| Message ID | 8dd245a0-c088-c5e1-a2be-913c96f44bc9@gmail.com (mailing list archive) |
|---|---|
| State | Not Applicable |
| Delegated to: | Bjorn Helgaas |
| Headers | show |
| Series | [RESEND] PCI/VPD: Add simple VPD read cache | expand |
On Thu, Nov 18, 2021 at 10:10:20PM +0100, Heiner Kallweit wrote: > VPD reads can be time-consuming operations, and when reading bytes > sequentially then worst-case we may read the same VPD dword four times. > Typically it happens in pci_vpd_size() already that the same VPD dword > is read more than once. Therefore let's add a simple read cache that > caches the last read VPD dword. This adds 8 more bytes to pci_dev for everybody. Can you quantify the benefit? It seems like well-formed VPD typically has 3-4 tags, so we'd be talking about maybe optimizing 20ish reads (4 bytes per tag, reading the same dword 4 times per tag) down to 3-4 dword reads. Not sure the cost/benefit really pays off here. For crappy devices with junk in their VPD, I certainly see how we could do a huge number of reads. But that doesn't seem like it would be worth optimizing. > Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com> > --- > drivers/pci/vpd.c | 13 +++++++++++++ > include/linux/pci.h | 2 ++ > 2 files changed, 15 insertions(+) > > diff --git a/drivers/pci/vpd.c b/drivers/pci/vpd.c > index 75e48df2e..2315e45f6 100644 > --- a/drivers/pci/vpd.c > +++ b/drivers/pci/vpd.c > @@ -194,6 +194,11 @@ static ssize_t pci_vpd_read(struct pci_dev *dev, loff_t pos, size_t count, > break; > } > > + if (vpd->cached_offset == (pos & ~3)) { > + val = vpd->cached_val; > + goto process; > + } > + > ret = pci_user_write_config_word(dev, vpd->cap + PCI_VPD_ADDR, > pos & ~3); > if (ret < 0) > @@ -206,6 +211,9 @@ static ssize_t pci_vpd_read(struct pci_dev *dev, loff_t pos, size_t count, > if (ret < 0) > break; > > + vpd->cached_val = val; > + vpd->cached_offset = pos & ~3; > +process: > skip = pos & 3; > for (i = 0; i < sizeof(u32); i++) { > if (i >= skip) { > @@ -242,6 +250,10 @@ static ssize_t pci_vpd_write(struct pci_dev *dev, loff_t pos, size_t count, > return -EINTR; > > while (pos < end) { > + /* invalidate read cache */ > + if (vpd->cached_offset == pos) > + vpd->cached_offset = -1; > + > ret = pci_user_write_config_dword(dev, vpd->cap + PCI_VPD_DATA, > get_unaligned_le32(buf)); > if (ret < 0) > @@ -270,6 +282,7 @@ void pci_vpd_init(struct pci_dev *dev) > > dev->vpd.cap = pci_find_capability(dev, PCI_CAP_ID_VPD); > mutex_init(&dev->vpd.lock); > + dev->vpd.cached_offset = -1; > } > > static ssize_t vpd_read(struct file *filp, struct kobject *kobj, > diff --git a/include/linux/pci.h b/include/linux/pci.h > index cd8aa6fce..fb123c248 100644 > --- a/include/linux/pci.h > +++ b/include/linux/pci.h > @@ -297,6 +297,8 @@ enum pcie_link_width pcie_get_width_cap(struct pci_dev *dev); > struct pci_vpd { > struct mutex lock; > unsigned int len; > + u32 cached_val; > + int cached_offset; > u8 cap; > }; > > -- > 2.33.0 >
diff --git a/drivers/pci/vpd.c b/drivers/pci/vpd.c index 75e48df2e..2315e45f6 100644 --- a/drivers/pci/vpd.c +++ b/drivers/pci/vpd.c @@ -194,6 +194,11 @@ static ssize_t pci_vpd_read(struct pci_dev *dev, loff_t pos, size_t count, break; } + if (vpd->cached_offset == (pos & ~3)) { + val = vpd->cached_val; + goto process; + } + ret = pci_user_write_config_word(dev, vpd->cap + PCI_VPD_ADDR, pos & ~3); if (ret < 0) @@ -206,6 +211,9 @@ static ssize_t pci_vpd_read(struct pci_dev *dev, loff_t pos, size_t count, if (ret < 0) break; + vpd->cached_val = val; + vpd->cached_offset = pos & ~3; +process: skip = pos & 3; for (i = 0; i < sizeof(u32); i++) { if (i >= skip) { @@ -242,6 +250,10 @@ static ssize_t pci_vpd_write(struct pci_dev *dev, loff_t pos, size_t count, return -EINTR; while (pos < end) { + /* invalidate read cache */ + if (vpd->cached_offset == pos) + vpd->cached_offset = -1; + ret = pci_user_write_config_dword(dev, vpd->cap + PCI_VPD_DATA, get_unaligned_le32(buf)); if (ret < 0) @@ -270,6 +282,7 @@ void pci_vpd_init(struct pci_dev *dev) dev->vpd.cap = pci_find_capability(dev, PCI_CAP_ID_VPD); mutex_init(&dev->vpd.lock); + dev->vpd.cached_offset = -1; } static ssize_t vpd_read(struct file *filp, struct kobject *kobj, diff --git a/include/linux/pci.h b/include/linux/pci.h index cd8aa6fce..fb123c248 100644 --- a/include/linux/pci.h +++ b/include/linux/pci.h @@ -297,6 +297,8 @@ enum pcie_link_width pcie_get_width_cap(struct pci_dev *dev); struct pci_vpd { struct mutex lock; unsigned int len; + u32 cached_val; + int cached_offset; u8 cap; };
VPD reads can be time-consuming operations, and when reading bytes sequentially then worst-case we may read the same VPD dword four times. Typically it happens in pci_vpd_size() already that the same VPD dword is read more than once. Therefore let's add a simple read cache that caches the last read VPD dword. Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com> --- drivers/pci/vpd.c | 13 +++++++++++++ include/linux/pci.h | 2 ++ 2 files changed, 15 insertions(+)