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

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Message-ID: <3cbd2d6c-20ba-49c2-af73-898ba28b5c9e@BN1BFFO11FD052.protection.gbl>
Date:	Wed, 21 May 2014 08:54:02 +0000
From:	Srikanth Thokala <srikanth.thokala@...inx.com>
To:	Arnd Bergmann <arnd@...db.de>
CC:	Bjorn Helgaas <bhelgaas@...gle.com>,
	"will.deacon@....com" <will.deacon@....com>,
	Michal Simek <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


> -----Original Message-----
> From: Arnd Bergmann [mailto:arnd@...db.de]
> Sent: Wednesday, May 21, 2014 1:23 PM
> To: Srikanth Thokala
> Cc: Bjorn Helgaas; will.deacon@....com; Michal Simek; linux-
> kernel@...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.

Yes, my hardware needs this additional logic.

Srikanth

>
>       Arnd


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