Re: [PATCH] PCI: Add support for multiple MSI

[Matthew Wilcox <matthew@wil.cx>]

On Fri, Jul 11, 2008 at 05:28:28PM +0900, Hidetoshi Seto wrote:
> Hi,
> 
> First of all, it seems that mask/unmask of MSI has problems.
>  - Per-vector masking is optional for MSI, so I think that allocating
>    multiple messages for a function without masking capability would be
>    not good idea, since all vector in the block will be masked/unmasked
>    at once without any agreement.
>  - Even if the function supports per-vector masking, current
>    mask/unmask_msi_irq() functions assume that MSI uses only one vector,
>    therefore they only set/unset the first bit of the maskbits which
>    for the first vector of the block.  The bits for other vectors are
>    initialized as 'masked' but no one unmask them.

Thank you for pointing out the problems with masking.  The device I am
testing with does not support per-vector masking, so I have not paid
attention to this.

To your first point, if the function does not support per-vector
masking, I think it's OK to mask/unmask all vectors at once.  But we
must be careful to manage this correctly in software; if we disable IRQ
496, disable IRQ 497, then enable IRQ 497, we must not enable IRQ 496 at
that time.  I think we can solve this problem, but I must think about it
some more.

The second point is a simple bug that should be easy to fix.  Thank you
for pointing it out.

> Matthew Wilcox wrote:
> > + * Allocate IRQs for a device with the MSI capability.
> > + * This function returns a negative errno if an error occurs.  If it
> > + * is unable to allocate the number of interrupts requested, it returns
> > + * the number of interrupts it might be able to allocate.  If it successfully
> > + * allocates at least the number of interrupts requested, it returns 0 and
> > + * updates the @dev's irq member to the lowest new interrupt number; the
> > + * other interrupt numbers allocated to this device are consecutive.
> > + */
> > +int pci_enable_msi_block(struct pci_dev *dev, unsigned int nvec)
> >  {
> >  	int status;
> >  
> > -	status = pci_msi_check_device(dev, 1, PCI_CAP_ID_MSI);
> > +	/* MSI only supports up to 32 interrupts */
> > +	if (nvec > 32)
> > +		return 32;
> 
> I think we should return -EINVAL here.
> No one guarantee that 32 interrupts is able to be allocate at this time.
> 
> And also I think -EINVAL should be returned if nvec is greater than
> the number of encoded in the function's "Multiple Message Capable", but
> I could not find any mention about handling of such over-capability request
> in PCI Bus Spec. 3.0.

It would be outside the scope of the PCI Bus Specification.  I think
you're right that we should check the MMC bits; but I think we should
tell the driver to request a lower number, not return -EINVAL.

Thanks for your comments.

-- 
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