linux-kernel - Re: [PATCH v2 14/24] PCI: rcar: Remove redundant error fabrication when device read fails

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Date:   Mon, 18 Oct 2021 17:21:48 +0530
From:   Naveen Naidu <naveennaidu479@...il.com>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:     Bjorn Helgaas <bhelgaas@...gle.com>,
        linux-kernel-mentees@...ts.linuxfoundation.org,
        linux-pci <linux-pci@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Marek Vasut <marek.vasut+renesas@...il.com>,
        Yoshihiro Shimoda <yoshihiro.shimoda.uh@...esas.com>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Rob Herring <robh@...nel.org>,
        Krzysztof Wilczyński <kw@...ux.com>,
        "open list:PCI DRIVER FOR RENESAS R-CAR" 
        <linux-renesas-soc@...r.kernel.org>
Subject: Re: [PATCH v2 14/24] PCI: rcar: Remove redundant error fabrication
 when device read fails

On 18/10, Geert Uytterhoeven wrote:
> Hi Naveen,
> 
> On Sat, Oct 16, 2021 at 5:33 PM Naveen Naidu <naveennaidu479@...il.com> 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.
> >
> > The host controller drivers sets the error response values (~0) and
> > returns an error when faulty hardware read occurs. But the error
> > response value (~0) is already being set in PCI_OP_READ and
> > PCI_USER_READ_CONFIG whenever a read by host controller driver fails.
> >
> > Thus, it's no longer necessary for the host controller drivers to
> > fabricate any error response.
> >
> > This helps unify PCI error response checking and make error check
> > consistent and easier to find.
> >
> > Signed-off-by: Naveen Naidu <naveennaidu479@...il.com>
> 
> Thanks for your patch!
> 
> > --- a/drivers/pci/controller/pcie-rcar-host.c
> > +++ b/drivers/pci/controller/pcie-rcar-host.c
> > @@ -161,10 +161,8 @@ static int rcar_pcie_read_conf(struct pci_bus *bus, unsigned int devfn,
> >
> >         ret = rcar_pcie_config_access(host, RCAR_PCI_ACCESS_READ,
> >                                       bus, devfn, where, val);
> > -       if (ret != PCIBIOS_SUCCESSFUL) {
> > -               *val = 0xffffffff;
> 
> I don't see the behavior you describe in PCI_OP_READ(), so dropping
> this will lead to returning an uninitialized value?
>

Hello Geert,

Thank you for looking into the patch.

The described behaviour for PCI_OP_READ is part of the 01/24 [1] patch of
the series. 

[1]:
https://lore.kernel.org/linux-pci/b913b4966938b7cad8c049dc34093e6c4b2fae68.1634306198.git.naveennaidu479@gmail.com/T/#u

It looks like, I did not add proper receipients for that patch and hence
is leading to confusion. I really apologize for that.

I do not know what the right approach here should be, should I resend
the entire patch series, adding proper receipients OR should I reply to
each of the patches for the drivers and add the link to the patch. I did
not want to spam people with a lot of mails so I was confused as to what
the right option is.

Thanks,
Naveen

> > +       if (ret != PCIBIOS_SUCCESSFUL)
> >                 return ret;
> > -       }
> >
> >         if (size == 1)
> >                 *val = (*val >> (BITS_PER_BYTE * (where & 3))) & 0xff;
> 
> Gr{oetje,eeting}s,
> 
>                         Geert
> 
> --
> Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org
> 
> In personal conversations with technical people, I call myself a hacker. But
> when I'm talking to journalists I just say "programmer" or something like that.
>                                 -- Linus Torvalds