[EXT] [PATCH 1/4] compressdev: add LZ4 algorithm support

[Akhil Goyal]

> -----Original Message-----
> From: Michael Baum <michaelba at nvidia.com>
> Sent: Tuesday, January 31, 2023 2:07 AM
> To: Akhil Goyal <gakhil at marvell.com>; dev at dpdk.org; Mahipal Challa
> <mchalla at marvell.com>; Fan Zhang <fanzhang.oss at gmail.com>; Ashish Gupta
> <ashishg at marvell.com>
> Cc: Matan Azrad <matan at nvidia.com>; Fiona Trahe <fiona.trahe at intel.com>;
> NBU-Contact-Thomas Monjalon (EXTERNAL) <thomas at monjalon.net>
> Subject: RE: [EXT] [PATCH 1/4] compressdev: add LZ4 algorithm support
> 
> On Mon, Jan 30, 2023 at 20:35 PM Akhil Goyal <gakhil at marvell.com> wrote:
> >
> > > +/**
> > > + * Block checksum flag.
> > > + * If this flag is set, each data block will be followed by a 4-bytes checksum,
> > > + * calculated by using the xxHash-32 algorithm on the raw (compressed)
> data
> > > + * block. The intention is to detect data corruption (storage or transmission
> > > + * errors) immediately, before decoding. Block checksum usage is optional.
> > > + */
> > > +#define RTE_COMP_LZ4_FLAG_BLOCK_CHECKSUM (1 << 4)
> > > +
> > > +/**
> > > + * Block Independence flag.
> > > + * If this flag is set to 1, blocks are independent.
> > > + * If this flag is set to 0, each block depends on previous ones (up to LZ4
> > > + * window size, which is 64 KB). In such case, it is necessary to decode all
> > > + * blocks in sequence.
> > > + * Block dependency improves compression ratio, especially for small
> blocks.
> > > On
> > > + * the other hand, it makes random access or multi-threaded decoding
> > > impossible.
> > > + */
> > > +#define RTE_COMP_LZ4_FLAG_BLOCK_INDEPENDENCE (1 << 5)
> >
> > Why did you start with 4th and 5th bit of the flags? Why not first two bits?
> 
> I didn't choose the values by myself, I took them from LZ4 standard:
> https://github.com/lz4/lz4/blob/dev/doc/lz4_Frame_format.md#frame-descriptor

Ok, Are we defining and making the frame descriptor inside library?
Also, why all the bits not added?
I believe these are just flags which PMD would use to make the frame descriptors.
If so, we should start with 0th bit and let the PMD handle bit shifting.