linux-kernel - Re: [PATCH v2 1/2] PCI: Use the correct bit in Link Training not active check

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Message-ID: <20240425175617.GA536953@bhelgaas>
Date: Thu, 25 Apr 2024 12:56:17 -0500
From: Bjorn Helgaas <helgaas@...nel.org>
To: Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>
Cc: Bjorn Helgaas <bhelgaas@...gle.com>, linux-pci@...r.kernel.org,
	"Maciej W. Rozycki" <macro@...am.me.uk>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/2] PCI: Use the correct bit in Link Training not
 active check

On Tue, Apr 23, 2024 at 04:08:19PM +0300, Ilpo Järvinen wrote:
> Two changes were made into link retraining logic independent of each
> other.
> 
> The commit e7e39756363a ("PCI/ASPM: Avoid link retraining race") added
> check to ensure no Link Training is currently active into
> pcie_retrain_link() to address the Implementation Note in PCIe r6.1 sec
> 7.5.3.7. At that time pcie_wait_for_retrain() only checked for Link
> Training (LT) bit being cleared.
> 
> The commit 680e9c47a229 ("PCI: Add support for polling DLLLA to
> pcie_retrain_link()") generalized pcie_wait_for_retrain() into
> pcie_wait_for_link_status() which can wait either for LT or Data Link
> Layer Link Active (DLLLA) bit with 'use_lt' argument and supporting
> waiting for either cleared or set using 'active' argument.
> 
> In the merge commit commit 1abb47390350 ("Merge branch
> 'pci/enumeration'"), those two divergent branches converged. The merge
> changed LT bit checking added in the commit e7e39756363a ("PCI/ASPM:
> Avoid link retraining race") to now wait for completion of any ongoing
> Link Training using DLLLA bit being set if 'use_lt' is false.
> 
> When 'use_lt' is false, the pseudo-code steps of what occurs in
> pcie_retrain_link():
> 
> 	1. Wait for DLLLA=1
> 	2. Trigger link to retrain
> 	3. Wait for DLLLA=1
> 
> Step 3 waits for the link to come up from the retraining triggered by
> Step 2. As Step 1 is supposed to wait for any ongoing retraining to
> end, using DLLLA also for it does not make sense because link training
> being active is still indicated using LT bit, not with DLLLA.
> 
> Correct the pcie_wait_for_link_status() parameters in Step 1 to only
> wait for LT=0 to ensure there is no ongoing Link Training.
> 
> This only impacts the Target Speed quirk, which is the only case where
> waiting for DLLLA bit is used. It currently works in the problematic
> case by means of link training getting initiated by hardware repeatedly
> and respecting the new link parameters set by the caller, which then
> make training succeed and bring the link up, setting DLLLA and causing
> pcie_wait_for_link_status() to return success. We are not supposed to
> rely on luck and need to make sure that LT transitioned through the
> inactive state though before we initiate link training by hand via RL
> (Retrain Link) bit.
> 
> Fixes: 1abb47390350 ("Merge branch 'pci/enumeration'")
> Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>

I applied both of these with minor typo fixes to pci/enumeration for
v6.10, thanks!

  73cb3a35f94d ("PCI: Wait for Link Training==0 before starting Link retrain")
  cdc6c4abcb31 ("PCI: Clarify intent of LT wait")

We can update if needed based on feedback from Maciej.

> ---
> 
> v2:
> - Improve commit message
> 
> NOTE: Maciej NAK'ed the v1 of this patch but has since retracted his
> NAK.
> 
> Maciej, if possible, could you please test this with your HW?
> 
> ---
>  drivers/pci/pci.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
> index e5f243dd4288..70b8c87055cb 100644
> --- a/drivers/pci/pci.c
> +++ b/drivers/pci/pci.c
> @@ -4629,7 +4629,7 @@ int pcie_retrain_link(struct pci_dev *pdev, bool use_lt)
>  	 * avoid LTSSM race as recommended in Implementation Note at the
>  	 * end of PCIe r6.0.1 sec 7.5.3.7.
>  	 */
> -	rc = pcie_wait_for_link_status(pdev, use_lt, !use_lt);
> +	rc = pcie_wait_for_link_status(pdev, true, false);
>  	if (rc)
>  		return rc;
>  
> -- 
> 2.39.2
>