ena: Add macros which represent LLQ header size in bytes

[osamaabb_amazon.com]

Summary

This commit adds macros which define LLQ entry's
header part size in bytes.

The header part size is calculated by

(LLQ_ENTRY_SIZE - DESC_NUMBER * DESC_LEN)

While DESC_LEN is constant (size of a struct) the other two variables
are received from device and can theoretically change.
For example:

• LLQ entry size can be either 128 or 256
• number of descriptors can vary from 1 to 8

Approved by: cperciva
MFC after: 2 weeks
Sponsored by: Amazon, Inc.

[cperciva]

number of descriptors can vary from 1 to 8

I'm a bit confused by this since in this commit you're fixing it at 2?

[osamaabb_amazon.com]

In D46326#1064437, @cperciva wrote:

number of descriptors can vary from 1 to 8

I'm a bit confused by this since in this commit you're fixing it at 2?

In some not so prevalent cases the device may decide that it needs more than two descriptors, if so then the driver will use as much descriptors in each LLQ entry as asked by the device, however the default value is 2 descriptors hence setting default to 2

[cperciva]

In D46326#1065841, @osamaabb_amazon.com wrote:

In D46326#1064437, @cperciva wrote:

number of descriptors can vary from 1 to 8

I'm a bit confused by this since in this commit you're fixing it at 2?

In some not so prevalent cases the device may decide that it needs more than two descriptors, if so then the driver will use as much descriptors in each LLQ entry as asked by the device, however the default value is 2 descriptors hence setting default to 2

Ok, maybe the comment /* we allow 2 DMA descriptors per LLQ entry */ should contain the words "by default" somewhere? Looking at the code makes me think "we're going to have a buffer overflow if we need >2 descriptors"; making it clear that these are just default sizes will help future readers to not be scared like I was.