Re: [PATCH] PCI: Remove pci_try_set_mwi

[Bjorn Helgaas <helgaas@kernel.org>]

On Sun, Mar 28, 2021 at 12:04:35AM +0100, Heiner Kallweit wrote:
> On 26.03.2021 22:26, Bjorn Helgaas wrote:
> > [+cc Randy, Andrew (though I'm sure you have zero interest in this
> > ancient question :))]
> > 
> > On Wed, Dec 09, 2020 at 09:31:21AM +0100, Heiner Kallweit wrote:
> >> pci_set_mwi() and pci_try_set_mwi() do exactly the same, just that the
> >> former one is declared as __must_check. However also some callers of
> >> pci_set_mwi() have a comment that it's an optional feature. I don't
> >> think there's much sense in this separation and the use of
> >> __must_check. Therefore remove pci_try_set_mwi() and remove the
> >> __must_check attribute from pci_set_mwi().
> >> I don't expect either function to be used in new code anyway.
> > 
> > There's not much I like better than removing things.  But some
> > significant thought went into adding pci_try_set_mwi() in the first
> > place, so I need a little more convincing about why it's safe to
> > remove it.
> > 
> 
> Thanks for the link to the 13 yrs old discussion. Unfortunately it
> doesn't mention any real argument for the __must_check, just:
> 
> "And one of the reasons for adding the __must_check annotation is to
> weed out design errors."
> And the very next response in the discussion calls this a "non-argument".
> Plus not mentioning what the other reasons could be.

I think you're referring to Alan's response [1]:

  akpm> And we *need* to be excessively anal in the PCI setup code.
  akpm> We have metric shitloads of bugs due to problems in that area,
  akpm> and the more formality and error handling and error reporting
  akpm> we can get in there the better off we will be.

  ac> No argument there

So Alan is actually *agreeing* that "we need to be excessively anal in
the PCI setup code,"  not saying that "weeding out design errors is
not an argument for __must_check."

> Currently we have three ancient drivers that bail out if the call fails.
> Most callers of pci_set_mwi() use the return code only to emit an
> error message, but they proceed normally. Majority of users calls
> pci_try_set_mwi(). And as stated in the commit message I don't expect
> any new usage of pci_set_mwi().

I would love to merge this patch.  We just need to clarify the commit
log.  Right now the only justification is "I don't think there's much
sense in the __must_check annotation," which may well be true but
could use some support.

If MWI is purely an optimization and there's never a functional
problem if pci_set_mwi() fails, we should say that (and maybe
update any drivers that bail out on failure).

Andrew and Alan both seem to agree that MSI *is* purely advisory:

  akpm> pci_set_mwi() is an advisory thing, and on certain platforms
  akpm> it might fail to set the cacheline size to the desired number.
  akpm> This is not a fatal error and the driver can successfully run
  akpm> at a lesser performance level.

  ac> Correct.

But even after that, Andrew proposed adding pci_try_set_mwi().  So it
makes sense to really understand what was going on there so we don't
break something in the name of cleaning it up.

[1] https://lore.kernel.org/linux-ide/20070405211609.5263d627@the-village.bc.nu/

> > The argument should cite the discussion about adding it.  I think one
> > of the earliest conversations is here:
> > https://lore.kernel.org/linux-ide/20070404213704.224128ec.randy.dunlap@oracle.com/