linux-kernel - Re: [PATCH v2 4/5] PCI: keystone: Add quirk to mark AM654 RC BAR flag as IORESOURCE

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Date:   Tue, 4 Jan 2022 15:57:41 +0000
From:   Lorenzo Pieralisi <lorenzo.pieralisi@....com>
To:     Kishon Vijay Abraham I <kishon@...com>,
        Bjorn Helgaas <bhelgaas@...gle.com>
Cc:     Rob Herring <robh@...nel.org>,
        Krzysztof Wilczyński <kw@...ux.com>,
        linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org,
        devicetree@...r.kernel.org
Subject: Re: [PATCH v2 4/5] PCI: keystone: Add quirk to mark AM654 RC BAR
 flag as IORESOURCE_UNSET

On Fri, Nov 26, 2021 at 02:01:18PM +0530, Kishon Vijay Abraham I wrote:
> AM654 RootComplex has a hard coded 64 bit BAR of size 1MB and also has
> both MSI and MSI-X capability in it's config space. If PCIEPORTBUS is
> enabled, it tries to configure MSI-X and msix_mask_all() adds about 10
> Second boot up delay when it tries to write to undefined location.
> 
> Add quirk to mark AM654 RC BAR flag as IORESOURCE_UNSET so that
> msix_map_region() returns NULL for Root Complex and avoid un-desirable
> writes to MSI-X table.

I don't think this is the right fix (it is not even a fix, just a
plaster to workaround an issue).

What do you mean by "writing to an undefined location" ?

What does "a hard coded BAR" mean ?

What happens if we _rightly_ write into it (ie to size it) ?

Lorenzo

> Signed-off-by: Kishon Vijay Abraham I <kishon@...com>
> ---
>  drivers/pci/controller/dwc/pci-keystone.c | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/pci/controller/dwc/pci-keystone.c b/drivers/pci/controller/dwc/pci-keystone.c
> index 52d20fe17ee9..73e6626a0d8f 100644
> --- a/drivers/pci/controller/dwc/pci-keystone.c
> +++ b/drivers/pci/controller/dwc/pci-keystone.c
> @@ -557,8 +557,14 @@ static void ks_pcie_quirk(struct pci_dev *dev)
>  		{ 0, },
>  	};
>  
> -	if (pci_is_root_bus(bus))
> +	if (pci_is_root_bus(bus)) {
>  		bridge = dev;
> +		if (pci_match_id(am6_pci_devids, bridge)) {
> +			struct resource *r = &dev->resource[0];
> +
> +			r->flags |= IORESOURCE_UNSET;
> +		}
> +	}
>  
>  	/* look for the host bridge */
>  	while (!pci_is_root_bus(bus)) {
> -- 
> 2.17.1
>