linux-kernel - Re: [PATCH 16/22] PCI: pciehp: Use RESPONSE_IS_PCI

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Date:   Wed, 13 Oct 2021 14:20:19 +0200
From:   Lukas Wunner <lukas@...ner.de>
To:     Pali Rohár <pali@...nel.org>
Cc:     Naveen Naidu <naveennaidu479@...il.com>, bhelgaas@...gle.com,
        linux-kernel-mentees@...ts.linuxfoundation.org,
        linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org,
        Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        Amey Narkhede <ameynarkhede03@...il.com>
Subject: Re: [PATCH 16/22] PCI: pciehp: Use RESPONSE_IS_PCI_ERROR() to check
 read from hardware

On Wed, Oct 13, 2021 at 01:12:01AM +0200, Pali Rohár wrote:
> > On 11/10, Lukas Wunner wrote:
> > > On Mon, Oct 11, 2021 at 11:37:33PM +0530, Naveen Naidu wrote:
> > > > An MMIO read from a PCI device that doesn't exist or doesn't respond
> > > > causes a PCI error.  There's no real data to return to satisfy the
> > > > CPU read, so most hardware fabricates ~0 data.
> > > > 
> > > > Use RESPONSE_IS_PCI_ERROR() to check the response we get when we read
> > > > data from hardware.
> > > 
> > > Actually what happens is that PCI read transactions *time out*,
> > > so the host controller fabricates a response.
> 
> This is not fully correct. 0xffffffff is returned when some error
> happens. It does not have to be timeout error. Errors like Unsupported
> Request, Completer Abort or Configuration Request Retry Status (when
> CRSSVE bit is disabled) are also reported as 0xffffffff and they do not
> represent timeout. For example Unsupported Request is returned when you
> try to read from non-existent device behind some PCIe switch.

This particular patch concerns pciehp and in that context,
"all ones" responses are predominantly timeouts caused by
hot-removed devices.

Thanks,

Lukas