Date:   Wed, 22 Jun 2022 09:38:31 -0700
From:   Ira Weiny <ira.weiny@...el.com>
To:     Bjorn Helgaas <helgaas@...nel.org>
CC:     Dan Williams <dan.j.williams@...el.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Jonathan Cameron <Jonathan.Cameron@...wei.com>,
        "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 V11 3/8] PCI: Create PCI library functions in support of
 DOE mailboxes.

On Fri, Jun 17, 2022 at 05:40:19PM -0500, Bjorn Helgaas wrote:
> On Fri, Jun 10, 2022 at 01:22:54PM -0700, ira.weiny@...el.com wrote:
> > From: Jonathan Cameron <Jonathan.Cameron@...wei.com>
> > 
> > Introduced in a PCI r6.0, sec 6.30, DOE provides a config space based
> > mailbox with standard protocol discovery.  Each mailbox is accessed
> > through a DOE Extended Capability.
> 
> > +/* Timeout of 1 second from 6.30.2 Operation, PCI Spec r6.0 */
> 
> s/PCI/PCIe/ (up in commit log, too, I guess :))
> 
> Not that there will ever be a conventional PCI r6.0 spec, but there
> was a PCI r3.0 well as a PCIe r3.0, so might as well keep them
> straight.

Done.

> 
> > +struct pci_doe_mb {
> > +	struct pci_dev *pdev;
> 
> Trivial, but I would put cap_offset here next to pdev because the
> (pdev, cap_offset) tuple is basically the identifier for the DOE
> instance.

Done.

> 
> > +	struct completion abort_c;
> > +	int irq;
> > +	struct pci_doe_protocol *prots;
> > +	int num_prots;
> > +	u16 cap_offset;
> 
> > +static void pci_doe_abort_start(struct pci_doe_mb *doe_mb)
> > +{
> > +	struct pci_dev *pdev = doe_mb->pdev;
> > +	int offset = doe_mb->cap_offset;
> > +	u32 val;
> > +
> > +	val = PCI_DOE_CTRL_ABORT;
> > +	if (doe_mb->irq >= 0)
> 
> Is zero a valid IRQ?  In general, I don't think it is, but maybe this
> is a special case.  Or maybe this is actually the "Interrupt Message
> Number" mentioned in sec 6.30.3?  If so maybe something other than
> "irq" would be a better name here.

Yes I think irq is a bad name.  I think 0 is valid here because this is the
Interrupt Message Number" from the DOE Capabilities Register (7.9.24.2).

At least with Qemu 0 is returned for the 1st mailbox.  I'm not sure if that is
valid or not but I think it is.

But reading that in detail I think there is even more complexity than Jonathan
or I realized with regard to MSI vs MSI-X.

I'm going to leave the irq support in this layer (changing 'irq' to
'irq_msg_num'?)  but I think the callers will need to resolve what support they
enable.

> 
> Possibly relevant: a85a6c86c25b ("driver core: platform: Clarify that
> IRQ 0 is invalid")
> 
> > +		pci_err(pdev,
> > +			"DOE [%x] expected [VID, Protocol] = [%04x, %02x], got [%04x, %02x]\n",
> 
> Wouldn't make a big difference, but could consider something like this
> for enforced consistency:
> 
>   #define dev_fmt(fmt) "DOE: " fmt

Good idea.

> 
> > +	case DOE_WAIT_ABORT:
> > +	case DOE_WAIT_ABORT_ON_ERR:
> > +		prev_state = doe_mb->state;
> > +
> > +		pci_read_config_dword(pdev, offset + PCI_DOE_STATUS, &val);
> > +
> > +		if (!FIELD_GET(PCI_DOE_STATUS_ERROR, val) &&
> > +		    !FIELD_GET(PCI_DOE_STATUS_BUSY, val)) {
> > +			doe_mb->state = DOE_IDLE;
> > +			/* Back to normal state - carry on */
> > +			retire_cur_task(doe_mb);
> > +		} else if (time_after(jiffies, doe_mb->timeout_jiffies)) {
> > +			/* Task has timed out and is dead - abort */
> > +			pci_err(pdev, "DOE [%x] ABORT timed out\n",
> > +				doe_mb->cap_offset);
> > +			set_bit(PCI_DOE_FLAG_DEAD, &doe_mb->flags);
> > +			retire_cur_task(doe_mb);
> > +		}
> > +
> > +		/*
> > +		 * For deliberately triggered abort, someone is
> > +		 * waiting.
> > +		 */
> > +		if (prev_state == DOE_WAIT_ABORT) {
> > +			if (task)
> > +				signal_task_complete(task, -EFAULT);
> > +			complete(&doe_mb->abort_c);
> > +		}
> > +
> > +		return;
> > +	}
> 
> The "return" in each case is perfectly correct, but it feels a little
> more conventional to make them "break" and return once here after the
> switch to make it clear that the only way to get to the labels is via
> an error path "goto".

Done.

