Re: [PATCH v4 06/21] perf arm-spe: Refactor printing string to buffer

[Leo Yan]

Hi Dave,

On Tue, Oct 27, 2020 at 02:31:54PM +0000, Dave Martin wrote:
> On Tue, Oct 27, 2020 at 03:09:02AM +0000, Leo Yan wrote:
> > When outputs strings to the decoding buffer with function snprintf(),
> > SPE decoder needs to detects if any error returns from snprintf() and if
> > so needs to directly bail out.  If snprintf() returns success, it needs
> > to update buffer pointer and reduce the buffer length so can continue to
> > output the next string into the consequent memory space.
> > 
> > This complex logics are spreading in the function arm_spe_pkt_desc() so
> > there has many duplicate codes for handling error detecting, increment
> > buffer pointer and decrement buffer size.
> > 
> > To avoid the duplicate code, this patch introduces a new helper function
> > arm_spe_pkt_snprintf() which is used to wrap up the complex logics, and
> > the caller arm_spe_pkt_desc() will call it and simply check the returns
> > value.
> > 
> > This patch also moves the variable 'blen' as the function's local
> > variable, this allows to remove the unnecessary braces and improve the
> > readability.
> > 
> > Suggested-by: Dave Martin <Dave.Martin@xxxxxxx>
> > Signed-off-by: Leo Yan <leo.yan@xxxxxxxxxx>
> > Reviewed-by: Andre Przywara <andre.przywara@xxxxxxx>
> > ---
> >  .../arm-spe-decoder/arm-spe-pkt-decoder.c     | 247 ++++++++++--------
> >  1 file changed, 135 insertions(+), 112 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 04fd7fd7c15f..b400636e6da2 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
> > @@ -9,6 +9,7 @@
> >  #include <endian.h>
> >  #include <byteswap.h>
> >  #include <linux/bitops.h>
> > +#include <stdarg.h>
> >  
> >  #include "arm-spe-pkt-decoder.h"
> >  
> > @@ -258,192 +259,214 @@ int arm_spe_get_packet(const unsigned char *buf, size_t len,
> >  	return ret;
> >  }
> >  
> > +static int arm_spe_pkt_snprintf(char **buf_p, size_t *blen,
> > +				const char *fmt, ...)
> > +{
> > +	va_list ap;
> > +	int ret;
> > +
> > +	va_start(ap, fmt);
> > +	ret = vsnprintf(*buf_p, *blen, fmt, ap);
> > +	va_end(ap);
> > +
> > +	if (ret < 0)
> > +		return ret;
> > +
> > +	*buf_p += ret;
> > +	*blen -= ret;
> > +	return ret;
> > +}
> > +
> 
> This approach seems OK, though I wonder whether all the
> "if (ret < 0) return;" logic is really needed.
> 
> In case of failure, it probably doesn't matter what ends up in buf.
> If not, we could just implement a cumulative error:
> 
> static int arm_spe_pkt_snprintf(int *err, char **buf_p, size_t *blen,
> {
> 
> 	/* ... */
> 
> 	if (ret < 0) {
> 		if (err && !*err)
> 			*err = ret;
> 	} else {
> 		*buf_p += ret;
> 		*blen -= ret;
> 	}
> 
> 	return err ? *err : ret;
> }
> 
> and just return the final value of err.
> 
> 
> 
> >  int arm_spe_pkt_desc(const struct arm_spe_pkt *packet, char *buf,
> >  		     size_t buf_len)
> >  {
> >  	int ret, ns, el, idx = packet->index;
> >  	unsigned long long payload = packet->payload;
> >  	const char *name = arm_spe_pkt_name(packet->type);
> > +	size_t blen = buf_len;
> >  
> >  	switch (packet->type) {
> >  	case ARM_SPE_BAD:
> >  	case ARM_SPE_PAD:
> >  	case ARM_SPE_END:
> > -		return snprintf(buf, buf_len, "%s", name);
> > -	case ARM_SPE_EVENTS: {
> > -		size_t blen = buf_len;
> > -
> > -		ret = 0;
> > -		ret = snprintf(buf, buf_len, "EV");
> > -		buf += ret;
> > -		blen -= ret;
> > +		return arm_spe_pkt_snprintf(&buf, &blen, "%s", name);
> > +	case ARM_SPE_EVENTS:
> > +		ret = arm_spe_pkt_snprintf(&buf, &blen, "EV");
> > +		if (ret < 0)
> > +			return ret;
> > +
> 
> ...
> 
> Then this becomes
> 
> 	case ARM_SPE_END:
> 		return arm_spe_pkt_snprintf(&err, &buf, &blen, "%s", name);
> 	case ARM_SPE_EVENTS:
> 		arm_spe_pkt_snprintf(&ret, &buf, &blen, "%s", name);
> 
> 		if (payload & 0x1)
> 			arm_spe_pkt_snprintf(&err, &buf, &blen, " EXCEPTION-GEN");
> 		if (payload & 0x2)
> 			arm_spe_pkt_snprintf(&err, &buf, &blen, " RETIRED");
> 
> 	/* ... */
> 
> 
> This might be over-engineering, but it does help to condense the code.

I agree this approach is more neat; will follow it to refine code.

Thanks a lot for detailed explaination.

Leo