netdev - Re: [PATCH v2 9/9] wifi: ath10k: Use RMW accessors for changing LNKCTL

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Date: Wed, 24 May 2023 10:10:54 -0500
From: Bjorn Helgaas <helgaas@...nel.org>
To: Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>
Cc: linux-pci@...r.kernel.org, Bjorn Helgaas <bhelgaas@...gle.com>,
	Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
	Rob Herring <robh@...nel.org>,
	Krzysztof Wilczyński <kw@...ux.com>,
	Emmanuel Grumbach <emmanuel.grumbach@...el.com>,
	"Rafael J . Wysocki" <rafael@...nel.org>,
	Heiner Kallweit <hkallweit1@...il.com>,
	Lukas Wunner <lukas@...ner.de>, Kalle Valo <kvalo@...nel.org>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	Michal Kazior <michal.kazior@...to.com>,
	Janusz Dziedzic <janusz.dziedzic@...to.com>,
	ath10k@...ts.infradead.org, linux-wireless@...r.kernel.org,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	Dean Luick <dean.luick@...nelisnetworks.com>,
	Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
	stable@...r.kernel.org
Subject: Re: [PATCH v2 9/9] wifi: ath10k: Use RMW accessors for changing
 LNKCTL

On Wed, May 17, 2023 at 01:52:35PM +0300, Ilpo Järvinen wrote:
> Don't assume that only the driver would be accessing LNKCTL. ASPM
> policy changes can trigger write to LNKCTL outside of driver's control.
> 
> Use RMW capability accessors which does proper locking to avoid losing
> concurrent updates to the register value. On restore, clear the ASPMC
> field properly.
> 
> Fixes: 76d870ed09ab ("ath10k: enable ASPM")
> Suggested-by: Lukas Wunner <lukas@...ner.de>
> Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>
> Cc: stable@...r.kernel.org
> ---
>  drivers/net/wireless/ath/ath10k/pci.c | 9 +++++----
>  1 file changed, 5 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/net/wireless/ath/ath10k/pci.c b/drivers/net/wireless/ath/ath10k/pci.c
> index a7f44f6335fb..9275a672f90c 100644
> --- a/drivers/net/wireless/ath/ath10k/pci.c
> +++ b/drivers/net/wireless/ath/ath10k/pci.c
> @@ -1963,8 +1963,9 @@ static int ath10k_pci_hif_start(struct ath10k *ar)
>  	ath10k_pci_irq_enable(ar);
>  	ath10k_pci_rx_post(ar);
>  
> -	pcie_capability_write_word(ar_pci->pdev, PCI_EXP_LNKCTL,
> -				   ar_pci->link_ctl);
> +	pcie_capability_clear_and_set_word(ar_pci->pdev, PCI_EXP_LNKCTL,
> +					   PCI_EXP_LNKCTL_ASPMC,
> +					   ar_pci->link_ctl & PCI_EXP_LNKCTL_ASPMC);
>  
>  	return 0;
>  }
> @@ -2821,8 +2822,8 @@ static int ath10k_pci_hif_power_up(struct ath10k *ar,
>  
>  	pcie_capability_read_word(ar_pci->pdev, PCI_EXP_LNKCTL,
>  				  &ar_pci->link_ctl);
> -	pcie_capability_write_word(ar_pci->pdev, PCI_EXP_LNKCTL,
> -				   ar_pci->link_ctl & ~PCI_EXP_LNKCTL_ASPMC);
> +	pcie_capability_clear_word(ar_pci->pdev, PCI_EXP_LNKCTL,
> +				   PCI_EXP_LNKCTL_ASPMC);

These ath drivers all have the form:

  1) read LNKCTL
  2) save LNKCTL value in ->link_ctl
  3) write LNKCTL with "->link_ctl & ~PCI_EXP_LNKCTL_ASPMC"
     to disable ASPM
  4) write LNKCTL with ->link_ctl, presumably to re-enable ASPM

These patches close the hole between 1) and 3) where other LNKCTL
updates could interfere, which is definitely a good thing.

But the hole between 1) and 4) is much bigger and still there.  Any
update by the PCI core in that interval would be lost.

Straw-man proposal:

  - Change pci_disable_link_state() so it ignores aspm_disabled and
    always disables ASPM even if platform firmware hasn't granted
    ownership.  Maybe this should warn and taint the kernel.

  - Change drivers to use pci_disable_link_state() instead of writing
    LNKCTL directly.

Bjorn