linux-kernel - Re: [PATCH v9 1/6] fpga: dfl: fix the definitions of type & feature

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Message-ID: <440b7d06-426f-86c6-cf3f-396a9cc6bff7@redhat.com>
Date:   Mon, 12 Oct 2020 07:07:55 -0700
From:   Tom Rix <trix@...hat.com>
To:     Xu Yilun <yilun.xu@...el.com>
Cc:     mdf@...nel.org, krzk@...nel.org, linux-fpga@...r.kernel.org,
        linux-kernel@...r.kernel.org, gregkh@...uxfoundation.org,
        lgoncalv@...hat.com, hao.wu@...el.com
Subject: Re: [PATCH v9 1/6] fpga: dfl: fix the definitions of type &
 feature_id for dfl devices


On 10/11/20 7:41 PM, Xu Yilun wrote:
> On Sat, Oct 10, 2020 at 08:07:07AM -0700, Tom Rix wrote:
>> On 10/10/20 12:09 AM, Xu Yilun wrote:
>>> The value of the field dfl_device.type comes from the 12 bits register
>>> field DFH_ID according to DFL spec. So this patch changes the definition
>>> of the type field to u16.
>>>
>>> Also it is not necessary to illustrate the valid bits of the type field
>>> in comments. Instead we should explicitly define the possible values in
>>> the enumeration type for it, because they are shared by hardware spec.
>>> We should not let the compiler decide these values.
>>>
>>> Similar changes are also applied to dfl_device.feature_id.
>>>
>>> This patch also fixed the MODALIAS format according to the changes
>>> above.
>>>
>>> Signed-off-by: Xu Yilun <yilun.xu@...el.com>
>>> ---
>>> v9: no change
>>> ---
>>>  drivers/fpga/dfl.c |  3 +--
>>>  drivers/fpga/dfl.h | 14 +++++++-------
>>>  2 files changed, 8 insertions(+), 9 deletions(-)
>>>
>>> diff --git a/drivers/fpga/dfl.c b/drivers/fpga/dfl.c
>>> index b450870..5a6ba3b 100644
>>> --- a/drivers/fpga/dfl.c
>>> +++ b/drivers/fpga/dfl.c
>>> @@ -298,8 +298,7 @@ static int dfl_bus_uevent(struct device *dev, struct kobj_uevent_env *env)
>>>  {
>>>  	struct dfl_device *ddev = to_dfl_dev(dev);
>>>  
>>> -	/* The type has 4 valid bits and feature_id has 12 valid bits */
>>> -	return add_uevent_var(env, "MODALIAS=dfl:t%01Xf%03X",
>>> +	return add_uevent_var(env, "MODALIAS=dfl:t%04Xf%04X",
>>>  			      ddev->type, ddev->feature_id);
>>>  }
>>>  
>>> diff --git a/drivers/fpga/dfl.h b/drivers/fpga/dfl.h
>>> index 5dc758f..ac373b1 100644
>>> --- a/drivers/fpga/dfl.h
>>> +++ b/drivers/fpga/dfl.h
>>> @@ -520,19 +520,19 @@ long dfl_feature_ioctl_set_irq(struct platform_device *pdev,
>>>   * enum dfl_id_type - define the DFL FIU types
>>>   */
>>>  enum dfl_id_type {
>>> -	FME_ID,
>>> -	PORT_ID,
>>> +	FME_ID = 0,
>>> +	PORT_ID = 1,
>> This is redundant, why make this change ?
> These values are shared by hardware spec, so it is suggested that the
> values of the enum type should be explicitly set, otherwise the compiler
> is in its right to do whatever it wants with them (within reason...)
>
> Please see the original discussion:
> https://lore.kernel.org/linux-fpga/20200923055436.GA2629915@kroah.com/

I don't believe this is undefined behavior for the compiler

from c11 6.7.2.2,3

The identifiers in an enumerator list are declared as constants that have type int and may appear wherever such are permitted.127) An enumerator with = defines its enumeration constant as the value of the constant expression. If the first enumerator has no =, the value of its enumeration constant is 0. Each subsequent enumerator with no = defines its enumeration constant as the value of the constant expression obtained by adding 1 to the value of the previous enumeration constant. (The use of enumerators with = may produce enumeration constants with values that duplicate other values in the same enumeration.) The enumerators of an enumeration are also known as its members.

setting them again has some use for documentation so this change is ok if you have strong feeling for it.

Tom

>
> Thanks,
> Yilun
>
>> Tom
>>
>>>  	DFL_ID_MAX,
>>>  };
>>>  
>>>  /**
>>>   * struct dfl_device_id -  dfl device identifier
>>> - * @type: contains 4 bits DFL FIU type of the device. See enum dfl_id_type.
>>> - * @feature_id: contains 12 bits feature identifier local to its DFL FIU type.
>>> + * @type: DFL FIU type of the device. See enum dfl_id_type.
>>> + * @feature_id: feature identifier local to its DFL FIU type.
>>>   * @driver_data: driver specific data.
>>>   */
>>>  struct dfl_device_id {
>>> -	u8 type;
>>> +	u16 type;
>>>  	u16 feature_id;
>>>  	unsigned long driver_data;
>>>  };
>>> @@ -543,7 +543,7 @@ struct dfl_device_id {
>>>   * @dev: generic device interface.
>>>   * @id: id of the dfl device.
>>>   * @type: type of DFL FIU of the device. See enum dfl_id_type.
>>> - * @feature_id: 16 bits feature identifier local to its DFL FIU type.
>>> + * @feature_id: feature identifier local to its DFL FIU type.
>>>   * @mmio_res: mmio resource of this dfl device.
>>>   * @irqs: list of Linux IRQ numbers of this dfl device.
>>>   * @num_irqs: number of IRQs supported by this dfl device.
>>> @@ -553,7 +553,7 @@ struct dfl_device_id {
>>>  struct dfl_device {
>>>  	struct device dev;
>>>  	int id;
>>> -	u8 type;
>>> +	u16 type;
>>>  	u16 feature_id;
>>>  	struct resource mmio_res;
>>>  	int *irqs;