linux-kernel - Re: [PATCH 1/2] PCI: Avoid pointless capability searches

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Message-ID: <20250213163850.GA114277@bhelgaas>
Date: Thu, 13 Feb 2025 10:38:50 -0600
From: Bjorn Helgaas <helgaas@...nel.org>
To: Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>
Cc: Alex Williamson <alex.williamson@...hat.com>,
	Christian König <ckoenig.leichtzumerken@...il.com>,
	linux-pci@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
	Bjorn Helgaas <bhelgaas@...gle.com>
Subject: Re: [PATCH 1/2] PCI: Avoid pointless capability searches

On Thu, Feb 13, 2025 at 03:52:05PM +0200, Ilpo Järvinen wrote:
> On Fri, 7 Feb 2025, Bjorn Helgaas wrote:
> > Many of the save/restore functions in the pci_save_state() and
> > pci_restore_state() paths depend on both a PCI capability of the device and
> > a pci_cap_saved_state structure to hold the configuration data, and they
> > skip the operation if either is missing.
> > 
> > Look for the pci_cap_saved_state first so if we don't have one, we can skip
> > searching for the device capability, which requires several slow config
> > space accesses.

> > +++ b/drivers/pci/vc.c
> > @@ -355,20 +355,17 @@ int pci_save_vc_state(struct pci_dev *dev)
> >  	int i;
> >  
> >  	for (i = 0; i < ARRAY_SIZE(vc_caps); i++) {
> > -		int pos, ret;
> >  		struct pci_cap_saved_state *save_state;
> > +		int pos, ret;
> > +
> > +		save_state = pci_find_saved_ext_cap(dev, vc_caps[i].id);
> > +		if (!save_state)
> > +			return -ENOMEM;
> >  
> >  		pos = pci_find_ext_capability(dev, vc_caps[i].id);
> >  		if (!pos)
> >  			continue;
> >  
> > -		save_state = pci_find_saved_ext_cap(dev, vc_caps[i].id);
> > -		if (!save_state) {
> > -			pci_err(dev, "%s buffer not found in %s\n",
> > -				vc_caps[i].name, __func__);
> > -			return -ENOMEM;
> > -		}
> 
> I think this order change will cause a functional change because 
> pci_allocate_vc_save_buffers() only allocated for those capabilities that 
> are exist for dev. Thus, the loop will prematurely exit.

Oof, thank you for catching this!  I'll drop this for now.

It would be nice to make pci_save_vc_state() parallel with
pci_restore_vc_state() (and with most other pci_save_*_state()
functions) and have it return void.  But pci_save_state() returns the
pci_save_vc_state() return value, and there are ~20 pci_save_state()
callers that pay attention to that return value.

I'm not convinced there's real value in pci_save_state() error
returns, given that so few callers check it, but it definitely
requires more analysis before removing it.

> >  		ret = pci_vc_do_save_buffer(dev, pos, save_state, true);
> >  		if (ret) {
> >  			pci_err(dev, "%s save unsuccessful %s\n",
> > @@ -392,12 +389,15 @@ void pci_restore_vc_state(struct pci_dev *dev)
> >  	int i;
> >  
> >  	for (i = 0; i < ARRAY_SIZE(vc_caps); i++) {
> > -		int pos;
> >  		struct pci_cap_saved_state *save_state;
> > +		int pos;
> > +
> > +		save_state = pci_find_saved_ext_cap(dev, vc_caps[i].id);
> > +		if (!save_state)
> > +			continue;
> >  
> >  		pos = pci_find_ext_capability(dev, vc_caps[i].id);
> > -		save_state = pci_find_saved_ext_cap(dev, vc_caps[i].id);
> > -		if (!save_state || !pos)
> > +		if (!pos)
> >  			continue;
> >  
> >  		pci_vc_do_save_buffer(dev, pos, save_state, false);
> > 
> 
> -- 
>  i.
>