On Fri, Oct 18, 2024 at 09:18:34AM -0700, Bart Van Assche wrote: > On 10/17/24 9:09 AM, Keith Busch wrote: > > Drivers with hardware that support write hints need a way to export how > > many are available so applications can generically query this. > > Something is missing from this patch, namely a change for the SCSI disk > (sd) driver that sets max_write_hints to sdkp->permanent_stream_count. Shouldn't someone who cares about scsi do that? I certainly don't care, nor have I been keeping up with what's happening there, so I'm also unqualified. > > +What: /sys/block/<disk>/queue/max_write_hints > > +Date: October 2024 > > +Contact: linux-block@xxxxxxxxxxxxxxx > > +Description: > > + [RO] Maximum number of write hints supported, 0 if not > > + supported. If supported, valid values are 1 through > > + max_write_hints, inclusive. > > That's a bit short. I think it would help to add a reference to the > aspects of the standards related to this attribute: permanent streams > for SCSI and FDP for NVMe. The specs regarding write hints have not historically been stable, so I'd rather not tie kernel docs to volatile external specifications. > > diff --git a/block/blk-settings.c b/block/blk-settings.c > > index a446654ddee5e..921fb4d334fa4 100644 > > --- a/block/blk-settings.c > > +++ b/block/blk-settings.c > > @@ -43,6 +43,7 @@ void blk_set_stacking_limits(struct queue_limits *lim) > > lim->seg_boundary_mask = BLK_SEG_BOUNDARY_MASK; > > /* Inherit limits from component devices */ > > + lim->max_write_hints = USHRT_MAX; > > lim->max_segments = USHRT_MAX; > > lim->max_discard_segments = USHRT_MAX; > > lim->max_hw_sectors = UINT_MAX; > > @@ -544,6 +545,8 @@ int blk_stack_limits(struct queue_limits *t, struct queue_limits *b, > > t->max_segment_size = min_not_zero(t->max_segment_size, > > b->max_segment_size); > > + t->max_write_hints = min(t->max_write_hints, b->max_write_hints); > > + > > alignment = queue_limit_alignment_offset(b, start); > > I prefer that lim->max_write_hints is initialized to zero in > blk_set_stacking_limits() and that blk_stack_limits() uses > min_not_zero(). How is a device supposed to report it doesn't support a write hint if 0 gets overridden?
