- Pete
-

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