linux-kernel - Re: [PATCH 2/2] PCI: Create device link for NVIDIA GPU

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Date:   Mon, 3 Jun 2019 12:07:07 +0200
From:   Lukas Wunner <lukas@...ner.de>
To:     Abhishek Sahu <abhsahu@...dia.com>
Cc:     Bjorn Helgaas <helgaas@...nel.org>, linux-kernel@...r.kernel.org,
        linux-pci@...r.kernel.org
Subject: Re: [PATCH 2/2] PCI: Create device link for NVIDIA GPU

On Mon, Jun 03, 2019 at 01:30:51PM +0530, Abhishek Sahu wrote:
> On 6/1/2019 2:09 AM, Bjorn Helgaas wrote:
> > > If there *is* spec language that allows dependencies like this, please
> > include the citation in your commit log.
> 
>  The PCIe specification treats each function separately but GPU case is
>  different. So, it won't be part of PCIe spec. in GPU, the different
>  kind of devices are internally coupled in HW but still needs to be
>  managed by different driver.

GPUs aren't the only devices which have these implicit dependencies.

E.g. Broadcom BCM57765 combines a Gigabit Ethernet chip in function 0
(14e4:16b4) with an SD card reader in function 1 (14e4:16bc) and AFAIK
for the latter to work, the former needs to be in D0.  I think we're
incorrectly not enforcing this dependency in the kernel yet.  That chip
was built into many MacBooks.

Indeed the inability to specify such inter-function power dependencies
in config space seems to be a gaping hole in the PCI spec.  Maybe we
should always assume that function 0 needs to be in D0 for any other
function to work?  Not sure if that would be safe.

Thanks,

Lukas