linux-kernel - Re: [PATCH v1 2/6] [media] cx23885: use generic power management

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Message-ID: <20200714105259.GA1792@gmail.com>
Date:   Tue, 14 Jul 2020 16:22:59 +0530
From:   Vaibhav Gupta <vaibhav.varodek@...il.com>
To:     Bjorn Helgaas <helgaas@...nel.org>
Cc:     Hans Verkuil <hverkuil@...all.nl>,
        Vaibhav Gupta <vaibhavgupta40@...il.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>, bjorn@...gaas.com,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        linux-kernel@...r.kernel.org, linux-media@...r.kernel.org,
        linux-kernel-mentees@...ts.linuxfoundation.org,
        skhan@...uxfoundation.org
Subject: Re: [PATCH v1 2/6] [media] cx23885: use generic power management

On Mon, Jul 13, 2020 at 07:38:14PM -0500, Bjorn Helgaas wrote:
> On Mon, Jul 13, 2020 at 12:01:51PM +0200, Hans Verkuil wrote:
> > On 29/06/2020 09:36, Vaibhav Gupta wrote:
> > 
> > I don't entirely understand this. Wouldn't it be sufficient to just
> > drop the .suspend/.resume assignments here? It is now required for
> > driver.pm to be non-NULL?
> > 
> > I'm not up to speed on the changes, but normally you can leave things
> > NULL if you don't support a feature (PM in this case).
> 
> I think this patch will break things.  Previously, we had:
> 
>   cx23885_pci_driver.suspend == NULL
>   cx23885_pci_driver.resume == NULL
>   cx23885_pci_driver.driver.pm == NULL
> 
> pci_pm_suspend() looks like:
> 
>   if (pci_has_legacy_pm_support(pci_dev))
>     return pci_legacy_suspend(dev, PMSG_SUSPEND);
> 
>   if (!pm) {
>     pci_pm_default_suspend(pci_dev);
>     return 0;
>   }
> 
> pci_has_legacy_pm_support() was false since drv->suspend and
> drv->resume are both NULL, so we'd take the pci_pm_default_suspend()
> path.  After this patch, driver.pm would no longer be NULL, so we'd
> take a different path that is clearly not equivalent.
> 
> I think you should do this:
> 
>   - 	/* TODO */
>   -	.suspend  = NULL,
>   -	.resume   = NULL,
> 
> and leave .driver.pm NULL by not mentioning it at all.  That should be
> identical at the object code level since those are the defaults
> anyway.
> 
> That almost looks like useless churn, but the point of this patch is
> to remove use of PCI legacy PM (pci_driver.suspend and .resume) so we
> can completely remove that infrastructure from the PCI core, including
> the .suspend and .resume members of struct pci_driver, so we really do
> need to do it.
Okay!

Thanks!
-- Vaibhav Gupta
> 
> Bjorn