linux-kernel - Re: [RFC PATCH 1/3] PCI: Wait for device readiness with Configuration RRS

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Message-ID: <Zs6kwHX7EIGvnf9_@wunner.de>
Date: Wed, 28 Aug 2024 06:17:04 +0200
From: Lukas Wunner <lukas@...ner.de>
To: Bjorn Helgaas <helgaas@...nel.org>
Cc: linux-pci@...r.kernel.org,
	Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>,
	"Maciej W . Rozycki" <macro@...am.me.uk>,
	Mika Westerberg <mika.westerberg@...ux.intel.com>,
	"Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
	Mario Limonciello <mario.limonciello@....com>,
	Duc Dang <ducdang@...gle.com>,
	Alex Williamson <alex.williamson@...hat.com>,
	linux-kernel@...r.kernel.org, Bjorn Helgaas <bhelgaas@...gle.com>
Subject: Re: [RFC PATCH 1/3] PCI: Wait for device readiness with
 Configuration RRS

On Tue, Aug 27, 2024 at 06:48:46PM -0500, Bjorn Helgaas wrote:
> @@ -1311,9 +1320,15 @@ static int pci_dev_wait(struct pci_dev *dev, char *reset_type, int timeout)
>  			return -ENOTTY;
>  		}
>  
> -		pci_read_config_dword(dev, PCI_COMMAND, &id);
> -		if (!PCI_POSSIBLE_ERROR(id))
> -			break;
> +		if (root && root->config_crs_sv) {
> +			pci_read_config_dword(dev, PCI_VENDOR_ID, &id);
> +			if (!pci_bus_crs_vendor_id(id))
> +				break;

There was an effort from Amazon back in 2020/2021 to improve CRS support:

https://lore.kernel.org/linux-pci/20200307172044.29645-1-stanspas@amazon.com/

One suggestion you raised in the subsequent discussion was to use a
16-bit (word) instead of a 32-bit (dword) read of the Vendor ID
register to avoid issues with devices that don't implement CRS SV
correctly:

https://lore.kernel.org/linux-pci/20210913160745.GA1329939@bjorn-Precision-5520/

I realize that pci_bus_crs_vendor_id() masks the Device ID bits,
so probably no biggie.  Just want to make sure all lessons learned
during previous discussions on this topic are considered. :)

Thanks,

Lukas