Re: [PATCH v3 2/4] PCI: of: Add API to retrieve equalization presets from device tree

From: Dmitry Baryshkov
Date: Mon Dec 23 2024 - 10:26:16 EST

On Mon, Dec 23, 2024 at 08:02:23PM +0530, Krishna Chaitanya Chundru wrote:
>
>
> On 12/23/2024 5:17 PM, Dmitry Baryshkov wrote:
> > On Mon, Dec 23, 2024 at 12:21:15PM +0530, Krishna Chaitanya Chundru wrote:
> > > PCIe equalization presets are predefined settings used to optimize
> > > signal integrity by compensating for signal loss and distortion in
> > > high-speed data transmission.
> > >
> > > As per PCIe spec 6.0.1 revision section 8.3.3.3 & 4.2.4 for data rates
> > > of 8.0 GT/s, 16.0 GT/s, 32.0 GT/s, and 64.0 GT/s, there is a way to
> > > configure lane equalization presets for each lane to enhance the PCIe
> > > link reliability. Each preset value represents a different combination
> > > of pre-shoot and de-emphasis values. For each data rate, different
> > > registers are defined: for 8.0 GT/s, registers are defined in section
> > > 7.7.3.4; for 16.0 GT/s, in section 7.7.5.9, etc. The 8.0 GT/s rate has
> > > an extra receiver preset hint, requiring 16 bits per lane, while the
> > > remaining data rates use 8 bits per lane.
> > >
> > > Based on the number of lanes and the supported data rate, this function
> > > reads the device tree property and stores in the presets structure.
> > >
> > > Signed-off-by: Krishna Chaitanya Chundru <krishna.chundru@xxxxxxxxxxxxxxxx>
> > > ---
> > > drivers/pci/of.c | 45 +++++++++++++++++++++++++++++++++++++++++++++
> > > drivers/pci/pci.h | 17 +++++++++++++++--
> > > 2 files changed, 60 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/drivers/pci/of.c b/drivers/pci/of.c
> > > index dacea3fc5128..99e0e7ae12e9 100644
> > > --- a/drivers/pci/of.c
> > > +++ b/drivers/pci/of.c
> > > @@ -826,3 +826,48 @@ u32 of_pci_get_slot_power_limit(struct device_node *node,
> > > return slot_power_limit_mw;
> > > }
> > > EXPORT_SYMBOL_GPL(of_pci_get_slot_power_limit);
> > > +
> >
> > kerneldoc? Define who should free the memory and how.
> >
> I will update this in next series.
> as we are allocating using devm_kzalloc it should be freed on driver
> detach, as no special freeing is required.
> > > +int of_pci_get_equalization_presets(struct device *dev,
> > > + struct pci_eq_presets *presets,
> > > + int num_lanes)
> > > +{
> > > + char name[20];
> > > + void **preset;
> > > + void *temp;
> > > + int ret;
> > > +
> > > + if (of_property_present(dev->of_node, "eq-presets-8gts")) {
> > > + presets->eq_presets_8gts = devm_kzalloc(dev, sizeof(u16) * num_lanes, GFP_KERNEL);
> > > + if (!presets->eq_presets_8gts)
> > > + return -ENOMEM;
> > > +
> > > + ret = of_property_read_u16_array(dev->of_node, "eq-presets-8gts",
> > > + presets->eq_presets_8gts, num_lanes);
> > > + if (ret) {
> > > + dev_err(dev, "Error reading eq-presets-8gts %d\n", ret);
> > > + return ret;
> > > + }
> > > + }
> > > +
> > > + for (int i = 1; i < sizeof(struct pci_eq_presets) / sizeof(void *); i++) {
> > > + snprintf(name, sizeof(name), "eq-presets-%dgts", 8 << i);
> > > + if (of_property_present(dev->of_node, name)) {
> > > + temp = devm_kzalloc(dev, sizeof(u8) * num_lanes, GFP_KERNEL);
> > > + if (!temp)
> > > + return -ENOMEM;
> > > +
> > > + ret = of_property_read_u8_array(dev->of_node, name,
> > > + temp, num_lanes);
> > > + if (ret) {
> > > + dev_err(dev, "Error %s %d\n", name, ret);
> > > + return ret;
> > > + }
> > > +
> > > + preset = (void **)((u8 *)presets + i * sizeof(void *));
> >
> > Ugh.
> >
> I was trying iterate over each element on the structure as presets holds the
> starting address of the structure and to that we are adding size of the void
> * point to go to each element. I did this way to reduce the
> redundant code to read all the gts which has same way of storing the data
> from the device tree. I will add comments here in the next series.

Please rewrite this in a cleaner way. The code shouldn't raise
questions.

> > > + *preset = temp;
> > > + }
> > > + }
> > > +
> > > + return 0;
> > > +}
> > > +EXPORT_SYMBOL_GPL(of_pci_get_equalization_presets);
> > > diff --git a/drivers/pci/pci.h b/drivers/pci/pci.h
> > > index 14d00ce45bfa..82362d58bedc 100644
> > > --- a/drivers/pci/pci.h
> > > +++ b/drivers/pci/pci.h
> > > @@ -731,7 +731,12 @@ static inline u64 pci_rebar_size_to_bytes(int size)
> > > }
> > > struct device_node;
> > > -
> > > +struct pci_eq_presets {
> > > + void *eq_presets_8gts;
> > > + void *eq_presets_16gts;
> > > + void *eq_presets_32gts;
> > > + void *eq_presets_64gts;
> >
> > Why are all of those void*? 8gts is u16*, all other are u8*.
> >
> To have common parsing logic I moved them to void*, as these are pointers
> actual memory is allocated by of_pci_get_equalization_presets()
> based upon the gts these should not give any issues.

Please, don't. They have types. void pointers are for the opaque data.

> > > +};
> >
> > Empty lines before and after the struct definition.
> >
> ack.
>
> - Krishna Chaitanya.
> > > #ifdef CONFIG_OF
> > > int of_pci_parse_bus_range(struct device_node *node, struct resource *res);
> > > int of_get_pci_domain_nr(struct device_node *node);
> > > @@ -746,7 +751,9 @@ void pci_set_bus_of_node(struct pci_bus *bus);
> > > void pci_release_bus_of_node(struct pci_bus *bus);
> > > int devm_of_pci_bridge_init(struct device *dev, struct pci_host_bridge *bridge);
> > > -
> > > +int of_pci_get_equalization_presets(struct device *dev,
> > > + struct pci_eq_presets *presets,
> > > + int num_lanes);
> >
> > Keep the empty line.
> >
> > > #else
> > > static inline int
> > > of_pci_parse_bus_range(struct device_node *node, struct resource *res)
> > > @@ -793,6 +800,12 @@ static inline int devm_of_pci_bridge_init(struct device *dev, struct pci_host_br
> > > return 0;
> > > }
> > > +static inline int of_pci_get_equalization_presets(struct device *dev,
> > > + struct pci_eq_presets *presets,
> > > + int num_lanes)
> > > +{
> > > + return 0;
> > > +}
> > > #endif /* CONFIG_OF */
> > > struct of_changeset;
> > >
> > > --
> > > 2.34.1
> > >
> >

--
With best wishes
Dmitry