Re: [PATCHv4 2/5] scsi: Export blacklist flags to sysfs

[Bart Van Assche <Bart.VanAssche@wdc.com>]

On Wed, 2017-08-30 at 12:50 +0200, Hannes Reinecke wrote:
> On 08/29/2017 06:02 PM, Bart Van Assche wrote:
> > On Tue, 2017-08-29 at 10:49 +0200, Hannes Reinecke wrote:
> > > +} sdev_bflags[] = {
> > > +#include "scsi_devinfo_tbl.c"
> > > +};
> > > +
> > > +static const char *sdev_bflags_name(unsigned int bflags)
> > > +{
> > > +	int i;
> > > +	const char *name = NULL;
> > > +
> > > +	for (i = 0; i < ARRAY_SIZE(sdev_bflags); i++) {
> > > +		if (sdev_bflags[i].value == bflags) {
> > > +			name = sdev_bflags[i].name;
> > > +			break;
> > > +		}
> > > +	}
> > > +	return name;
> > > +}
> > 
> > How about using ilog2() of the BLIST_* values as index of the table such that
> > an array lookup can be used instead of a for-loop over the entire array?
> 
> Not sure if that'll work. The BLIST_* values in fact form a sparse
> array, so the lookup array 'sdev_bflags' actually an associative array.
> And I dare not convert it to a normal array as then I couldn't to the
> automatice table creation anymore.
> 
> So I'm not sure if ilog2 will save me anything here.

Hello Hannes,

Are you aware that there is already code upstream for converting a bitmask
into a list of names of flags that supports sparse bit definitions? From
block/blk-mq-debugfs.c:

#define HCTX_FLAG_NAME(name) [ilog2(BLK_MQ_F_##name)] = #name
static const char *const hctx_flag_name[] = {
	HCTX_FLAG_NAME(SHOULD_MERGE),
	HCTX_FLAG_NAME(TAG_SHARED),
	HCTX_FLAG_NAME(SG_MERGE),
	HCTX_FLAG_NAME(BLOCKING),
	HCTX_FLAG_NAME(NO_SCHED),
};
#undef HCTX_FLAG_NAME

static int blk_flags_show(struct seq_file *m, const unsigned long flags,
			  const char *const *flag_name, int flag_name_count)
{
	bool sep = false;
	int i;

	for (i = 0; i < sizeof(flags) * BITS_PER_BYTE; i++) {
		if (!(flags & BIT(i)))
			continue;
		if (sep)
			seq_puts(m, "|");
		sep = true;
		if (i < flag_name_count && flag_name[i])
			seq_puts(m, flag_name[i]);
		else
			seq_printf(m, "%d", i);
	}
	return 0;
}

[ ... ]
	blk_flags_show(m, hctx->flags ^ BLK_ALLOC_POLICY_TO_MQ_FLAG(alloc_policy),
		       hctx_flag_name, ARRAY_SIZE(hctx_flag_name));
[ ... ]

Bart.