linux-kernel - Re: [PATCH v2] AHCI: Add generic MSI-X interrupt support to SATA PCI driver

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Message-ID: <20150513143353.GU11388@htj.duckdns.org>
Date:	Wed, 13 May 2015 10:33:53 -0400
From:	Tejun Heo <tj@...nel.org>
To:	Robert Richter <robert.richter@...iumnetworks.com>
Cc:	Robert Richter <rric@...nel.org>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will.deacon@....com>,
	Sunil Goutham <sgoutham@...ium.com>,
	Jiang Liu <jiang.liu@...ux.intel.com>,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	linux-ide@...r.kernel.org, Alexander Gordeev <agordeev@...hat.com>
Subject: Re: [PATCH v2] AHCI: Add generic MSI-X interrupt support to SATA PCI
 driver

Hello, Robert.

On Mon, May 11, 2015 at 07:18:10PM +0200, Robert Richter wrote:
> static int ahci_init_msix(struct pci_dev *pdev, unsigned int n_ports,
> 			  struct ahci_host_priv *hpriv)
> {
> 	int rc, nvec;
> 	struct msix_entry entry = {};
> 
> 	/* check if msix is supported */
> 	nvec = pci_msix_vec_count(pdev);
> 	if (nvec <= 0)
> 		return 0;
> 
> 	/*
> 	 * Per-port msix interrupts are not supported. Assume single
> 	 * port interrupts for:
> 	 *
> 	 *  n_ports == 1, or
> 	 *  nvec < n_ports.
> 	 *
> 	 * We also need to check for n_ports != 0 which is implicitly
> 	 * covered here since nvec > 0.
> 	 */
> 	if (n_ports != 1 && nvec >= n_ports)
> 		return -ENOSYS;

Why are failing the whole thing when nvec >= n_ports?  Can't we just
print some warning and configure it for single interrupt mode?

> > Also, shouldn't we be printing a warning message here explaining why
> > probing is failing?
> 
> I didn't want to print a warning in case -ENOSYS for backward
> compatability. Only if msi-x code fails there is a message, see
> __ahci_init_interrupts(). In any other case the behaviour is as
> before, thus no message is printed.

I'm confused here.  Why are we implementing msix support at all if it
only support single interrupt mode?  I kinda assumed that that was
because you're trying to support a controller which does only msix,
no?  At any rate, I don't think it's wrong to print an informational /
warning message when a controller declares msix support but has wacko
parameters.

> > > +
> > > +   /* only enable the first entry (entry.entry = 0) */
> > > +   rc = pci_enable_msix_exact(pdev, &entry, 1);
> >
> > So, enabling the first msix works if nvec > 1 && nvec < n_ports but
> > not if nvec >= n_ports?
> 
> For n_ports > 1 && nvec >= n_ports we need to assume per-port
> interrupts. There are enough vectors for all ports then.

Again, and we fail irq init in that case?

Thanks.

-- 
tejun
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