[RFC] PCI/MSI: Clarify usage of pci_msix_free_irq()

pci_msix_free_irq() is used to free an interrupt on
a PCI/MSI-X interrupt domain.

The API description specifies that the interrupt to
be freed was allocated via pci_msix_alloc_irq_at().
This description limits the usage of pci_msix_free_irq()
since pci_msix_free_irq() can also be used to free
MSI-X interrupts allocated with, for example,
pci_alloc_irq_vectors().

Remove the text stating that the interrupt to be freed had
to be allocated with pci_msix_alloc_irq_at(). The needed
struct msi_map need not be from pci_msix_alloc_irq_at()
but can be created from scratch using pci_irq_vector()
to obtain the Linux IRQ number. Highlight that
pci_msix_free_irq() cannot be used to disable MSI-X to
guide users that, for example, pci_free_irq_vectors()
remains to be needed.

Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
---
Dear Maintainers,

Is my understanding correct?

For example, from my understanding the following is
correct API usage:

/*
 * Enable MSI-X and allocate num interrupts.
 */
pci_alloc_irq_vectors()

/*
 * Free one of the interrupts allocated via pci_alloc_irq_vectors().
 * Possibly called num times, but not required.
 */
pci_msix_free_irq()

/*
 * Ensure all remaining interrupts freed and MSI-X disabled.
 */
pci_free_irq_vectors()

This could be interpreted as an asymmetric usage of the API, yet
found to be practical when trying to use these new calls.

Your feedback is appreciated.

Regards,

Reinette

 drivers/pci/msi/api.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

> From: Chatre, Reinette <reinette.chatre@intel.com>
> Sent: Wednesday, February 15, 2023 5:13 AM
> 
> pci_msix_free_irq() is used to free an interrupt on
> a PCI/MSI-X interrupt domain.
> 
> The API description specifies that the interrupt to
> be freed was allocated via pci_msix_alloc_irq_at().
> This description limits the usage of pci_msix_free_irq()
> since pci_msix_free_irq() can also be used to free
> MSI-X interrupts allocated with, for example,
> pci_alloc_irq_vectors().
> 
> Remove the text stating that the interrupt to be freed had
> to be allocated with pci_msix_alloc_irq_at(). The needed
> struct msi_map need not be from pci_msix_alloc_irq_at()
> but can be created from scratch using pci_irq_vector()
> to obtain the Linux IRQ number. Highlight that
> pci_msix_free_irq() cannot be used to disable MSI-X to
> guide users that, for example, pci_free_irq_vectors()
> remains to be needed.
> 
> Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
> ---
> Dear Maintainers,
> 
> Is my understanding correct?
> 
> For example, from my understanding the following is
> correct API usage:
> 
> /*
>  * Enable MSI-X and allocate num interrupts.
>  */
> pci_alloc_irq_vectors()
> 
> /*
>  * Free one of the interrupts allocated via pci_alloc_irq_vectors().
>  * Possibly called num times, but not required.
>  */
> pci_msix_free_irq()
> 
> /*
>  * Ensure all remaining interrupts freed and MSI-X disabled.
>  */
> pci_free_irq_vectors()
> 
> This could be interpreted as an asymmetric usage of the API, yet
> found to be practical when trying to use these new calls.
> 
> Your feedback is appreciated.

I asked same question before. Thomas gave exactly same thought. [1]

"
The preallocated descriptors are only relevant during setup, but not
post setup. So yes, you can do:

pci_alloc_irq_vectors(dev, 10, 10, PCI_IRQ_MSIX);

and then

    map = { .index = 9, .virq = $IRQ };
    pci_msix_free_irq(dev, &map);

It just works.
"

but looks the comment was not adjusted accordingly.

So,

Reviewed-by: Kevin Tian <kevin.tian@intel.com> 

[1] https://lore.kernel.org/lkml/87r0xsd8j4.ffs@tglx/

> 
> Regards,
> 
> Reinette
> 
>  drivers/pci/msi/api.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/pci/msi/api.c b/drivers/pci/msi/api.c
> index b8009aa11f3c..be679aa5db64 100644
> --- a/drivers/pci/msi/api.c
> +++ b/drivers/pci/msi/api.c
> @@ -163,11 +163,11 @@ EXPORT_SYMBOL_GPL(pci_msix_alloc_irq_at);
> 
>  /**
>   * pci_msix_free_irq - Free an interrupt on a PCI/MSIX interrupt domain
> - *		      which was allocated via pci_msix_alloc_irq_at()
>   *
>   * @dev:	The PCI device to operate on
>   * @map:	A struct msi_map describing the interrupt to free
> - *		as returned from the allocation function.
> + *
> + * Undo an interrupt vector allocation. Does not disable MSI-X.
>   */
>  void pci_msix_free_irq(struct pci_dev *dev, struct msi_map map)
>  {
> --
> 2.34.1