linux-kernel - Re: [PATCH] PCI: Allocate maximum available buses to help extending the daisy chain

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Message-ID: <0aa6cb16-27af-345d-7e6c-cf985290d1b4@amd.com>
Date:   Fri, 18 Aug 2023 09:01:50 +0530
From:   Sanath S <sanaths2@....com>
To:     Mika Westerberg <mika.westerberg@...ux.intel.com>
Cc:     bhelgaas@...gle.com, linux-pci@...r.kernel.org,
        linux-kernel@...r.kernel.org, mario.limonciello@....com,
        Sanjay R Mehta <sanju.mehta@....com>
Subject: Re: [PATCH] PCI: Allocate maximum available buses to help extending
 the daisy chain


On 8/17/2023 3:54 PM, Mika Westerberg wrote:
> On Wed, Aug 16, 2023 at 06:48:35PM +0530, Sanath S wrote:
>> Adding Mika.
> Thanks!
>
>> On 8/16/2023 10:49 AM, Sanath S wrote:
>>> In the case of Thunderbolt, it contains a PCIe switch and one or
>>> more hotplug-capable PCIe downstream ports where the daisy chain
>>> can be extended.
>>>
>>> Currently when a Thunderbolt Dock is plugged in during S5/Reboot,
>>> System BIOS allocates a very minimal number of buses for bridges and
>>> hot-plug capable PCIe downstream ports to enumerate the dock during
>>> boot. Because of this, we run out of bus space pretty quickly when
>>> more PCIe devices are attached to hotplug downstream ports in order
>>> to extend the chain.
>>>
>>> Before:
>>>              +-04.0
>>>              +-04.1-[63-c1]----00.0-[64-69]--+-00.0-[65]--
>>>              |                               +-01.0-[66]--
>>>              |                               +-02.0-[67]--
>>>              |                               +-03.0-[68]--
>>>              |                               \-04.0-[69]--
>>>              +-08.0
> This is something the BIOS should be doing but for some reason it is
> not on that particular system.
Yes, BIOS should be doing it. Idea here is if BIOS has not distributed 
it correctly, OS
can reallocate and distribute it correctly.
>>> In case of a thunderbolt capable bridge, reconfigure the buses allocated
> Thunderbolt
Will correct it.
>
>>> by BIOS to the maximum available buses. So that the hot-plug bridges gets
>>> maximum buses and chain can be extended to accommodate more PCIe devices.
>>> This fix is necessary for all the PCIe downstream ports where the daisy
>>> chain can be extended.
> This is necessary only when there is no proper BIOS allocation for the
> resources.
Yes, will send out a v2 with updated commit message.
>
>>> After:
>>>              +-04.0
>>>              +-04.1-[63-c1]----00.0-[64-c1]--+-00.0-[65]--
>>>              |                               +-01.0-[66-84]--
>>>              |                               +-02.0-[85-a3]--
>>>              |                               +-03.0-[a4-c0]--
>>>              |                               \-04.0-[c1]--
>>>              +-08.0
>>>
>>> Link: https://bugzilla.kernel.org/show_bug.cgi?id=216000
> Did you get confirmation that this actually solves the issue?
I've tested this on my setup, it is resolving the issue.
>
>>> Signed-off-by: Sanjay R Mehta <sanju.mehta@....com>
>>> Signed-off-by: Sanath S <Sanath.S@....com>
>>> ---
>>>    drivers/pci/probe.c | 9 +++++++++
>>>    1 file changed, 9 insertions(+)
>>>
>>> diff --git a/drivers/pci/probe.c b/drivers/pci/probe.c
>>> index 8bac3ce02609..ab7e90ef2382 100644
>>> --- a/drivers/pci/probe.c
>>> +++ b/drivers/pci/probe.c
>>> @@ -1263,6 +1263,8 @@ static int pci_scan_bridge_extend(struct pci_bus *bus, struct pci_dev *dev,
>>>    	bool fixed_buses;
>>>    	u8 fixed_sec, fixed_sub;
>>>    	int next_busnr;
>>> +	int start = bus->busn_res.start;
>>> +	int end = bus->busn_res.end;
>>>    	/*
>>>    	 * Make sure the bridge is powered on to be able to access config
>>> @@ -1292,6 +1294,13 @@ static int pci_scan_bridge_extend(struct pci_bus *bus, struct pci_dev *dev,
>>>    		broken = 1;
>>>    	}
>>> +	/* Reconfigure, If maximum buses are not allocated */
>>> +	if (!pass && start != 0 && end != 0xff && subordinate != end) {
>>> +		pci_info(dev, "Bridge has subordinate 0x%x but max busn 0x%x, reconfiguring\n",
>>> +			 subordinate, end);
>>> +		broken = 1;
>>> +	}
>>> +
>>>    	/*
>>>    	 * Disable Master-Abort Mode during probing to avoid reporting of
>>>    	 * bus errors in some architectures.