linux-kernel - Re: [PATCH V12 6/9] cxl/port: Read CDAT table

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Date:   Tue, 28 Jun 2022 15:47:27 +0100
From:   Jonathan Cameron <Jonathan.Cameron@...wei.com>
To:     <ira.weiny@...el.com>
CC:     Dan Williams <dan.j.williams@...el.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Lukas Wunner <lukas@...ner.de>,
        Alison Schofield <alison.schofield@...el.com>,
        Vishal Verma <vishal.l.verma@...el.com>,
        "Dave Jiang" <dave.jiang@...el.com>,
        Ben Widawsky <bwidawsk@...nel.org>,
        <linux-kernel@...r.kernel.org>, <linux-cxl@...r.kernel.org>,
        <linux-pci@...r.kernel.org>
Subject: Re: [PATCH V12 6/9] cxl/port: Read CDAT table

On Mon, 27 Jun 2022 21:15:24 -0700
ira.weiny@...el.com wrote:

> From: Ira Weiny <ira.weiny@...el.com>
> 
> The OS will need CDAT data from CXL devices to properly set up
> interleave sets.  Currently this is supported through a DOE mailbox
> which supports CDAT.
> 
> Search the DOE mailboxes available, query CDAT data, and cache the data
> for later parsing.
> 
> Provide a sysfs binary attribute to allow dumping of the CDAT.
> 
> Binary dumping is modeled on /sys/firmware/ACPI/tables/
> 
> The ability to dump this table will be very useful for emulation of real
> devices once they become available as QEMU CXL type 3 device emulation will
> be able to load this file in.
> 
> This does not support table updates at runtime. It will always provide
> whatever was there when first cached. Handling of table updates can be
> implemented later.
> 
> Finally create a complete list of CDAT defines within cdat.h for code
> wishing to decode the CDAT table.
> 
> Signed-off-by: Jonathan Cameron <Jonathan.Cameron@...wei.com>
> Co-developed-by: Jonathan Cameron <Jonathan.Cameron@...wei.com>
> Signed-off-by: Ira Weiny <ira.weiny@...el.com>

One query inline, though I'm fine with it either way, just want to
understand your logic in keeping completion when Dan suggested using
flush_work() to achieve the same thing.

> 
> ---
> Changes from V11:
> 	Adjust for the use of DOE mailbox xarray
> 	Dan Williams:
> 		Remove unnecessary get/put device
> 		Use new BIN_ATTR_ADMIN_RO macro
> 		Flag that CDAT was supported
> 			If there is a read error then the CDAT sysfs
> 			will return a 0 length entry
> 
...
> diff --git a/drivers/cxl/core/pci.c b/drivers/cxl/core/pci.c
> index c4c99ff7b55e..4bd479ec0253 100644
> --- a/drivers/cxl/core/pci.c
> +++ b/drivers/cxl/core/pci.c
> @@ -4,10 +4,12 @@

> +static int cxl_cdat_get_length(struct device *dev,
> +			       struct pci_doe_mb *cdat_mb,
> +			       size_t *length)
> +{
> +	u32 cdat_request_pl = CDAT_DOE_REQ(0);
> +	u32 cdat_response_pl[32];
> +	DECLARE_COMPLETION_ONSTACK(c);
> +	struct pci_doe_task task = {
> +		.prot.vid = PCI_DVSEC_VENDOR_ID_CXL,
> +		.prot.type = CXL_DOE_PROTOCOL_TABLE_ACCESS,
> +		.request_pl = &cdat_request_pl,
> +		.request_pl_sz = sizeof(cdat_request_pl),
> +		.response_pl = cdat_response_pl,
> +		.response_pl_sz = sizeof(cdat_response_pl),
> +		.complete = cxl_doe_task_complete,
> +		.private = &c,
> +	};
> +	int rc = 0;
> +
> +	rc = pci_doe_submit_task(cdat_mb, &task);
> +	if (rc < 0) {
> +		dev_err(dev, "DOE submit failed: %d", rc);
> +		return rc;
> +	}
> +	wait_for_completion(&c);

Dan mentioned in his review that we could just use
flush_work() and drop the completion logic and callback.
Why didn't you go that way?  Would want to wrap it up
in pci_doe_wait_task() or something like that.

> +
> +	if (task.rv < 1)
> +		return -EIO;
> +
> +	*length = cdat_response_pl[1];
> +	dev_dbg(dev, "CDAT length %zu\n", *length);
> +
> +	return rc;
> +}
> +