linux-kernel - Re: [RFC PATCH 00/35] Move all PCIBIOS* definitions into arch/x86

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Message-ID: <2629240c07af7fec5989127ed6da405e12cf77a3.camel@kernel.crashing.org>
Date:   Wed, 15 Jul 2020 12:19:46 +1000
From:   Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:     Bjorn Helgaas <helgaas@...nel.org>, Arnd Bergmann <arnd@...db.de>
Cc:     "Saheed O. Bolarinwa" <refactormyself@...il.com>,
        bjorn@...gaas.com, Shuah Khan <skhan@...uxfoundation.org>,
        linux-pci <linux-pci@...r.kernel.org>,
        linux-kernel-mentees@...ts.linuxfoundation.org,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Richard Henderson <rth@...ddle.net>,
        Ivan Kokshaysky <ink@...assic.park.msu.ru>,
        Matt Turner <mattst88@...il.com>,
        Greg Ungerer <gerg@...ux-m68k.org>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Russell King <linux@...linux.org.uk>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Juergen Gross <jgross@...e.com>,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        Michael Ellerman <mpe@...erman.id.au>,
        Paul Mackerras <paulus@...ba.org>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        "David S. Miller" <davem@...emloft.net>,
        sparclinux <sparclinux@...r.kernel.org>,
        Guenter Roeck <linux@...ck-us.net>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Heiner Kallweit <hkallweit1@...il.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Keith Busch <kbusch@...nel.org>, Jens Axboe <axboe@...com>,
        Christoph Hellwig <hch@....de>,
        Sagi Grimberg <sagi@...mberg.me>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Rob Herring <robh@...nel.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Kevin Hilman <khilman@...libre.com>,
        Philipp Zabel <p.zabel@...gutronix.de>,
        Jingoo Han <jingoohan1@...il.com>,
        Gustavo Pimentel <gustavo.pimentel@...opsys.com>,
        Toan Le <toan@...amperecomputing.com>,
        Ray Jui <rjui@...adcom.com>,
        Scott Branden <sbranden@...adcom.com>,
        Ley Foon Tan <ley.foon.tan@...el.com>,
        Marek Vasut <marek.vasut+renesas@...il.com>,
        Kjetil Oftedal <oftedal@...il.com>
Subject: Re: [RFC PATCH 00/35] Move all PCIBIOS* definitions into arch/x86

On Tue, 2020-07-14 at 18:46 -0500, Bjorn Helgaas wrote:
> Yes.  I have no problem with that.  There are a few cases where it's
> important to check for errors, e.g., we read a status register and do
> something based on a bit being set.  A failure will return all bits
> set, and we may do the wrong thing.  But most of the errors we care
> about will be on MMIO reads, not config reads, so we can probably
> ignore most config read errors.

And in both cases, we don't have the plumbing to provide accurate
and reliable error returns for all platforms anyways (esp. not for
MMIO).

I think it makes sense to stick to the good old "if all 1's, then go
out of line" including for config space.

 ../..

> Yep, except for things like device removal or other PCI errors.

A whole bunch of these are reported asynchronously, esp for writes (and
yes, including config writes, they are supposed to be non-posted but
more often than not, the path  from the CPU to the PCI bridge remains
posted for writes including config ones).

> So maybe a good place to start is by removing some of the useless
> error checking for pci_read_config_*() and pci_write_config_*().
> That's a decent-sized but not impractical project that could be done
> per subsystem or something:
> 
>   git grep -E "(if|return|=).*\<pci_(read|write)_config" drivers
> 
> finds about 400 matches.
> 
> Some of those callers probably really *do* want to check for errors,
> and I guess we'd have to identify them and do them separately as you
> mentioned.

I'd be curious about these considering how unreliable our error return
is accross the board.

Cheers,
Ben.