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

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Message-ID: <CAL_JsqKYP+6Bzm5hvcVbAz5R3+omREDJoOspJ4eTBeMwBSfkfw@mail.gmail.com>
Date:   Tue, 12 Oct 2021 13:02:39 -0500
From:   Rob Herring <robh@...nel.org>
To:     Naveen Naidu <naveennaidu479@...il.com>
Cc:     Bjorn Helgaas <bhelgaas@...gle.com>,
        linux-kernel-mentees@...ts.linuxfoundation.org,
        PCI <linux-pci@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 02/22] PCI: Unify PCI error response checking

On Tue, Oct 12, 2021 at 11:21 AM Naveen Naidu <naveennaidu479@...il.com> wrote:
>
> 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.

Generally, we don't do macros if a static inline function will work
because you get more type checking with a function. There's exceptions
like struct initializers which need to work in declarations.

Second, I think the above obfuscates the code. I know exactly what the
original line is doing to val. With SET_PCI_ERROR_RESPONSE(), I have
to go look and it hasn't saved us any LOC to make the caller more
readable. The downside of the original way, is I don't know why we set
val to ~0, but just a define would tell me that.

> 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]

He's no longer at Arm nor active upstream.

> 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.

I don't either. My guess is either just too many places to fix or
somehow it trickles up to userspace (but probably not since the error
codes aren't in a uapi header).

> > 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.

I'm expecting Bjorn to chime in.

Rob