linux-kernel - Re: [PATCH v3 2/2] PCI: pciehp: Clear the optional capabilities in DEVCTL2 on a hot-plug

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Date:   Fri, 23 Jun 2023 08:06:28 +0200
From:   Lukas Wunner <lukas@...ner.de>
To:     Felix Kuehling <felix.kuehling@....com>
Cc:     Smita Koralahalli <Smita.KoralahalliChannabasappa@....com>,
        linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org,
        Bjorn Helgaas <bhelgaas@...gle.com>, oohall@...il.com,
        Mahesh J Salgaonkar <mahesh@...ux.ibm.com>,
        Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        Yazen Ghannam <yazen.ghannam@....com>,
        Fontenot Nathan <Nathan.Fontenot@....com>,
        Jay Cornwall <Jay.Cornwall@....com>
Subject: Re: [PATCH v3 2/2] PCI: pciehp: Clear the optional capabilities in
 DEVCTL2 on a hot-plug

On Fri, Jun 23, 2023 at 05:59:55AM +0200, Felix Kuehling wrote:
> On 2023-06-22 23:42, Lukas Wunner wrote:
> > On Thu, Jun 22, 2023 at 02:02:12PM -0700, Smita Koralahalli wrote:
> > > Would it be fair to just reuse pci_enable_atomic_ops_to_root() for
> > > Atomic_Ops configuration?
> > 
> > Hm, that's a good question.  I'm not an expert on that corner of
> > the PCI core.
> > 
> > But indeed what you could try is amend that function to not only
> > *set* PCI_EXP_DEVCTL2_ATOMIC_REQ if it's supported, but to also
> > *clear* it if it's not supported.
> > 
> > And you'd have to call pci_enable_atomic_ops_to_root() on enumeration,
> > e.g. from pci_init_capabilities().
> > 
> > That should obviate the need to call pci_enable_atomic_ops_to_root()
> > from drivers, so you could probably remove the call from all the
> > drivers which currently call it (amdgpu, infiniband, mellanox),
> > in one separate patch per driver.
> > 
> > An then you could drop the EXPORT clause for pci_enable_atomic_ops_to_root()
> > and make it private to the PCI core.
> 
> Then our driver would need an alternative way to determine whether atomic
> capabilities are enabled for a device. We currently use the return value
> from pci_enable_atomic_ops_to_root to determine this.

Just read PCI_EXP_DEVCTL2 and check whether PCI_EXP_DEVCTL2_ATOMIC_REQ
is set.  (I.e. has been set by the PCI core on device enumeration.)

Problem solved, I guess?

Thanks,

Lukas