Re: pci error recovery procedure

["Zhang, Yanmin" <yanmin_zhang@linux.intel.com>]

On Fri, 2006-09-01 at 01:50, Linas Vepstas wrote:
> On Thu, Aug 31, 2006 at 03:10:12PM +0800, Zhang, Yanmin wrote:
> > Linas,
> > 
> > I am reviewing the error handlers of e1000 driver and got some ideas. My
> > startpoint is to simplify the err handler implementations for drivers, or
> > driver developers are *not willing* to add it if it's too complicated.
> 
> I don't see that its to complicated ... 
Originally, I didn't think so, but after I try to add err_handlers to some
drivers, I feel it's too complicated.

> 
> > 1) Callback mmio_enabled looks useless. Documentation/pci-error-recovery.txt
> > says the current powerpc implementation does not implement this callback.
> 
> I don't know if its useless or not. I have not needed it yet for the
> symbios, ipr and e1000 drivers, but its possible that some more
> sophisticated device may want it. I'm tempted to keep it a while 
> longer befoe discarding it.
> 
> The scenario is this: the device driver decides that, rather than asking
> for a full electical reset of the card, instead, it wants to perform 
> its own recovery. It can do this as follows:
> 
> a) enable MMIO
> b) issue reset command to adapter
> c) enable DMA.
> 
> If we enabled both DMA and MMIO at the same time, there are mnay 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.  Withouth the mmio_enabled() reset, there
> is no way of doing this.
The new error_resume, or the old slot_reset could take care of it. The specific
device driver knows all the details about how to initiate the devices. The 
error_resume could call the step a) b) c) sequencially while doing checking among
steps.

If there is really a device having specific requirement to reinitiate it (very rarely),
it could use walkaround, such like schedule a WORKER. No need to provide a generic
mmio_enabled.

> 
> > 2) Callback slot_reset could be merged with resume. The new resume could be:
> > int (*error_resume)(struct pci_dev *dev); I checked e1000 and e100 drivers and
> > think there is no actual reason to have both slot_reset and resume.
> 
> The idea here was to handle multi-function cards.  On a multi-function card, 
> *all* devices need to indicate that they were able to reset. Once all devices 
> have been successfuly reset, then operation can be resumed. If the reset 
> of one function fails, then operation is not resumed for any f the
> functions.
I don't think we need slot_reset to coordinate multi-function devices. The new
error_resume could take care of multi-function card. 'reset' here means driver
need do I/O to detect if the device (function) still works well. If a function
of a multi-function device couldn't reset while other functions could reset,
other functions could just go on to reinitiate. In the end, the error recovery
procedure (handle_eeh_events in PowerPC implementation) could check all the
returning values of error_resume. If there is a failure value, then removes
all the functions' pci_dev of the device from the bus.

> 
> > 3) link_reset is not used in pci express aer implementation, so it could be
> > deleted also.
> 
> OK. Link reset was added explicitly to support PCI-E, so if its not wanted,
> we can eliminate it.
> 
> > How did you test e1000 err_handler? 
> 
> We have three methods (I thought these were documented). In one, a
> technician brushes a grounding strap to some of the signal pins. 
> In the second, slots are populated with known-bad cards. The third test
> involes sending a command down to the pci bridge chip, telling it to 
> behave as if it detected an error. For development, the last is
> quick-n-easy.
Thanks for your explanation.

> 
> > In the simulated enviroment, the testing might be
> > incorrect. 
> 
> Why would it be incorrect?  I mean, we don't simulate having someone pour a
> cup of coffee into the guts of the machine ... but my understanding is
> the machines do get standard vibration/thermal/humidity testing, which
> is good enough for me.
> 
> > For example, e1000_io_error_detected would call e1000_down to reset NIC. 
> 
> Why would that be incorrect?
> 
> > During
> > our last discussion on LKML, you said PowerPC will block further I/O if the platform captures
> > a pci error, so the all I/O in e1000_down will be blocked. Later on, e1000_io_slot_reset
> > will reenable pci device and initiate NIC. I guess late initiate might fail because prior
> > e1000_down I/O don't reach NIC.
> 
> Why would it fail? The e1000_down serves primarily to get the Linux
> kernel into a known state. It doesn't matter what happens to the card,
> since the next step will be to perform an electrical reset of the card.
Who will perform the electrical reset of the card? Function e1000_reset or the platform?
If it's the platform, I agree with you, but if it's e1000_reset, it might not work because
e1000_reset uses a e1000-specific approach to reset the card. I'm not sure if the e1000_reset
will restore the NIC to fresh system power-on state. At least, from the source codes, e1000_reset
couldn't.

Yanmin
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