On Wed, Sep 18, 2013 at 12:30:23AM +1000, Michael Ellerman wrote:
> How about no?
>
> We have a small number of MSIs available, limited by hardware &
> firmware, if we don't impose a quota then the first device that probes
> will get most/all of the MSIs and other devices miss out.
Out of curiosity - how pSeries has had done it without quotas before
448e2ca ("powerpc/pseries: Implement a quota system for MSIs")?
> Anyway I don't see what problem you're trying to solve? I agree the
> -ve/0/+ve return value pattern is ugly, but it's hardly the end of the
> world.
Well, the interface recently has been re-classified from "ugly" to
"unnecessarily complex and actively harmful" in Tejun's words ;)
Indeed, I checked most of the drivers and it is incredible how people
are creative in misusing the interface: from innocent pci_disable_msix()
calls when if pci_enable_msix() failed to assuming MSI-Xs were enabled
if pci_enable_msix() returned a positive value (apparently untested).
Roughly third of the drivers just do not care and bail out once
pci_enable_msix() has not succeeded. Not sure how many of these are
mandated by the hardware.
Another quite common pattern is a call to pci_enable_msix() to figure out
the number of MSI-Xs available and a repeated call of pci_enable_msix()
to enable those MSI-Xs, this time.
The last pattern makes most of sense to me and could be updated with a more
clear sequence - a call to (bit modified) pci_msix_table_size() followed
by a call to pci_enable_msix(). I think this pattern can effectively
supersede the currently recommended "loop" practice.
But as pSeries quota is still required I propose to introduce a new interface
pci_get_msix_limit() that combines pci_msix_table_size() and (also new)
arch_get_msix_limit(). The latter would check the quota thing in case of
pSeries and none in case of all other architectures.
The recommended practice would be:
/*
* Retrieving 'nvec' by means other than pci_msix_table_size()
*/
rc = pci_get_msix_limit(pdev);
if (rc < 0)
return rc;
/*
* nvec = min(rc, nvec);
*/
for (i = 0; i < nvec; i++)
msix_entry[i].entry = i;
rc = pci_enable_msix(pdev, msix_entry, nvec);
if (rc)
return rc;
Thoughts?
> cheers
--
Regards,
Alexander Gordeev
agordeev@xxxxxxxxxx
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