netdev - Re: [Intel-wired-lan] [PATCH iwl-next v1] idpf: refactor some missing field get/prep conversions

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Date: Fri, 1 Dec 2023 21:43:45 +0100 (CET)
From: Julia Lawall <julia.lawall@...ia.fr>
To: Jesse Brandeburg <jesse.brandeburg@...el.com>
cc: Alexander Lobakin <aleksander.lobakin@...el.com>, 
    intel-wired-lan@...ts.osuosl.org, netdev@...r.kernel.org, 
    anthony.l.nguyen@...el.com
Subject: Re: [Intel-wired-lan] [PATCH iwl-next v1] idpf: refactor some missing
 field get/prep conversions



On Fri, 1 Dec 2023, Jesse Brandeburg wrote:

> On 12/1/2023 6:32 AM, Alexander Lobakin wrote:
> > From: Jesse Brandeburg <jesse.brandeburg@...el.com>
>
> >> --- a/drivers/net/ethernet/intel/idpf/idpf_txrx.c
> >> +++ b/drivers/net/ethernet/intel/idpf/idpf_txrx.c
> >> @@ -505,7 +505,7 @@ static void idpf_rx_post_buf_refill(struct idpf_sw_queue *refillq, u16 buf_id)
> >>
> >>  	/* store the buffer ID and the SW maintained GEN bit to the refillq */
> >>  	refillq->ring[nta] =
> >> -		((buf_id << IDPF_RX_BI_BUFID_S) & IDPF_RX_BI_BUFID_M) |
> >> +		FIELD_PREP(IDPF_RX_BI_BUFID_M, buf_id) |
> >>  		(!!(test_bit(__IDPF_Q_GEN_CHK, refillq->flags)) <<
> >>  		 IDPF_RX_BI_GEN_S);
> >
> > Why isn't that one converted as well?
>
> Because it's not a constant, and it's not checking a mask with "&", so
> the automation ignored it. I *did* a test, and we could convert the
> return value from test_bit (a bool) into the IDPF_RX_BI_GEN_M mask with
> FIELD_PREP, since C-code allows the luxury of converting a bool to a
> "1", even though it's a bit type ugly in this age of strict typing.
>
> >
> >>
> >> @@ -1825,14 +1825,14 @@ static bool idpf_tx_clean_complq(struct idpf_queue *complq, int budget,
> >>  		u16 gen;
> >>
> >>  		/* if the descriptor isn't done, no work yet to do */
> >> -		gen = (le16_to_cpu(tx_desc->qid_comptype_gen) &
> >> -		      IDPF_TXD_COMPLQ_GEN_M) >> IDPF_TXD_COMPLQ_GEN_S;
> >> +		gen = FIELD_GET(IDPF_TXD_COMPLQ_GEN_M,
> >> +				le16_to_cpu(tx_desc->qid_comptype_gen));
> >
> > The definition:
> >
> > #define IDPF_TXD_COMPLQ_GEN_M		BIT_ULL(IDPF_TXD_COMPLQ_GEN_S)
> >
> > Please don't use FIELD_*() API for 1 bit.
>
> Did you mean that gen is effectively used as a bool? I think that has
> nothing to do with my change over to FIELD_GET, but I could see how
> redesigning this code would be useful, but not as part of this
> conversion series.
>
> >
> > 		gen = !!(le16_to_cpu(tx_desc->qid_comptype_gen) &
> > 			 IDPF_TXD_COMPLQ_GEN_M);
> >
> > is enough.
>
> Generally I'd prefer that the kind of check above returned a bool with a
> constant conversion of the mask (compile time) to an LE16 mask, and then
> use that, which is why all of our other drivers do that instead.
>
> >
> > Moreover, you could use le*_{get,encode,replace}_bits() to get/set LE
> > values right away without 2-step operation (from/to CPU + masks), but
> > you didn't do that here (see below for an example).
>
> Those aren't widely used yet in our drivers so I wasn't picking them up
> yet. But thank you for pointing that out.
>
> <snip>
>
>
> > In general, I would say those two issues are very common in IDPF and
> > also the whole your series converting the Intel drivers. The scripts
> > won't check whether the mask has only 1 bit or whether the value gets
> > converted from/to LE, so they won't help here.
>
> I had been hoping to do some more followup work. it's possible that with
> some tweaking the coccinelle script could learn how to detect non-pow2
> constants, and therefore possibly one bit constants as well. Maybe
> @Julia can help us refine the script and possibly get it into the
> scripts/coccinelle directory to help other drivers as well.

I don't have the original script handy.

If it is an explicit constant, like 8, then you can match it with
something like:

constant pow2 : script:python() { is_pow2(pow2) };

where is_pow2 is a python function that first converts pow2 to an integer,
and then, if that succeeds, checks if it is a power of two.  The result of
is_pow2 should be true or false.

When there is a #define constant, then you can to match the #define to
determine the value.  This may require options like --all-includes or
--recursive-includes.

Then you can write:

@r@
constant pow2 : script:python() { is_pow2(pow2) };
identifier i;
@@

#define i pow2

and then in some later rule, you can have

identifier r.i;

and then use i in a place where you expect a power of two constant.

julia


>
> > Could you maybe manually recheck all the places where bitfield masks are
> > used at least in IDPF (better in ice, iavf, i40e, ..., as well) and
> > posted a series that would address them? At the end, manual work is more
> > valuable than automated conversions :p
>
> I think a followup series would work better for this, do you agree?
>
> Thanks,
> Jesse
>