linux-kernel - Re: [PATCH] PCI: Generic Configuration Access Mechanism support

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Date:	Wed, 21 May 2014 09:52:57 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	Srikanth Thokala <sthokal@...inx.com>
Cc:	Bjorn Helgaas <bhelgaas@...gle.com>, will.deacon@....com,
	michals@...inx.com,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>
Subject: Re: [PATCH] PCI: Generic Configuration Access Mechanism support

On Tuesday 20 May 2014 20:01:01 Srikanth Thokala wrote:
> On Mon, May 19, 2014 at 10:33 PM, Arnd Bergmann <arnd@...db.de> wrote:
> > On Sunday 18 May 2014 19:38:45 Srikanth Thokala wrote:
> >> +
> >> +     if (cfg->ops->is_valid_cfg_access) {
> >> +             if (!cfg->ops->is_valid_cfg_access(bus, devfn)) {
> >> +                     *val = PCI_CFG_INVALID_DEVFN;
> >> +                     return PCIBIOS_DEVICE_NOT_FOUND;
> >> +             }
> >> +     }
> >
> > Can you explain why this callback is needed? If the space for the
> > bus is mapped, any access should just work.
> 
> As I was explaining to Will, there are some controllers which doesn't
> return FF's
> when a device is not found on the bus (as per the PCI specification) and
> accessing such a device address from the kernel results in an external abort.
> So, I added this additional logic in my driver to bypass this and
> return FF's. Our IP
> and even other controllers like Tegra, Renesas have similar implementation.
> We cant think of a better solution and please let you us know if you
> have any inputs.

Does your hardware need this? My first response would otherwise be to
treat that as noncompliant and not handle this case in the generic implementation.

	Arnd
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