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Subject: Re: [PATCH] PCI: Add CRS timeout for pci_device_is_present()
To:     Sinan Kaya <okaya@kernel.org>, Thierry Reding <treding@nvidia.com>
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From:   Vidya Sagar <vidyas@nvidia.com>
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Date:   Tue, 15 Oct 2019 17:04:01 +0530
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On 10/15/2019 1:51 AM, Sinan Kaya wrote:
> On 10/14/2019 1:20 AM, Thierry Reding wrote:
>> I think this makes sense, so:
>>
>> Reviewed-by: Thierry Reding <treding@nvidia.com>
>>
>> However, it looks like Sinan has researched this extensively in the past
>> and gave a presentation on this at Plumbers in 2017:
>>
>> 	https://blog.linuxplumbersconf.org/2017/ocw/system/presentations/4732/original/crs.pdf
>>
>> Adding Sinan to see if he has any concerns about this, since resume time
>> is explicitly mentioned in the above slides.
> 
> 
> Thanks for including me. Let me catch up here.
> 
> pci_dev_wait() is supposed to handle this case via pci_pm_reset().
> 
> /**
>   * pci_pm_reset - Put device into PCI_D3 and back into PCI_D0.
>   * @dev: Device to reset.
>   * @probe: If set, only check if the device can be reset this way.
>   */
> 
> Do you know if your execution path hits this function? We might have
> missed a use case.
> 
Nope. It doesn't.
Following is the stack dump showing how pci_update_current_state() is called in resume() path.
And pci_device_is_present() is called from inside pci_update_current_state() API.
My understanding is that pci_device_is_present() is the API we hit first in the resume() path
for any PCIe device.

[   36.380726] Call trace:
[   36.383270]  dump_backtrace+0x0/0x158
[   36.386802]  show_stack+0x14/0x20
[   36.389749]  dump_stack+0xb0/0xf8
[   36.393451]  pci_update_current_state+0x58/0xe0
[   36.398178]  pci_power_up+0x60/0x70
[   36.401672]  pci_pm_resume_noirq+0x6c/0x130
[   36.405669]  dpm_run_callback.isra.16+0x20/0x70
[   36.410248]  device_resume_noirq+0x120/0x238
[   36.414364]  async_resume_noirq+0x24/0x58
[   36.418364]  async_run_entry_fn+0x40/0x148
[   36.422418]  process_one_work+0x1e8/0x360
[   36.426525]  worker_thread+0x40/0x488
[   36.430201]  kthread+0x118/0x120
[   36.433843]  ret_from_fork+0x10/0x1c

Also, I don't see pci_pm_reset() getting called in resume() path at all.

Thanks,
Vidya Sagar