linux-kernel - Re: [PATCH v2] PCI: pciehp: Report degraded links via link bandwidth notification

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Date:   Wed, 27 Feb 2019 20:21:58 +0000
From:   <Alex_Gagniuc@...lteam.com>
To:     <lukas@...ner.de>, <mr.nuke.me@...il.com>
Cc:     <helgaas@...nel.org>, <Austin.Bolen@...l.com>,
        <Shyam.Iyer@...l.com>, <bhelgaas@...gle.com>,
        <rafael.j.wysocki@...el.com>, <keith.busch@...el.com>,
        <poza@...eaurora.org>, <mika.westerberg@...ux.intel.com>,
        <fred@...dlawl.com>, <linux-kernel@...r.kernel.org>,
        <linux-pci@...r.kernel.org>
Subject: Re: [PATCH v2] PCI: pciehp: Report degraded links via link bandwidth
 notification

On 2/24/19 8:29 PM, Lukas Wunner wrote:
> On Fri, Dec 07, 2018 at 12:20:00PM -0600, Alexandru Gagniuc wrote:
> 
> 
>> Q: Why is this unconditionally compiled in?
>> A: The symmetrical check in pci probe() is also always compiled in.
> 
> Hm, it looks like the convention is to provide a separate Kconfig entry
> for each port service.

Does the convention still make sense in light of the symmetry reason?

>> Q: Why call module_init() instead of adding a call in pcie_init_services() ?
>> A: A call in pcie_init_services() also requires a prototype in portdrv.h, a
>> non-static implementation in bw_notification.c. Using module_init() is
>> functionally equivalent, and takes less code.
> 
> Commit c29de84149ab ("PCI: portdrv: Initialize service drivers directly")
> moved away from module_init() on purpose, apparently to fix a race
> condition.

*GROWL*


> What if the link is retrained at the same speed/width?  Intuitively
> I'd compare the speed in the Link Status Register to what is cached
> in the cur_bus_speed member of struct pci_bus and print a message
> only if the speed has changed.  (Don't we need to cache the width as
> well?)

There are two mechanisms to bring a degraded link back to full BW.
   1. Secondary bus reset, which results in the device being tore down 
along with our cached speed value.
   2. Set the PCI_EXP_LNKCTL_LD (Link Disable) bit and clear it. We do 
that as part of the pciehp teardown path. We'd lose our cached value 
just like in the first case.

>> +static irqreturn_t pcie_bw_notification_irq(int irq, void *context)
>> [...]
> 
> You need to hold pci_bus_sem [...]
> This may sleep, so request the IRQ with request_threaded_irq() [...]

Good catch! Thanks!
All other issues you pointed out should be resolved in next version.

Alex