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Date: Thu, 20 Jan 2022 07:26:09 +0100
From: Greg KH <gregkh@...uxfoundation.org>
To: Vikash Bansal <bvikas@...are.com>
Cc: bhelgaas@...gle.com, linux-pci@...r.kernel.org,
linux-kernel@...r.kernel.org, srivatsab@...are.com,
srivatsa@...il.mit.edu, amakhalov@...are.com, srinidhir@...are.com,
anishs@...are.com, vsirnapalli@...are.com, akaher@...are.com
Subject: Re: [PATCH] PCI: Speed up device init by parsing capabilities all at
once
On Tue, Jan 18, 2022 at 09:16:01AM -0800, Vikash Bansal wrote:
> In the current implementation, the PCI capability list is parsed from
> the beginning to find each capability, which results in a large number
> of redundant PCI reads.
>
> Instead, we can parse the complete list just once, store it in the
> pci_dev structure, and get the offset of each capability directly from
> the pci_dev structure.
>
> This implementation improves pci devices initialization time by ~2-3% in
> case of bare metal and 7-8% in case of VM running on ESXi.
What is that in terms of "wall clock" time? % is hard to know here, and
of course it will depend on the PCI bus speed, right?
> It also adds a memory overhead of 20bytes (value of PCI_CAP_ID_MAX) per
> PCI device.
>
> Signed-off-by: Vikash Bansal <bvikas@...are.com>
> ---
> drivers/pci/pci.c | 43 ++++++++++++++++++++++++++++++++++++-------
> drivers/pci/probe.c | 5 +++++
> include/linux/pci.h | 2 ++
> 3 files changed, 43 insertions(+), 7 deletions(-)
>
> diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
> index 3d2fb394986a..8e024db30262 100644
> --- a/drivers/pci/pci.c
> +++ b/drivers/pci/pci.c
> @@ -468,6 +468,41 @@ static u8 __pci_bus_find_cap_start(struct pci_bus *bus,
> return 0;
> }
>
> +
> +/**
> + * pci_find_all_capabilities - Read all capabilities
> + * @dev: the PCI device
> + *
> + * Read all capabilities and store offsets in cap_off
> + * array in pci_dev structure.
> + */
> +void pci_find_all_capabilities(struct pci_dev *dev)
> +{
> + int ttl = PCI_FIND_CAP_TTL;
> + u16 ent;
> + u8 pos;
> + u8 id;
> +
> + pos = __pci_bus_find_cap_start(dev->bus, dev->devfn, dev->hdr_type);
> + if (!pos)
> + return;
> + pci_bus_read_config_byte(dev->bus, dev->devfn, pos, &pos);
> + while (ttl--) {
> + if (pos < 0x40)
What is this magic value of 0x40?
> + break;
> + pos &= ~3;
Why ~3?
> + pci_bus_read_config_word(dev->bus, dev->devfn, pos, &ent);
> + id = ent & 0xff;
Do you really need the & if you are truncating it?
> + if (id == 0xff)
> + break;
> +
> + /* Read first instance of capability */
> + if (!(dev->cap_off[id]))
> + dev->cap_off[id] = pos;
Shouldn't you have checked this before you read the value?
> + pos = (ent >> 8);
What about walking the list using __pci_find_next_cap() like before?
Why is this somehow the same as the old function?
> + }
> +}
> +
> /**
> * pci_find_capability - query for devices' capabilities
> * @dev: PCI device to query
> @@ -489,13 +524,7 @@ static u8 __pci_bus_find_cap_start(struct pci_bus *bus,
> */
> u8 pci_find_capability(struct pci_dev *dev, int cap)
> {
> - u8 pos;
> -
> - pos = __pci_bus_find_cap_start(dev->bus, dev->devfn, dev->hdr_type);
> - if (pos)
> - pos = __pci_find_next_cap(dev->bus, dev->devfn, pos, cap);
> -
> - return pos;
> + return dev->cap_off[cap];
> }
> EXPORT_SYMBOL(pci_find_capability);
>
> diff --git a/drivers/pci/probe.c b/drivers/pci/probe.c
> index 087d3658f75c..bacab12cedbb 100644
> --- a/drivers/pci/probe.c
> +++ b/drivers/pci/probe.c
> @@ -1839,6 +1839,11 @@ int pci_setup_device(struct pci_dev *dev)
> dev->hdr_type = hdr_type & 0x7f;
> dev->multifunction = !!(hdr_type & 0x80);
> dev->error_state = pci_channel_io_normal;
> + /*
> + * Read all capabilities and store offsets in cap_off
> + * array in pci_dev structure.
> + */
Comment is not needed if the function name is descriptive.
> + pci_find_all_capabilities(dev);
And it is, so no need for the comment.
> set_pcie_port_type(dev);
>
> pci_set_of_node(dev);
> diff --git a/include/linux/pci.h b/include/linux/pci.h
> index 18a75c8e615c..d221c73e67f8 100644
> --- a/include/linux/pci.h
> +++ b/include/linux/pci.h
> @@ -326,6 +326,7 @@ struct pci_dev {
> unsigned int class; /* 3 bytes: (base,sub,prog-if) */
> u8 revision; /* PCI revision, low byte of class word */
> u8 hdr_type; /* PCI header type (`multi' flag masked out) */
> + u8 cap_off[PCI_CAP_ID_MAX]; /* Offsets of all pci capabilities */
Did you run 'pahole' to ensure you are not adding extra padding bytes
here?
> #ifdef CONFIG_PCIEAER
> u16 aer_cap; /* AER capability offset */
> struct aer_stats *aer_stats; /* AER stats for this device */
> @@ -1128,6 +1129,7 @@ void pci_sort_breadthfirst(void);
>
> u8 pci_bus_find_capability(struct pci_bus *bus, unsigned int devfn, int cap);
> u8 pci_find_capability(struct pci_dev *dev, int cap);
> +void pci_find_all_capabilities(struct pci_dev *dev);
Why is this now a global function and not one just local to the pci
core? Who else would ever need to call it?
thanks,
greg k-h
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