From: Lukas Wunner <lukas@wunner.de>
To: Smita Koralahalli <Smita.KoralahalliChannabasappa@amd.com>
Cc: linux-pci@vger.kernel.org, linux-kernel@vger.kernel.org,
Bjorn Helgaas <helgaas@kernel.org>,
Mahesh J Salgaonkar <mahesh@linux.ibm.com>,
Yazen Ghannam <yazen.ghannam@amd.com>,
Ilpo Jarvinen <ilpo.jarvinen@linux.intel.com>,
Bowman Terry <terry.bowman@amd.com>,
Hagan Billy <billy.hagan@amd.com>,
Simon Guinot <simon.guinot@seagate.com>,
"Maciej W . Rozycki" <macro@orcam.me.uk>,
Kuppuswamy Sathyanarayanan
<sathyanarayanan.kuppuswamy@linux.intel.com>
Subject: Re: [PATCH] PCI: pciehp: Clear LBMS on hot-remove to prevent link speed reduction
Date: Mon, 6 May 2024 09:29:20 +0200 [thread overview]
Message-ID: <ZjiG0JJC4iS_9VHb@wunner.de> (raw)
In-Reply-To: <20240424033339.250385-1-Smita.KoralahalliChannabasappa@amd.com>
On Wed, Apr 24, 2024 at 03:33:39AM +0000, Smita Koralahalli wrote:
> Clear Link Bandwidth Management Status (LBMS) if set, on a hot-remove event.
>
> The hot-remove event could result in target link speed reduction if LBMS
> is set, due to a delay in Presence Detect State Change (PDSC) happening
> after a Data Link Layer State Change event (DLLSC).
>
> In reality, PDSC and DLLSC events rarely come in simultaneously. Delay in
> PDSC can sometimes be too late and the slot could have already been
> powered down just by a DLLSC event. And the delayed PDSC could falsely be
> interpreted as an interrupt raised to turn the slot on. This false process
> of powering the slot on, without a link forces the kernel to retrain the
> link if LBMS is set, to a lower speed to restablish the link thereby
> bringing down the link speeds [2].
>
> According to PCIe r6.2 sec 7.5.3.8 [1], it is derived that, LBMS cannot
> be set for an unconnected link and if set, it serves the purpose of
> indicating that there is actually a device down an inactive link.
> However, hardware could have already set LBMS when the device was
> connected to the port i.e when the state was DL_Up or DL_Active. Some
> hardwares would have even attempted retrain going into recovery mode,
> just before transitioning to DL_Down.
>
> Thus the set LBMS is never cleared and might force software to cause link
> speed drops when there is no link [2].
Is this an issue introduced by commit a89c82249c37 ("PCI: Work around
PCIe link training failures")?
If so, could you add a Fixes tag?
I might be mistaken but my impression is that we're seeing a lot of
fallout as a result of that commit.
Thanks,
Lukas
prev parent reply other threads:[~2024-05-06 7:38 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-04-24 3:33 [PATCH] PCI: pciehp: Clear LBMS on hot-remove to prevent link speed reduction Smita Koralahalli
2024-04-24 9:32 ` Ilpo Järvinen
2024-04-25 20:23 ` Smita Koralahalli
2024-04-26 8:42 ` Ilpo Järvinen
2024-04-25 3:10 ` Kuppuswamy Sathyanarayanan
2024-04-25 20:49 ` Smita Koralahalli
2024-05-06 7:29 ` Lukas Wunner [this message]
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