Re: [PATCH] PCI: Remove pci_try_set_mwi

From: Bjorn Helgaas <helgaas@kernel.org>
To: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: Heiner Kallweit <hkallweit1@gmail.com>,
	Bjorn Helgaas <bhelgaas@google.com>,
	Jonathan Corbet <corbet@lwn.net>, Jens Axboe <axboe@kernel.dk>,
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Subject: Re: [PATCH] PCI: Remove pci_try_set_mwi
Date: Fri, 26 Mar 2021 16:55:16 -0500	[thread overview]
Message-ID: <20210326215516.GA916324@bjorn-Precision-5520> (raw)
In-Reply-To: <YF5VVjQ7q/JBSR1Z@smile.fi.intel.com>

On Fri, Mar 26, 2021 at 11:42:46PM +0200, Andy Shevchenko wrote:
> On Fri, Mar 26, 2021 at 04:26:55PM -0500, 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.
> > 
> > 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/
> 
> It's solely PCI feature which is absent on PCIe.
>
> So, if there is a guarantee that the driver never services a device connected
> to old PCI bus, it's okay to remove the call (it's no-op on PCIe anyway).

Yes, I'm aware that MWI is a no-op on PCIe.  If we want to argue that
we don't need to support Conventional PCI devices, that should be
explicit, and we could remove pci_set_mwi() completely.  But I don't
think we're ready to drop Conventional PCI support.

> OTOH, PCI core may try MWI itself for every device (but this is an opposite,
> what should we do on broken devices that do change their state based on that
> bit while violating specification).
> 
> In any case
> 
> Acked-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>

Thanks!

Bjorn