linux-kernel - Re: [PATCH 2/2] PCI,pciehp: avoid add a device already exist during pciehp

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Date:	Thu, 11 Jul 2013 10:33:20 +0800
From:	Yijing Wang <wangyijing@...wei.com>
To:	Bjorn Helgaas <bhelgaas@...gle.com>
CC:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
	Rafael <rjw@...k.pl>, Hanjun Guo <guohanjun@...wei.com>,
	Jiang Liu <jiang.liu@...wei.com>,
	Paul Bolle <pebolle@...cali.nl>,
	Oliver Neukum <oneukum@...e.de>,
	Gu Zheng <guz.fnst@...fujitsu.com>
Subject: Re: [PATCH 2/2] PCI,pciehp: avoid add a device already exist during
 pciehp_resume

>> If the slot support surprise hot remove, this action maybe safe. right?
> 
> If there's no device, config space accesses performed by .remove()
> will fail (reads will return -1 data or error; writes will be
> dropped).  MMIO or I/O port accesses may fail with machine checks or
> similar bad things.  But I don't see a way around that except by
> fixing drivers as we encounter issues like that.
> 
> Since you're not changing anything here, I don't think we should worry
> about it for now.

OK.

> 
>>>> remove the old card firstly, then add the new card.
>>>
>>> With your patch, I think we'll call the old driver's .remove() method
>>> on the new device.  This seems bad; see below.
>>
>> Ah, this is issue.
>> What about power off slot first, then call the old driver's remove() method
>> will not touch the new physical device. After the old driver's remove() cleanup,
>> we call pciehp_enable_slot() to power on and enable the new device.
> 
> Turning off power to the slot seems like a reasonable approach.  Then
> we can run the old .remove() method in basically the same way we would
> in case 2.

Hmmm, I will follow this method to rework this patch in next version.

> 
>>> With your patch, if we remove and reinsert the same device while
>>> suspended, we do nothing because the DSN didn't change.  Previously we
>>> called pciehp_enable_slot().  I don't know if we need to do anything
>>> here or not.
>>
>> Mainly to avoid the redundant device add, the same like the changes for case 4
> 
> I don't know whether it's redundant or not.  Obviously if we remove
> and reinsert a device, we lose *all* state that was in the device.  If
> we lose everything even if the card stayed inserted the whole time we
> were suspended, we must already deal with that and the "add" would be
> redundant.  But if the state of the card is different if it got pulled
> and reinserted, the "add" would be necessary.

This is a key issue, sorry, I'm not familiar with PM :(
In my opinion, the device driver .suspend() method will be called
regardless of system enter in suspend to RAM(S3) or suspend to Disk(S4). Driver will
save the pci/pcie/pci-x state in .suspend() method. So once device driver .resume()
method be called, the pci/pcie/pci-x state willl be restored.
Because suspend to disk will power off whole system, so I thought if the device
was removed and inserted same device(DSN) again, maybe .resume will enable this device ok
regardless of the pci config space state has been changed.

If I have any thing above understanding wrong, please correct me, thanks!

> 
>>>> 4. slot is non empty before suspend, no action during suspend.
>>>>         We should do nothing in pciehp_resume, but we call
>>>> pciehp_enable_slot(), so some uncomfortable messages show like above.
>>>> In this case, we can improve it a little by add a guard
>>>> if (!list_empty(bus->devices)).
>>>
>>> This is the common case.  Previously we called pciehp_enable_slot(),
>>> and with your patch we do nothing.  I think that seems sensible, but
>>> this part should be split into a separate patch.  That way we can keep
>>> the benefit of this change even if we trip over something with the
>>> other changes.
>>
>> OK, I will split this changes into a new patch.
> 
> Actually, without your DSN changes, I don't think we can distinguish
> this from case 3.  So I doubt it really could be split out.

I will try, but I think this is not a big deal :)

> 
> .
> 


-- 
Thanks!
Yijing

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