Net: ucc_geth ethernet driver optimization space

Net: ucc_geth ethernet driver optimization space

[Liu Dave-R63238]

Guys,

The ucc_geth ethernet driver have dozens of strong sync read/write
operation, such as in_be32/16/8, out_be32/16/8.

all of them is sync read/write, it is very expensive for performance.

For the critical patch, we can remove some unnecessary in_be(x),
out_be(x) with normal memory operation, and keep some necessary
memory barrier.

eg: BD access in the interrupt handler and start_xmit.

The BD operation only need the memory barrier between length/buffer
and status.

struct buffer descriptor {
	u16 status;
	u16 length;
	u32 buffer;
} __attribute__ ((packed));

struct buffer descriptor *BD;

BD->length =3D xxxx;
BD->buffer =3D yyyy;
wmb();
BD->status =3D zzzz;

For powerpc, eieio is enough for 60x, mbar 1 is enough for e500.
Of couse, also need the memory clobber to avoid the compiler
reorder between them.

Thanks, Dave

Re: Net: ucc_geth ethernet driver optimization space

[Joakim Tjernlund]

linuxppc-dev-bounces+joakim.tjernlund=transmode.se@ozlabs.org wrote on 27/05/2009 07:08:07:
>
> Guys,
>
> The ucc_geth ethernet driver have dozens of strong sync read/write
> operation, such as in_be32/16/8, out_be32/16/8.
>
> all of them is sync read/write, it is very expensive for performance.
>
> For the critical patch, we can remove some unnecessary in_be(x),
> out_be(x) with normal memory operation, and keep some necessary
> memory barrier.
>
> eg: BD access in the interrupt handler and start_xmit.
>
> The BD operation only need the memory barrier between length/buffer
> and status.
>
> struct buffer descriptor {
>    u16 status;
>    u16 length;
>    u32 buffer;
> } __attribute__ ((packed));
>
> struct buffer descriptor *BD;
>
> BD->length = xxxx;
> BD->buffer = yyyy;
> wmb();
> BD->status = zzzz;
>
> For powerpc, eieio is enough for 60x, mbar 1 is enough for e500.
> Of couse, also need the memory clobber to avoid the compiler
> reorder between them.
>
> Thanks, Dave

Yes, pretty please :)

You might want to combine status and length into one U32 though:
BD->buffer = yyyy;
wmb();
BD->stat_len = zzzz << 16 | xxxx;

  Jocke

Re: Net: ucc_geth ethernet driver optimization space

[Li Yang]

On Wed, May 27, 2009 at 1:08 PM, Liu Dave-R63238 <DaveLiu@freescale.com> wr=
ote:
> Guys,
>
> The ucc_geth ethernet driver have dozens of strong sync read/write
> operation, such as in_be32/16/8, out_be32/16/8.
>
> all of them is sync read/write, it is very expensive for performance.
>

Totally agree.  That's one of my concerns right from the beginning.

> For the critical patch, we can remove some unnecessary in_be(x),
> out_be(x) with normal memory operation, and keep some necessary
> memory barrier.
>
> eg: BD access in the interrupt handler and start_xmit.
>
> The BD operation only need the memory barrier between length/buffer
> and status.
>
> struct buffer descriptor {
> =C2=A0 =C2=A0 =C2=A0 =C2=A0u16 status;
> =C2=A0 =C2=A0 =C2=A0 =C2=A0u16 length;
> =C2=A0 =C2=A0 =C2=A0 =C2=A0u32 buffer;
> } __attribute__ ((packed));
>
> struct buffer descriptor *BD;
>
> BD->length =3D xxxx;
> BD->buffer =3D yyyy;
> wmb();
> BD->status =3D zzzz;

The BD can reside either in memory or memory mapped region, which
makes the case more complex.

MMIO accesses need to use IO accessors for the sparse checking.  We
might make use of the __raw_*() accessors, but I'm not sure if it's
suitable for non-PCI buses on powerpc.  And also we need to pay
special attention to the problem described here:
http://lwn.net/Articles/198988/

- Leo