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Subject: Re: [PATCH] PCI: Check for PCIe downtraining conditions
To:     Sinan Kaya <okaya@codeaurora.org>, Alex_Gagniuc@Dellteam.com,
        bhelgaas@google.com
Cc:     Austin.Bolen@dell.com, Shyam.Iyer@dell.com, keith.busch@intel.com,
        linux-pci@vger.kernel.org, linux-kernel@vger.kernel.org
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From:   "Alex G." <mr.nuke.me@gmail.com>
Message-ID: <54071f83-5d0d-04a0-d448-0c99ec0ffc4f@gmail.com>
Date:   Thu, 31 May 2018 11:49:21 -0500
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On 05/31/2018 11:13 AM, Sinan Kaya wrote:
> On 5/31/2018 12:01 PM, Alex G. wrote:
>>>       PCI: Add pcie_print_link_status() to log link speed and whether it's limited
>> This one, I have, but it's not what I need. This looks at the available
>> bandwidth from root port to endpoint, whereas I'm only interested in
>> downtraining between endpoint and upstream port.
> 
> I see what you are saying. 
> 
> With a little bit of effort, you can reuse the same code.
> 
> Here is an attempt.
> 
> You can probably extend pcie_bandwidth_available() to put an optional parent bridge
> device for your own use case and terminate the loop around here.
> 
> https://elixir.bootlin.com/linux/v4.17-rc7/source/drivers/pci/pci.c#L5182
> 
> Then, you can use the existing code to achieve what you are looking for via
> pcie_print_link_status() by adding an optional parent parameter.
> 
> 	bw_cap = pcie_bandwidth_capable(dev, &speed_cap, &width_cap);
> 	bw_avail = pcie_bandwidth_available(dev, &limiting_dev, &speed, &width, *parent*);

That's confusing. I'd expect _capable() and _available() to be
symmetrical. They either both look at one link only, or both go down to
the root port. Though it seems _capable() is link-local, and
_available() is down to root port.

> 
> If parent parameter is NULL, code can walk all the way to root as it is doing today.
> If it is not, then will terminate the loop on the first iteration.
>