linux-kernel - Re: [PATCH v1 3/7] PCI: New Primary to Sideband (P2SB) bridge support library

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Date:   Thu, 1 Apr 2021 18:43:11 +0300
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     Henning Schild <henning.schild@...mens.com>
Cc:     Wolfram Sang <wsa+renesas@...g-engineering.com>,
        Jean Delvare <jdelvare@...e.de>,
        Lee Jones <lee.jones@...aro.org>,
        Tan Jui Nee <jui.nee.tan@...el.com>,
        Jim Quinlan <james.quinlan@...adcom.com>,
        Jonathan Yong <jonathan.yong@...el.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        linux-kernel@...r.kernel.org, linux-i2c@...r.kernel.org,
        linux-pci@...r.kernel.org, Jean Delvare <jdelvare@...e.com>,
        Peter Tyser <ptyser@...-inc.com>, hdegoede@...hat.com,
        Mika Westerberg <mika.westerberg@...ux.intel.com>
Subject: Re: [PATCH v1 3/7] PCI: New Primary to Sideband (P2SB) bridge
 support library

On Sat, Mar 13, 2021 at 10:45:57AM +0100, Henning Schild wrote:
> Am Mon, 8 Mar 2021 14:20:16 +0200
> schrieb Andy Shevchenko <andriy.shevchenko@...ux.intel.com>:

...

> > + * pci_p2sb_bar - Get Primary to Sideband bridge (P2SB) device BAR
> > + * @pdev:	PCI device to get a PCI bus to communicate with
> > + * @devfn:	PCI slot and function to communicate with
> > + * @mem:	memory resource to be filled in
> 
> Do we really need that many arguments to it?
> 
> Before i had, in a platform driver that never had its own pci_dev or bus
> 
>   res->start = simatic_ipc_get_membase0(PCI_DEVFN(13, 0));
>   if (res-start == 0)
>     return -ENODEV;
> 
> So helper only asked for the devfn, returned base and no dedicated
> error code.
> 
> With this i need
> 
>   struct pci_bus *bus = pci_find_bus(0, 0);
>   struct pci_dev *pci_dev = bus->self;
>   unsigned int magic_i_do_not_want =  PCI_DEVFN(13, 0);

What confuses me is the use for SPI NOR controller on Broxton. And I think
we actually can indeed hide all this under the hood by exposing P2SB to the OS.

Mika, what do you think?

> I guess that second devfn is for devices behind that bridge. So
> unhiding it might reveal several devices?

Good question. I need a device where actually this happens (hidden P2SB stuff
with let's say SPI NOR there) to see how it looks like in such case. I only
understood the GPIO part. But I'm not so sure anymore.

> But when caring about that
> p2sb do i really need to know its devfn. If so i would like to get

-- 
With Best Regards,
Andy Shevchenko