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

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Message-ID: <20211012162054.rxx7aubwdvhl2eqj@theprophet>
Date:   Tue, 12 Oct 2021 21:51:08 +0530
From:   Naveen Naidu <naveennaidu479@...il.com>
To:     Rob Herring <robh@...nel.org>
Cc:     bhelgaas@...gle.com,
        linux-kernel-mentees@...ts.linuxfoundation.org,
        linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org,
        andrew.murray@....com
Subject: Re: [PATCH 02/22] PCI: Unify PCI error response checking

On 11/10, Rob Herring wrote:
> 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.
>

Apologies, if this is a lame question. Why is the macro
SET_PCI_ERROR_RESPONSE not a kernel style. I ask this so that I do not
end up making the same mistake again.

Bjorn, did initally make a patch with only replacing '~0' but then
Andrew suggested in the patch [1] that we should use the macro. 

[1]:
https://lore.kernel.org/linux-pci/20190823104415.GC14582@e119886-lin.cambridge.arm.com/
[Adding Andrew in the CC for this]

Apologies, I should have added this link in the cover letter but I
completely forgot about it. 

That's why I decided to go with the macro. If that is not the right
approach please let me know and I can fix it up.

> >  		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.
> 

I am sorry, but I do not have any answer for this. I really do not know
why we return custom error codes instead of standard Linux errno. Maybe
someone else can pitch in on this.

> 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.
> 

This seems like a really good idea :) But again, I am not entirely sure
if doing so would give us any unexpected behaviour. I'll wait for some
one to reply to this and if people agree to it, I would be glad to make
the changes to PCI_OP_READ and PCI_USER_READ_CONFIG and send a new
patch.

Thank you very much for the review :-)

> Rob