Re: pci error recovery procedure

[linas@austin.ibm.com (Linas Vepstas)]

On Wed, Sep 06, 2006 at 10:04:31AM +0800, Zhang, Yanmin wrote:
> On Wed, 2006-09-06 at 03:17, Linas Vepstas wrote:
> > On Mon, Sep 04, 2006 at 01:47:30PM +0800, Zhang, Yanmin wrote:
> > > > 
> > > > Again, consider the multi-function cards. On pSeries, I can  only enable 
> > > > DMA on a per-slot basis, not a per-function basis. So if one driver
> > > > enables DMA before some other driver has reset appropriately, everything
> > > > breaks.
> > > Does here 'reset' mean hardware slot reset? 
> > 
> > I should have said: If one driver of a multi-function card enables DMA before 
> > another driver has stabilized its harware, then everything breaks.
> What's another driver's hardware? A function of the previous multi-function
> card? Or a function of another device?

Yes. Either. Both. Doesn't matter.  Enabling DMA is "granular" at a 
different size scale than pci functions, and possibly even pci devices 
or slots, dependeing on the architecture. Before DMA can be enabled, 
*all* affected device drivers have to be approve, and have to be ready
for it. 

> > If we enabled both DMA and MMIO at the same time, there are many cases
> > where the card will immediately trap again -- for example, if its
> > DMA'ing to some crazy address. Thus, typically, one wants DMA disabled 
> > until after the card reset.  Without the mmio_enabled() reset, there
> > is no way of doing this.
>
> Did you asume the card reset is executed by callback mmio_enabled?

I am assuming that, when a driver receives the mmio_enabled() callback,
it will perform some sort of register i/o.  For example, I am currently
planning to modify the e1000 driver to do the following:

-- The error_occurred() callback returns PCI_ERS_RESULT_CAN_RECOVER
-- The arch enables mmio, and then calls the mmio_enabled() callback.
-- The mmio_enabled() callback in the driver takes a full dump of all 
   of the regsters on the card.  It then returns PCI_ERS_RESULT_NEED_RESET
-- The arch performs the full electrical #RST of device. Recovery from
   this point proceeds as before.

> > Again, consider the multi-function cards. On pSeries, I can only enable 
> > DMA on a per-slot basis, not a per-function basis. So if one driver
> > enables DMA before some other driver has reset appropriately, everything
> > breaks.
>
> What does 'I' above stand for? The platform error recovery procedure

Yes. The pSeries platform error recovery procedure can only enable DMA
on a per-slot basis.

> I guess it means platform, that is,
> only platform enables DMA for the whole slot. 

Yes.

> But why does the last sentence
> become driver enables DMA? 

In your proposal, you were suggesting that MMIO and DMA be enabled with 
one and the same routine, and I was attempting to explain why that can't
work.

> Could driver enable DMA for a function?

No, not on pSeries hardware.

> > > If mmio_enabled is not used currently, I think we could delete it firstly. Later on,
> > > if a platform really need it, we could add it, so we could keep the simplied codes.
> > 
> > It would be very difficult to add it later. And it would be especially
> > silly, given that someone would find this discussion in the mailing list 
> > archives.
> You stick to keep mmio_enabled which is not used currently, but if there will be
> a new platform who uses a more fine-grained steps to recover pci/pci-e, would
> you say 'it would be very difficut' and refuse add new callbacks?

Yes. 

> It doesn't prevent software from merging some steps. And, we want
> to implement pci/pci-e error recovery for more platforms instead of just
> pSeries.

Yes. The API was designed so that it could be supported on every
current and future platform we could think of. You haven't yet
claimed that "pci-e can't be supported".  Based on what 
I understand, changing the API wouldn't make the implementation 
any easier. (It is very easy to call a callback, and then 
examine its return value. Removing a few callbacks does not
materially simplify the recovery mechanism. Managing these
callbacks is *not* the hard part of implementing this thing.)

--linas
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