linux-kernel - Re: [PATCH] PCI: Remove pci_try_set

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Message-ID: <20210326215516.GA916324@bjorn-Precision-5520>
Date:   Fri, 26 Mar 2021 16:55:16 -0500
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Cc:     Heiner Kallweit <hkallweit1@...il.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Jonathan Corbet <corbet@....net>, Jens Axboe <axboe@...nel.dk>,
        Viresh Kumar <vireshk@...nel.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Vinod Koul <vkoul@...nel.org>,
        David Miller <davem@...emloft.net>,
        Lee Jones <lee.jones@...aro.org>,
        Ion Badulescu <ionut@...ula.org>,
        Jakub Kicinski <kuba@...nel.org>,
        Lino Sanfilippo <LinoSanfilippo@....de>,
        Christian Lamparter <chunkeey@...glemail.com>,
        Kalle Valo <kvalo@...eaurora.org>,
        Luis Chamberlain <mcgrof@...nel.org>,
        Adam Radford <aradford@...il.com>,
        "James E.J. Bottomley" <jejb@...ux.ibm.com>,
        "Martin K. Petersen" <martin.petersen@...cle.com>,
        James Smart <james.smart@...adcom.com>,
        Dick Kennedy <dick.kennedy@...adcom.com>,
        Nilesh Javali <njavali@...vell.com>,
        GR-QLogic-Storage-Upstream@...vell.com,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jirislaby@...nel.org>,
        Peter Chen <Peter.Chen@....com>,
        Felipe Balbi <balbi@...nel.org>,
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        Randy Dunlap <rdunlap@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] PCI: Remove pci_try_set_mwi

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@...ux.intel.com>

Thanks!

Bjorn