linux-kernel - Re: [patch 1/8] PCI/MSI: Enable and mask MSIX early

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Date:   Thu, 22 Jul 2021 16:43:29 -0500
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Alex Williamson <alex.williamson@...hat.com>,
        "Raj, Ashok" <ashok.raj@...el.com>,
        "David S. Miller" <davem@...emloft.net>,
        Bjorn Helgaas <bhelgaas@...gle.com>, linux-pci@...r.kernel.org,
        Kevin Tian <kevin.tian@...el.com>,
        Marc Zyngier <maz@...nel.org>, Ingo Molnar <mingo@...nel.org>,
        x86@...nel.org
Subject: Re: [patch 1/8] PCI/MSI: Enable and mask MSIX early

s/MSIX/MSI-X/ in subject

On Wed, Jul 21, 2021 at 09:11:27PM +0200, Thomas Gleixner wrote:
> The ordering of MSI-X enable in hardware is disfunctional:

s/disfunctional/dysfunctional/, isn't English wonderful ;)

>  1) MSI-X is disabled in the control register
>  2) Various setup functions
>  3) pci_msi_setup_msi_irqs() is invoked which ends up accessing
>     the MSI-X table entries
>  4) MSI-X is enabled and masked in the control register with the
>     comment that enabling is required for some hardware to access
>     the MSI-X table
> 
> #4 obviously contradicts #3. The history of this is an issue with the NIU

Annoyingly, if you "git rebase" and reword this commit log, it drops
this line and the one a few lines below because they start with "#".
Should be obvious, but took me a few iterations to see what was
happening.

> hardware. When #4 was introduced the table access actually happened in
> msix_program_entries() which was invoked after enabling and masking MSI-X.
> 
> This was changed in commit d71d6432e105 ("PCI/MSI: Kill redundant call of
> irq_set_msi_desc() for MSI-X interrupts") which removed the table write
> from msix_program_entries().
> 
> Interestingly enough nobody noticed and either NIU still works or it did
> not get any testing with a kernel 3.19 or later.
> 
> Nevertheless this is inconsistent and there is no reason why MSI-X can't be
> enabled and masked in the control register early on, i.e. move #4 above to
> #1. This preserves the NIU workaround and has no side effects on other
> hardware.
>
> Fixes: d71d6432e105 ("PCI/MSI: Kill redundant call of irq_set_msi_desc() for MSI-X interrupts")
> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
> Cc: David S. Miller <davem@...emloft.net>
> Cc: Bjorn Helgaas <bhelgaas@...gle.com>
> Cc: linux-pci@...r.kernel.org
> ---
>  drivers/pci/msi.c |   28 +++++++++++++++-------------
>  1 file changed, 15 insertions(+), 13 deletions(-)
> 
> --- a/drivers/pci/msi.c
> +++ b/drivers/pci/msi.c
> @@ -772,18 +772,25 @@ static int msix_capability_init(struct p
>  	u16 control;
>  	void __iomem *base;
>  
> -	/* Ensure MSI-X is disabled while it is set up */
> -	pci_msix_clear_and_set_ctrl(dev, PCI_MSIX_FLAGS_ENABLE, 0);
> +	/*
> +	 * Some devices require MSI-X to be enabled before the MSI-X
> +	 * registers can be accessed.  Mask all the vectors to prevent
> +	 * interrupts coming in before they're fully set up.
> +	 */
> +	pci_msix_clear_and_set_ctrl(dev, 0, PCI_MSIX_FLAGS_MASKALL |
> +				    PCI_MSIX_FLAGS_ENABLE);
>  
>  	pci_read_config_word(dev, dev->msix_cap + PCI_MSIX_FLAGS, &control);
>  	/* Request & Map MSI-X table region */
>  	base = msix_map_region(dev, msix_table_size(control));
> -	if (!base)
> -		return -ENOMEM;
> +	if (!base) {
> +		ret = -ENOMEM;
> +		goto out_disable;
> +	}
>  
>  	ret = msix_setup_entries(dev, base, entries, nvec, affd);
>  	if (ret)
> -		return ret;
> +		goto out_disable;
>  
>  	ret = pci_msi_setup_msi_irqs(dev, nvec, PCI_CAP_ID_MSIX);
>  	if (ret)
> @@ -794,14 +801,6 @@ static int msix_capability_init(struct p
>  	if (ret)
>  		goto out_free;
>  
> -	/*
> -	 * Some devices require MSI-X to be enabled before we can touch the
> -	 * MSI-X registers.  We need to mask all the vectors to prevent
> -	 * interrupts coming in before they're fully set up.
> -	 */
> -	pci_msix_clear_and_set_ctrl(dev, 0,
> -				PCI_MSIX_FLAGS_MASKALL | PCI_MSIX_FLAGS_ENABLE);
> -
>  	msix_program_entries(dev, entries);
>  
>  	ret = populate_msi_sysfs(dev);
> @@ -836,6 +835,9 @@ static int msix_capability_init(struct p
>  out_free:
>  	free_msi_irqs(dev);
>  
> +out_disable:
> +	pci_msix_clear_and_set_ctrl(dev, PCI_MSIX_FLAGS_ENABLE, 0);
> +
>  	return ret;
>  }
>  
>