RE: [PATCH 01/16] hpsa: do readl after writel in main i/o path toensure commands don't get lost.

[Miller, Mike (OS Dev)]

> -----Original Message-----
> From: Tomas Henzl [mailto:thenzl@xxxxxxxxxx]
> Sent: Thursday, May 26, 2011 7:14 AM
> To: Miller, Mike (OS Dev)
> Cc: Valdis.Kletnieks@xxxxxx; scameron@xxxxxxxxxxxxxxxxxx; Andrew Morton;
> LKML; LKML-scsi; Jens Axboe
> Subject: Re: [PATCH 01/16] hpsa: do readl after writel in main i/o path
> to ensure commands don't get lost.
> 
> On 05/25/2011 05:20 PM, Miller, Mike (OS Dev) wrote:
> > Tomas wrote:
> >
> >
> >> -----Original Message-----
> >> From: Tomas Henzl [mailto:thenzl@xxxxxxxxxx]
> >> Sent: Monday, May 23, 2011 6:38 AM
> >> To: Miller, Mike (OS Dev)
> >> Cc: Valdis.Kletnieks@xxxxxx; scameron@xxxxxxxxxxxxxxxxxx; Andrew
> Morton;
> >> LKML; LKML-scsi; Jens Axboe
> >> Subject: Re: [PATCH 01/16] hpsa: do readl after writel in main i/o
> path
> >> to ensure commands don't get lost.
> >>
> >> On 05/05/2011 08:35 PM, Mike Miller wrote:
> >>
> >>> On Wed, May 04, 2011 at 01:54:22PM -0400, Valdis.Kletnieks@xxxxxx
> >>>
> >> wrote:
> >>
> >>>
> >>>> On Wed, 04 May 2011 11:37:35 MDT, Matthew Wilcox said:
> >>>>
> >>>>
> >>>>>> This probably needs a comment like
> >>>>>> 	/* don't care - dummy read just to force write posting to
> chipset
> >>>>>>
> >> */
> >>
> >>>>>> or similar.  I'm assuming it's just functioning as a barrier-type
> >>>>>>
> >> flush of some sort?
> >>
> >>>>>>
> >>>>> It's a PCI write flush.  It's not clear to me why it's needed
> here,
> >>>>> though.  The write will eventually get to the device; why we need
> to
> >>>>> make the CPU wait around for it to actually get there doesn't make
> >>>>>
> >> sense.
> >>
> >>>>>
> >>>> Exactly why I think it needs a one-liner comment. :)
> >>>>
> >>>>
> >>>>
> >>> So we're not exactly sure why it's needed either. We've had reports
> of
> >>> commands getting "lost" or "stuck" under some workloads. The extra
> >>>
> >> readl
> >>
> >>> works around the issue but certainly may have negative side effects.
> >>>
> >>> I'm not sure I understand how writel works.
> >>>
> >>> From linux-2.6/arch/x86/include/asm/io.h:
> >>>
> >>> #define build_mmio_write(name, size, type, reg, barrier) \
> >>> static inline void name(type val, volatile void __iomem *addr) \
> >>> { asm volatile("mov" size " %0,%1": :reg (val), \
> >>> "m" (*(volatile type __force *)addr) barrier); }
> >>>
> >>> This implies (at least to me) that a barrier is part of writel. I
> >>>
> >> don't know
> >>
> >>> why a write operation needs a barrier but thats essentially what
> we've
> >>>
> >> done
> >>
> >>> by adding the extra readl. Can someone confirm or deny that a
> barrier
> >>>
> >> is
> >>
> >>> actually built into writel? Or used by writel? If so, does this
> >>>
> >> indicate
> >>
> >>> that barrier is broken?
> >>>
> >>> At this point we (the software guys) are pretty much at a loss as to
> >>>
> >> how to
> >>
> >>> continue debugging. We don't know what to trigger on for the PCIe
> >>>
> >> analyzer.
> >>
> >>> If we track outstanding commands then trigger on one that doesn't
> >>>
> >> complete in
> >>
> >>> some amount of time the problem could conceivably be far in the past
> >>>
> >> and
> >>
> >>> difficult to correlate to the data in the trace.
> >>>
> >>>
> >> I'd look at the firmware part, you could check what happens for
> example
> >> when
> >> the firmware gets send a command it doesn't understand.
> >> You could also change the communication with the fw by adding a count
> >> field, which can
> >> be then checked for the !(next_value == previous_value + 1) and raise
> an
> >> event.
> >> tomas
> >>
> > Tomas,
> > We've tried something very similar to the counter idea in fw. It
> doesn't help because the controller thinks he's done with the request.
> We have a (pretty crude) counter in the driver but no timing mechanism.
> We could add a timer. But what's a suitable timeout value? Is 2 seconds
> too short, too long? Suggestions, please.
> >
> I know that a counter isn't a ground-breaking idea, just wanted to show
> some interest :)

:)

> The command can be either eaten by the firmware or during the
> communication in or out from the device.
> I'd would start by the communication, by adding some fields to the
> command to detect if a command in the row(s) isn't
> missing - I know even that isn't easy. The same could be done
> independently done for the other direction.
> 
> tomash

Thanks, Tomas.

> 
> > -- mikem
> >
> >
> >
> >>
> >>
> >>> If anyone has any thoughts, suggestions, or flames they would be
> >>>
> >> greatly
> >>
> >>> appreciated.
> >>>
> >>> -- mikem
> >>> --
> >>> To unsubscribe from this list: send the line "unsubscribe linux-
> scsi"
> >>>
> >> in
> >>
> >>> the body of a message to majordomo@xxxxxxxxxxxxxxx
> >>> More majordomo info at  http://vger.kernel.org/majordomo-info.html
> >>>
> >>>
> >

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