[dpdk-dev] [PATCH 01/38] eal: add support for 24 40 and 48 bit operations

[Hemant Agrawal]

On 6/19/2017 7:22 PM, Wiles, Keith wrote:
>
>> On Jun 19, 2017, at 6:00 AM, Shreyansh Jain <shreyansh.jain at nxp.com> wrote:
>>
>> Hello Adrien,
>>
>> On Friday 16 June 2017 04:04 PM, Adrien Mazarguil wrote:
>>> Hi Shreyansh,
>>> On Fri, Jun 16, 2017 at 09:21:35AM +0000, Shreyansh Jain wrote:
>>>> Hi Bruce,
>>>>
>>>>> -----Original Message-----
>>>>> From: Bruce Richardson [mailto:bruce.richardson at intel.com]
>>>>> Sent: Friday, June 16, 2017 2:27 PM
>>>>> To: Shreyansh Jain <shreyansh.jain at nxp.com>
>>>>> Cc: dev at dpdk.org; ferruh.yigit at intel.com; Hemant Agrawal
>>>>> <hemant.agrawal at nxp.com>
>>>>> Subject: Re: [dpdk-dev] [PATCH 01/38] eal: add support for 24 40 and 48 bit
>>>>> operations
>>>>>
>>>>> On Fri, Jun 16, 2017 at 11:10:31AM +0530, Shreyansh Jain wrote:
>>>>>> From: Hemant Agrawal <hemant.agrawal at nxp.com>
>>>>>>
>>>>>> Bit Swap and LE<=>BE conversions for 23, 40 and 48 bit width
>>>>>>
>>>>>> Signed-off-by: Hemant Agrawal <hemant.agrawal at nxp.com>
>>>>>> ---
>>>>>>  .../common/include/generic/rte_byteorder.h         | 78
>>>>> ++++++++++++++++++++++
>>>>>>  1 file changed, 78 insertions(+)
>>>>>>
>>>>> Are these really common enough for inclusion in an generic EAL file?
>>>>> Would they be better being driver specific, so that we don't end up with
>>>>> lots of extra byte-swap routines for each possible size used by a
>>>>> driver.
>>>>  Reasoning was to keep all bit/byte swap at a single place and if it is
>>>> useful for others.
>>>>
>>>> From DPAA perspective, these macro can be anywhere. In case someone else too
>>>> has use of this (now or in near-future), probably then we can consider this
>>>> in EAL.
>>>> Else, if I don't get much responses in a few days, I will shift them to
>>>> DPAA driver in next version of this series.
>>> While I'm not against exposing exotic byte swapping functions, they are not
>>> completely safe and I'm not sure they should be part of public header files
>>> on that basis.
>>> Problem is their storage size is larger than the number of bytes they deal
>>> with, which raises the question: are filler bytes prepended or appended to
>>> the converted value? How about input values in non-native order? Answering
>>> that is not so easy as it depends on the use case. We actually had a similar
>>> issue when defining VXLAN's VNI field for rte_flow, which is 24-bit in
>>> network order.
>>> Take rte_constant_bswap48() for instance, assuming input value is
>>> little-endian, output is supposed to be big-endian. While the shifts are
>>> correct, filler bytes are not in the right place for a big-endian system,
>>> and the resulting value stored on uint64_t cannot be used as-is. Again, that
>>> depends on the use case, it could be correct if the resulting value was to
>>> be used as is on a little-endian system.
>>
>> I understand what you have stated - the application or any user needs to be context aware about what they are using and the side-effect of such conversions.
>>
>>> I think the only safe way to deal with that is by defining specific types of
>>> the proper size, e.g.:
>>>  typedef uint8_t uint48_t[6];
>>> These are cumbersome and cannot be used like normal integers though. With
>>> such types, byte-swapping functions become meaningless.
>>> Since these are supposed to be rather simple functions, I'm not sure
>>> handling/documenting all this complexity in rte_byteorder.h makes sense.
>>
>> I have no issues moving these into DPAA specific code. Hemant added them in generic just in case they would be of use to others.
>>
>> -
>> Shreyansh
>
> These are all static inline functions, so no real code increase unless used and having them in one spot is the best place IMO.
>
>
> Regards,
> Keith

Yes! these are simple static inline functions with no core unless used.
Many hardware accelerators usages 40 bit & 48 bits data. we thought, it 
can be usable by others as well.

currently we are seeing a mixed opinion.

In next revision, We will move them within our driver code. If someone 
need them in future, we can always bring them to eal.

Regards,
Hemant

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