Date:   Thu, 7 Apr 2022 17:32:15 -0500
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Shlomo Pongratz <shlomopongratz@...il.com>
Cc:     linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org,
        andrew.maier@...eticom.com, logang@...tatee.com,
        bhelgaas@...gle.com, jgg@...dia.com,
        Shlomo Pongratz <shlomop@...ops.com>
Subject: Re: [PATCH V6 1/1] Intel Sky Lake-E host root ports check.

On Sun, Apr 03, 2022 at 01:20:08PM +0300, Shlomo Pongratz wrote:
> In commit 7b94b53db34f ("PCI/P2PDMA: Add Intel Sky Lake-E Root Ports B, C,
>  D to the whitelist")
> Andrew Maier added the Sky Lake-E additional devices
> 2031, 2032 and 2033 root ports to the already existing 2030 device.
> 
> The Intel devices 2030, 2031, 2032 and 2033 which are root ports A, B, C
> and D, respectively and if all exist they will occupy slots 0 till 3 in
> that order.
> 
> The original code handled only the case where the devices in the whitelist
> are host bridges and assumed that they will be found on slot 0.
> 
> Since this assumption doesn't hold for root ports, add a test to cover this
> case.
> 
> Signed-off-by: Shlomo Pongratz <shlomop@...ops.com>
> ---
>  drivers/pci/p2pdma.c | 25 ++++++++++++++++++-------
>  1 file changed, 18 insertions(+), 7 deletions(-)
> 
> diff --git a/drivers/pci/p2pdma.c b/drivers/pci/p2pdma.c
> index 30b1df3c9d2f..c281bf5b304a 100644
> --- a/drivers/pci/p2pdma.c
> +++ b/drivers/pci/p2pdma.c
> @@ -327,15 +327,19 @@ static const struct pci_p2pdma_whitelist_entry {
>  
>  /*
>   * This lookup function tries to find the PCI device corresponding to a given
> - * host bridge.
> + * host bridge or a root port.
>   *
>   * It assumes the host bridge device is the first PCI device in the
> - * bus->devices list and that the devfn is 00.0. These assumptions should hold
> - * for all the devices in the whitelist above.
> + * bus->devices list and that the devfn is 00.0. The first assumption should
> + * hold for all the devices in the whitelist above, however the second
> + * assumption doesn't always hold for root ports.
> + * For example for Intel Skylake devices 2030, 2031, 2032 and 2033,
> + * which are root ports (A, B, C and D respectively).
> + * So the function checks explicitly that the device is a root port.
>   *

> - * This function is equivalent to pci_get_slot(host->bus, 0), however it does
> - * not take the pci_bus_sem lock seeing __host_bridge_whitelist() must not
> - * sleep.
> + * This function is equivalent to pci_get_slot(host->bus, 0) (except for
> + * the root port test), however it does not take the pci_bus_sem lock seeing
> + * __host_bridge_whitelist() must not sleep.
>   *
>   * For this to be safe, the caller should hold a reference to a device on the
>   * bridge, which should ensure the host_bridge device will not be freed
> @@ -350,7 +354,14 @@ static struct pci_dev *pci_host_bridge_dev(struct pci_host_bridge *host)
>  
>  	if (!root)
>  		return NULL;
> -	if (root->devfn != PCI_DEVFN(0, 0))
> +
> +	/* Verify that the device is a host bridge or a root port
> +	 * It is assumed that host bridges have a 0 devfn, (common practice)
> +	 * but some of the entries in the whitelist are root ports that can
> +	 * have any devfn
> +	 */
> +	if (root->devfn != PCI_DEVFN(0, 0) &&
> +	    pci_pcie_type(root) != PCI_EXP_TYPE_ROOT_PORT)
>  		return NULL;
>  
>  	return root;

The negative logic here makes this hard to read.  The previous code
was the same as:

  if (root->devfn == PCI_DEVFN(0, 0))
    return root;

  return NULL;

I think this patch would be easier to read if you made it:

  if (root->devfn == PCI_DEVFN(0, 0))
    return root;

  if (pci_pcie_type(root) == PCI_EXP_TYPE_ROOT_PORT)
    return root;

  return NULL;

IIUC, this patch tweaks it so we take the first device on the bus and
if it is either 00.0 or a Root Port, we search pci_p2pdma_whitelist[]
for it.

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