[PATCHv8 3/6] block: introduce max_write_hints queue limit

[Bart Van Assche]

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.

> +What:		/sys/block/<disk>/queue/max_write_hints
> +Date:		October 2024
> +Contact:	linux-block at vger.kernel.org
> +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.

> 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().

Thanks,

Bart.