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 13:32:31 +0200
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Naveen Naidu <naveennaidu479@...il.com>
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

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?

> +       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