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 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;
                         }
                 }
         }
- Krishna Chaitanya.

>