> 
> > +err_abort:
> > +	doe_mb->state = DOE_WAIT_ABORT_ON_ERR;
> > +	pci_doe_abort_start(doe_mb);
> > +err_busy:
> > +	signal_task_complete(task, rc);
> > +	if (doe_mb->state == DOE_IDLE)
> > +		retire_cur_task(doe_mb);
> > +}
> 
> > +	 * Enabling bus mastering is required for MSI/MSIx.  It is safe to call
> 
> s/MSIx/MSI-X/ (typical spelling in spec)
> 
> > +	 * this multiple times and thus is called here to ensure that mastering
> > +	 * is enabled even if the driver has done so.
> > +	 */
> > +	pci_set_master(pdev);
> > +	rc = pci_request_irq(pdev, irq, pci_doe_irq_handler, NULL, doe_mb,
> > +			     "DOE[%d:%s]", irq, pci_name(pdev));
> 
> I assume the "DOE[%d:%s]" part appears in /proc/interrupts?

Yes

> Is it
> redundant to include "irq", since /proc/interrupts already prints it,
> or is there somewhere else where "irq" is useful?

As you pointed out irq is the wrong name here.  This is just the message
number.

> 
> How does the user associate this IRQ in /proc/interrupts with a
> specific DOE capability?  Should we include the cap_offset along with
> the pci_name()?

Good idea, cap_offset is much more useful.  In my testing the irq's were all
unique but as Dan pointed out I did not realize that the message number could
be shared.

> 
> > + * pci_doe_get_irq_num() - Return the irq number for the mailbox at offset
> > + *
> > + * @pdev: The PCI device
> > + * @offset: Offset of the DOE mailbox
> > + *
> > + * Returns: irq number on success
> > + *	    -errno if irqs are not supported on this mailbox
> 
> I normally capitalize IRQ/IRQs in comments.  There are probably others
> throughout the file.  I notice some are already capitalized but not all.

Done.

> 
> > + */
> > +int pci_doe_get_irq_num(struct pci_dev *pdev, int offset)
> > +{
> > +	u32 val;
> > +
> > +	pci_read_config_dword(pdev, offset + PCI_DOE_CAP, &val);
> > +	if (!FIELD_GET(PCI_DOE_CAP_INT, val))
> > +		return -EOPNOTSUPP;
> > +
> > +	return FIELD_GET(PCI_DOE_CAP_IRQ, val);
> > +}
> > +EXPORT_SYMBOL_GPL(pci_doe_get_irq_num);
> 
> Confusing function name (and comment) since PCI_DOE_CAP_IRQ is an
> Interrupt Message Number that has nothing to do with Linux IRQ
> numbers.

Agreed.  Changed to pci_doe_get_irq_msg_num();  With corresponding fixups to
the kdoc.

> 
> I see we already have PCI_EXP_FLAGS_IRQ, PCI_ERR_ROOT_AER_IRQ,
> PCI_EXP_DPC_IRQ, so I guess you're in good company.
> 
> At least maybe update the comment to say "Interrupt Message Number"
> instead of "irq".

Yea I did that too.

> 
> > + * pci_doe_supports_prot() - Return if the DOE instance supports the given
> > + *			     protocol
> > + * @doe_mb: DOE mailbox capability to query
> > + * @vid: Protocol Vendor ID
> > + * @type: Protocol type
> > + *
> > + * RETURNS: True if the DOE mailbox supports the protocol specified
> 
> Is the typical use that the caller has a few specific protocols it
> cares about?

That is how CXL needs it right now yes.

> There's no case where a caller might want to enumerate
> them all?

Not at this time.

> I guess they're all in prots[], but that's supposed to be
> opaque to users.

Agreed.  Something else would be needed in that use case.

> 
> > + */
> > +bool pci_doe_supports_prot(struct pci_doe_mb *doe_mb, u16 vid, u8 type)
> > +{
> > +	int i;
> > +
> > +	/* The discovery protocol must always be supported */
> > +	if (vid == PCI_VENDOR_ID_PCI_SIG && type == PCI_DOE_PROTOCOL_DISCOVERY)
> > +		return true;
> > +
> > +	for (i = 0; i < doe_mb->num_prots; i++)
> > +		if ((doe_mb->prots[i].vid == vid) &&
> > +		    (doe_mb->prots[i].type == type))
> > +			return true;
> > +
> > +	return false;
> > +}
> > +EXPORT_SYMBOL_GPL(pci_doe_supports_prot);
> 
> > + * struct pci_doe_task - represents a single query/response
> > + *
> > + * @prot: DOE Protocol
> > + * @request_pl: The request payload
> > + * @request_pl_sz: Size of the request payload
> 
> Size is in dwords, not bytes, I guess?

No.  Those are in bytes and the DOE layer takes care of the DW conversion.
I'll update the kdoc to make that clear.

Thanks again for the review,
Ira

> 
> > + * @response_pl: The response payload
> > + * @response_pl_sz: Size of the response payload
> > + * @rv: Return value.  Length of received response or error
> > + * @complete: Called when task is complete
> > + * @private: Private data for the consumer
> > + */
> > +struct pci_doe_task {
> > +	struct pci_doe_protocol prot;
> > +	u32 *request_pl;
> > +	size_t request_pl_sz;
> > +	u32 *response_pl;
> > +	size_t response_pl_sz;
> > +	int rv;
> > +	void (*complete)(struct pci_doe_task *task);
> > +	void *private;
> > +};

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