linux-kernel - Re: [PATCH 02/22] PCI: Unify PCI error response checking

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Date:   Mon, 11 Oct 2021 17:05:54 -0500
From:   Rob Herring <robh@...nel.org>
To:     Naveen Naidu <naveennaidu479@...il.com>
Cc:     bhelgaas@...gle.com,
        linux-kernel-mentees@...ts.linuxfoundation.org,
        linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 02/22] PCI: Unify PCI error response checking

On Mon, Oct 11, 2021 at 11:08:32PM +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 SET_PCI_ERROR_RESPONSE() to set the error response and
> RESPONSE_IS_PCI_ERROR() to check the error response during hardware
> read.
> 
> These definitions make error checks consistent and easier to find.
> 
> Signed-off-by: Naveen Naidu <naveennaidu479@...il.com>
> ---
>  drivers/pci/access.c | 22 +++++++++++-----------
>  1 file changed, 11 insertions(+), 11 deletions(-)
> 
> diff --git a/drivers/pci/access.c b/drivers/pci/access.c
> index 46935695cfb9..e1954bbbd137 100644
> --- a/drivers/pci/access.c
> +++ b/drivers/pci/access.c
> @@ -81,7 +81,7 @@ int pci_generic_config_read(struct pci_bus *bus, unsigned int devfn,
>  
>  	addr = bus->ops->map_bus(bus, devfn, where);
>  	if (!addr) {
> -		*val = ~0;
> +		SET_PCI_ERROR_RESPONSE(val);

This to me doesn't look like kernel style. I'd rather see a define 
replace just '~0', but I defer to Bjorn.

>  		return PCIBIOS_DEVICE_NOT_FOUND;

Neither does this using custom error codes rather than standard Linux 
errno. I point this out as I that's were I'd start with the config 
accessors. Though there are lots of occurrences so we'd need a way to do 
this in manageable steps.

Can't we make PCI_OP_READ and PCI_USER_READ_CONFIG set the data value 
and delete the drivers all doing this? Then we have 2 copies (in source) 
rather than the many this series modifies. Though I'm not sure if there 
are other cases of calling pci_bus.ops.read() which expect to get ~0.

Rob