linux-kernel - Re: [PATCH v11 3/4] PCI/DOE: Expose the DOE features via sysfs

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Date: Fri, 14 Jun 2024 09:59:24 +0100
From: Jonathan Cameron <Jonathan.Cameron@...wei.com>
To: Alistair Francis <alistair23@...il.com>
CC: <bhelgaas@...gle.com>, <linux-pci@...r.kernel.org>, <lukas@...ner.de>,
	<alex.williamson@...hat.com>, <christian.koenig@....com>, <kch@...dia.com>,
	<gregkh@...uxfoundation.org>, <logang@...tatee.com>,
	<linux-kernel@...r.kernel.org>, <chaitanyak@...dia.com>,
	<rdunlap@...radead.org>, Alistair Francis <alistair.francis@....com>
Subject: Re: [PATCH v11 3/4] PCI/DOE: Expose the DOE features via sysfs

On Fri, 14 Jun 2024 10:12:43 +1000
Alistair Francis <alistair23@...il.com> wrote:

> The PCIe 6 specification added support for the Data Object
> Exchange (DOE).
> When DOE is supported the DOE Discovery Feature must be implemented per
> PCIe r6.1 sec 6.30.1.1. The protocol allows a requester to obtain
> information about the other DOE features supported by the device.
> 
> The kernel is already querying the DOE features supported and cacheing
> the values. Expose the values in sysfs to allow user space to
> determine which DOE features are supported by the PCIe device.
> 
> By exposing the information to userspace tools like lspci can relay the
> information to users. By listing all of the supported features we can
> allow userspace to parse the list, which might include
> vendor specific features as well as yet to be supported features.
> 
> As the DOE Discovery feature must always be supported we treat it as a
> special named attribute case. This allows the usual PCI attribute_group
> handling to correctly create the doe_features directory when registering
> pci_doe_sysfs_group (otherwise it doesn't and sysfs_add_file_to_group()
> will seg fault).
> 
> After this patch is supported you can see something like this when
> attaching a DOE device
> 
> $ ls /sys/devices/pci0000:00/0000:00:02.0//doe*
> 0001:01        0001:02        doe_discovery
> 
> Signed-off-by: Alistair Francis <alistair.francis@....com>
Hi Alistair,

One question inline.  Feels like I'm either missing something or
the code has evolved in a fashion that left us with a pointless check
on attr visibility.

Jonathan

> diff --git a/drivers/pci/doe.c b/drivers/pci/doe.c
> index defc4be81bd4..9858b709c020 100644
> --- a/drivers/pci/doe.c
> +++ b/drivers/pci/doe.c

> +static umode_t pci_doe_features_sysfs_attr_visible(struct kobject *kobj,
> +						   struct attribute *a, int n)
> +{
> +	struct pci_dev *pdev = to_pci_dev(kobj_to_dev(kobj));
> +	struct pci_doe_mb *doe_mb;
> +	unsigned long index, j;
> +	unsigned long vid, type;
> +	void *entry;
> +
> +	xa_for_each(&pdev->doe_mbs, index, doe_mb) {
> +		xa_for_each(&doe_mb->feats, j, entry) {

I'm confused.  What is the intent here?

Given every DOE should have the discovery entry any call of this function
that actually finds a DOE should return a->mode, so why search the
actual entries? 

Given absence of the files anyway (due to the directory visible checks)
if there are no DOEs, why not drop this function completely?

> +			vid = xa_to_value(entry) >> 8;
> +			type = xa_to_value(entry) & 0xFF;
> +
> +			if (vid == 0x01 && type == 0x00) {
> +				/*
> +				 * This is the DOE discovery protocol
> +				 * Every DOE instance must support this, so we
> +				 * give it a useful name.
> +				 */
> +				return a->mode;
> +			}
> +		}
> +	}
> +
> +	return 0;
> +}