linux-kernel - RE: [patch V2 07/33] genirq/msi: Provide msi_create/free_device_irq

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Date:   Mon, 28 Nov 2022 01:47:32 +0000
From:   "Tian, Kevin" <kevin.tian@...el.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>
CC:     "x86@...nel.org" <x86@...nel.org>, Joerg Roedel <joro@...tes.org>,
        "Will Deacon" <will@...nel.org>,
        "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        "Lorenzo Pieralisi" <lorenzo.pieralisi@....com>,
        Marc Zyngier <maz@...nel.org>,
        "Greg Kroah-Hartman" <gregkh@...uxfoundation.org>,
        Jason Gunthorpe <jgg@...lanox.com>,
        "Jiang, Dave" <dave.jiang@...el.com>,
        Alex Williamson <alex.williamson@...hat.com>,
        "Williams, Dan J" <dan.j.williams@...el.com>,
        Logan Gunthorpe <logang@...tatee.com>,
        "Raj, Ashok" <ashok.raj@...el.com>, Jon Mason <jdmason@...zu.us>,
        Allen Hubbe <allenbh@...il.com>
Subject: RE: [patch V2 07/33] genirq/msi: Provide
 msi_create/free_device_irq_domain()

> From: Thomas Gleixner <tglx@...utronix.de>
> Sent: Thursday, November 24, 2022 4:36 PM
> 
> On Thu, Nov 24 2022 at 01:07, Kevin Tian wrote:
> >> From: Thomas Gleixner <tglx@...utronix.de>
> > I looked at it from the outmost invocation:
> >
> > @@ -436,6 +436,9 @@ int __pci_enable_msi_range(struct pci_de
> >  	if (rc)
> >  		return rc;
> >
> > +	if (!pci_setup_msi_device_domain(dev))
> > +		return -ENODEV;
> > +
> >
> > the current style kind of converts meaningful -EINVAL/-ENOMEM/etc.
> > into -ENODEV.
> 
> But go to the call sites of the various places in drivers which set up
> MSI or MSI-X and check whether anything evaluates those error codes in a
> meaningful way.
> 
> Some of them print the error code, but that does not help much because
> the error code does not allow you to pin point the place which returns
> that. If you just analyze the pci_alloc_irq_vectors_affinity() call then
> you find at least 10 places, which can return -ENOMEM. So how is that
> meaningful and helpful?
> 
> All it tells you is that some memory allocation failed. In that case the
> failure of the PCI/MSI[-X] setup is the least of the problems.
> 
> Where error codes are mandatory are user space interfaces, but in the
> kernel a simple fail/success like we have with many interfaces which
> just return a NULL pointer on fail is sufficient.
> 
> Just because the kernel historically propagated error codes all over the
> place does not make them useful or meaningful.
> 

Good learning. Thanks.