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

[Wiles, Keith]

> 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