linux-kernel - Re: [PATCH v3 03/20] perf arm-spe: Refactor payload size calculation

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Date:   Mon, 26 Oct 2020 09:41:55 +0800
From:   Leo Yan <leo.yan@...aro.org>
To:     André Przywara <andre.przywara@....com>
Cc:     Arnaldo Carvalho de Melo <acme@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Wei Li <liwei391@...wei.com>,
        James Clark <james.clark@....com>, Al Grant <Al.Grant@....com>,
        Dave Martin <Dave.Martin@....com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 03/20] perf arm-spe: Refactor payload size calculation

Hi Andre,

On Fri, Oct 23, 2020 at 06:08:53PM +0100, André Przywara wrote:
> On 22/10/2020 15:57, Leo Yan wrote:
> 
> Hi Leo,
> 
> > This patch defines macro to extract "sz" field from header, and renames
> > the function payloadlen() to arm_spe_payload_len().
> > 
> > Signed-off-by: Leo Yan <leo.yan@...aro.org>
> > ---
> >  .../util/arm-spe-decoder/arm-spe-pkt-decoder.c | 18 +++++++++---------
> >  .../util/arm-spe-decoder/arm-spe-pkt-decoder.h |  3 +++
> >  2 files changed, 12 insertions(+), 9 deletions(-)
> > 
> > diff --git a/tools/perf/util/arm-spe-decoder/arm-spe-pkt-decoder.c b/tools/perf/util/arm-spe-decoder/arm-spe-pkt-decoder.c
> > index 7c7b5eb09fba..4294c133a465 100644
> > --- a/tools/perf/util/arm-spe-decoder/arm-spe-pkt-decoder.c
> > +++ b/tools/perf/util/arm-spe-decoder/arm-spe-pkt-decoder.c
> > @@ -69,22 +69,22 @@ const char *arm_spe_pkt_name(enum arm_spe_pkt_type type)
> >  	return arm_spe_packet_name[type];
> >  }
> >  
> > -/* return ARM SPE payload size from its encoding,
> > - * which is in bits 5:4 of the byte.
> > - * 00 : byte
> > - * 01 : halfword (2)
> > - * 10 : word (4)
> > - * 11 : doubleword (8)
> > +/*
> > + * Extracts the field "sz" from header bits and converts to bytes:
> > + *   00 : byte (1)
> > + *   01 : halfword (2)
> > + *   10 : word (4)
> > + *   11 : doubleword (8)
> >   */
> > -static int payloadlen(unsigned char byte)
> > +static unsigned int arm_spe_payload_len(unsigned char hdr)
> >  {
> > -	return 1 << ((byte & 0x30) >> 4);
> > +	return 1 << SPE_HEADER_SZ(hdr);
> 
> I know, I know, I asked for this, but now looking again at it - and
> after having seen the whole series:
> This is now really trivial, and there are just two users? And
> SPE_HEADER_SZ() is only used in here?
> 
> So either you just stuff the "1U << .." into the callers of
> arm_spe_payload_len(), or indeed put all of this into one macro (as you
> had originally).

Okay, will stuff the "1U << .." into the function.

> Apologies for this forth and back, but I didn't realise how this is
> really used eventually, and I just saw the transition from function to
> macro.

Actually you considered more than me and I learned many details from the
reivewing process, so no worries :)

> But please use 1U << .., signed shifts are treacherous.
> 
> >  }
> >  
> >  static int arm_spe_get_payload(const unsigned char *buf, size_t len,
> >  			       struct arm_spe_pkt *packet)
> >  {
> > -	size_t payload_len = payloadlen(buf[0]);
> > +	size_t payload_len = arm_spe_payload_len(buf[0]);
> >  
> >  	if (len < 1 + payload_len)
> >  		return ARM_SPE_NEED_MORE_BYTES;
> > diff --git a/tools/perf/util/arm-spe-decoder/arm-spe-pkt-decoder.h b/tools/perf/util/arm-spe-decoder/arm-spe-pkt-decoder.h
> > index 4c870521b8eb..e9ea8e3ead5d 100644
> > --- a/tools/perf/util/arm-spe-decoder/arm-spe-pkt-decoder.h
> > +++ b/tools/perf/util/arm-spe-decoder/arm-spe-pkt-decoder.h
> > @@ -9,6 +9,7 @@
> >  
> >  #include <stddef.h>
> >  #include <stdint.h>
> > +#include <linux/bits.h>
> >  
> >  #define ARM_SPE_PKT_DESC_MAX		256
> >  
> > @@ -36,6 +37,8 @@ struct arm_spe_pkt {
> >  	uint64_t		payload;
> >  };
> >  
> > +#define SPE_HEADER_SZ(val)			((val & GENMASK_ULL(5, 4)) >> 4)
> 
> If you should keep this, please put parentheses around "val".

I went through your suggestions in this patch and other patches, looks
reasonable to me, will follow them and respin patch set.

Thanks a lot for timely reviewing!

Leo