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Message-ID: <7busek7zgost2s7mjklgvlccaef3lgz4k7btki72nkr5et7fdn@wkv2z6zbicdj>
Date: Tue, 24 Dec 2024 11:55:39 +0200
From: Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
To: Krishna Chaitanya Chundru <quic_krichai@...cinc.com>
Cc: Konrad Dybcio <konradybcio@...nel.org>, 
	Krishna Chaitanya Chundru <krishna.chundru@....qualcomm.com>, Rob Herring <robh@...nel.org>, 
	Krzysztof Kozlowski <krzk+dt@...nel.org>, Conor Dooley <conor+dt@...nel.org>, 
	Bjorn Helgaas <bhelgaas@...gle.com>, Jingoo Han <jingoohan1@...il.com>, 
	Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>, Lorenzo Pieralisi <lpieralisi@...nel.org>, 
	Krzysztof WilczyƄski <kw@...ux.com>, linux-arm-msm@...r.kernel.org, devicetree@...r.kernel.org, 
	linux-kernel@...r.kernel.org, linux-pci@...r.kernel.org, konrad.dybcio@....qualcomm.com, 
	quic_mrana@...cinc.com, quic_vbadigan@...cinc.com, 
	Bjorn Andersson <andersson@...nel.org>
Subject: Re: [PATCH v3 2/4] PCI: of: Add API to retrieve equalization presets
 from device tree

On Tue, Dec 24, 2024 at 02:47:00PM +0530, Krishna Chaitanya Chundru wrote:
> 
> 
> On 12/24/2024 12:00 AM, Dmitry Baryshkov wrote:
> > On Mon, Dec 23, 2024 at 10:13:29PM +0530, Krishna Chaitanya Chundru wrote:
> > > 
> > > 
> > > On 12/23/2024 8:56 PM, Dmitry Baryshkov wrote:
> > > > 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@....qualcomm.com>
> > > > > > > ---
> > > > > > >     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.
> > > > 
> > > ok.
> > > 
> > > I think then better to use v1 patch
> > > https://lore.kernel.org/all/20241116-presets-v1-2-878a837a4fee@quicinc.com/
> > > 
> > > konrad, any objection on using v1 as that will be cleaner way even if we
> > > have some repetitive code.
> > 
> > Konrad had a nice suggestion about using the array of values. Please use
> > such an array for 16gts and above. This removes most of repetitive code.
> > 
> I don't feel having array in the preset structure looks good, I have
> come up with this logic if you feel it is not so good I will go to the
> suggested way by having array for 16gts and above.
> 
>        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 = EQ_PRESET_TYPE_16GTS; i < EQ_PRESET_TYPE_64GTS; 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;
>                         }
> 
>                         switch (i) {
>                                 case EQ_PRESET_TYPE_16GTS:
>                                         presets->eq_presets_16gts = temp;
>                                         break;
>                                 case EQ_PRESET_TYPE_32GTS:
>                                         presets->eq_presets_32gts = temp;
>                                         break;
>                                 case EQ_PRESET_TYPE_64GTS:
>                                         presets->eq_presets_64gts = temp;
>                                         break;
>                         }

This looks like 'presets->eq_presets[i] = temp;', but I won't insist on
that.

Also, a strange thought came to my mind: we know that there won't be
more than 16 lanes. Can we have the following structure instead:

#define MAX_LANES 16
enum pcie_gts {
	PCIE_GTS_16GTS,
	PCIE_GTS_32GTS,
	PCIE_GTS_64GTS,
	PCIE_GTS_MAX,
};
struct pci_eq_presets {
	u16 eq_presets_8gts[MAX_LANES];
	u8 eq_presets_Ngts[PCIE_GTS_MAX][MAX_LANES];
};

This should allow you to drop the of_property_present() and
devm_kzalloc(). Just read DT data into a corresponding array.

>                 }
>         }
> - Krishna Chaitanya.
> 
> > 

-- 
With best wishes
Dmitry

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