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

[Krishna Chaitanya Chundru <quic_krichai@quicinc.com>]

On 12/24/2024 3:25 PM, Dmitry Baryshkov wrote:
> 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@oss.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.
> 
in the dwc driver patch I was using pointers and memory allocation
to known if the property is present or not. If I use this way I might
end up reading dt property again. I think better to switch to have a
array for above 16gts.

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