Re: [PATCH v3 2/2] ata: libata-scsi: scale DSM TRIM payload by MAX PAGES PER DSM COMMAND

[Thomas Huth <thuth@redhat.com>]

On 02/07/2026 12.59, Niklas Cassel wrote:
> DSM TRIM currently always emits a single 512-byte page of LBA Range Entries
> (ATA_MAX_TRIM_RNUM == 64 ranges), regardless of how many pages the device
> can accept in one DATA SET MANAGEMENT command.
> 
> The maximum is reported by MAX PAGES PER DSM COMMAND (IDENTIFY DEVICE word
> 105).  Honour it: size the TRIM descriptor as a whole number of 512-byte
> pages, bounded by that limit and by the logical sector size (the WRITE SAME
> data-out buffer is a single logical block).  Build and transfer only as
> many pages as the request needs, and set the DSM COUNT field, qc->nbytes
> and the maximum WRITE SAME length in the Block Limits VPD page accordingly.
> 
> Build the descriptor straight into the WRITE SAME data-out buffer using an
> atomic sg_miter mapping, instead of staging it in the shared ata_scsi_rbuf
> and copying it out.  This removes the global ata_scsi_rbuf_lock and a
> memcpy from the TRIM path.
> 
> While commit 9379e6b8e0f9 ("libata: Safely overwrite attached page in WRITE
> SAME xlat") replaced direct access to the data-out buffer with an
> intermediate step that writes the entries in the ata_scsi_rbuf buffer, this
> solution writes to the data-out buffer using sg_miter, which maps each
> segment with kmap_atomic (SG_MITER_ATOMIC), so it's highmem- and
> multi-segment-safe, and it's usable from the non-sleeping
> command-submission path (unlike the page_address() access that
> ata_scsi_rbuf originally replaced).
> 
> A 512-byte-sector device still uses a single page, so its behaviour is
> unchanged.
> 
> Add ata_id_dsm_max_pages() to read IDENTIFY DEVICE word 105.
> 
> Reviewed-by: Hannes Reinecke <hare@kernel.org>
> Signed-off-by: Niklas Cassel <cassel@kernel.org>
> ---
>   drivers/ata/libata-scsi.c | 135 ++++++++++++++++++++++++++------------
>   include/linux/ata.h       |  13 ++++
>   2 files changed, 107 insertions(+), 41 deletions(-)
> 
> diff --git a/drivers/ata/libata-scsi.c b/drivers/ata/libata-scsi.c
> index 429b03a08071..5d56ad66af77 100644
> --- a/drivers/ata/libata-scsi.c
> +++ b/drivers/ata/libata-scsi.c
> @@ -2198,6 +2198,39 @@ static unsigned int ata_scsiop_inq_89(struct ata_device *dev,
>   	return get_unaligned_be16(&rbuf[2]) + 4;
>   }
>   
> +/**
> + * ata_dsm_trim_pages - maximum DSM TRIM payload for a device, in 512-byte pages
> + * @dev: ATA device the DATA SET MANAGEMENT TRIM command will be sent to
> + *
> + * A DATA SET MANAGEMENT TRIM payload is a list of 512-byte pages, each holding
> + * up to ATA_MAX_TRIM_RNUM (64) LBA Range Entries; the format is page-based and
> + * unrelated to the logical sector size.
> + *
> + * The logical sector size still bounds it, though: the descriptor is written
> + * directly into the WRITE SAME data-out buffer, which sd sizes to a single
> + * logical block, so it can hold at most sector_size / 512 pages.
> + *
> + * Return: the maximum number of 512-byte pages a single translated WRITE SAME
> + * command may send to @dev (never less than one), that is the smaller of:
> + *   - MAX PAGES PER DSM COMMAND (IDENTIFY DEVICE word 105), when the device
> + *     reports a non-zero limit; and
> + *   - the logical sector size expressed in 512-byte pages (see above).
> + */

  Hi,

not sure whether anybody noticed it already (I did not spot a patch on 
lore), but this causes an ERROR during "make htmldoc":

Documentation/driver-api/libata:604: drivers/ata/libata-scsi.c:2301: ERROR: 
Unexpected indentation. [docutils]

  Thomas