lkml.org 
[lkml]   [2013]   [Oct]   [1]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH v2 2/6] PCI/MSI: Factor out pci_get_msi_cap() interface
On Wed, Sep 18, 2013 at 09:22:31AM -0500, Tejun Heo wrote:
> Hello,
>
> On Wed, Sep 18, 2013 at 11:48:00AM +0200, Alexander Gordeev wrote:
> > 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")?
>
> Hmmm... do we need to treat this any differently? If the platform
> can't allocate full range of requested MSIs, just failing should be
> enough regardless of why such allocation can't be met, no?
>
> > > 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 ;)
>
> LOL. :)
>
> > 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.
>
> Yeah, I mean, this type of interface is a trap. People have to
> actively resist to avoid doing silly stuff which is a lot to ask.

I really think you're overstating the complexity here.

Functions typically return a boolean -> nothing to see here
This function returns a tristate value -> brain explosion!


> > /*
> > * 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;
>
> I really think what we should do is
>
> * Determine the number of MSIs the controller wants. Don't worry
> about quotas or limits or anything. Just determine the number
> necessary to enable enhanced interrupt handling.
>
> * Try allocating that number of MSIs. If it fails, then just revert
> to single interrupt mode. It's not the end of the world and mostly
> guaranteed to work. Let's please not even try to do partial
> multiple interrupts. I really don't think it's worth the risk or
> complexity.

It will potentially break existing setups on our hardware.

Can I make that any clearer?

cheers


\
 
 \ /
  Last update: 2013-10-01 10:01    [W:0.158 / U:7.020 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site