Re: [PATCH v5 3/6] ata: libata-scsi: route non-zero LUN commands for multi-LUN ATAPI

[Hannes Reinecke]

On 5/12/26 22:27, Phil Pemberton wrote:
> Two changes are required to route commands to ATAPI LUNs other than 0:
>
> 1. __ata_scsi_find_dev():  The existing code rejects any scsi_device
>     with a non-zero LUN, returning NULL and dropping the command on
>     the floor.  Hoist a non-zero LUN early-exit ahead of the original
>     channel/id checks: when scsidev->lun is non-zero, allow it through
>     only if the underlying ata_device is ATAPI class.  The original
>     LUN-0 path is left structurally unchanged.
>
> 2. atapi_xlat():  Older ATAPI devices (SCSI-2 era) expect the LUN in
>     CDB byte 1 bits 7:5 rather than relying on transport-level LUN
>     addressing.  Encode scmd->device->lun into those bits, preserving
>     the existing command-specific bits in 4:0.  This is required by
>     both the Panasonic PD/CD combos and Nakamichi CD changers.
>
>     The SCSI layer caps the LUN at shost->max_lun, so a value beyond
>     the device's nr_luns should never reach this point; guard with
>     WARN_ON_ONCE() and return AC_ERR_INVALID if it does, since the
>     3-bit CDB field cannot represent it.
>
> Signed-off-by: Phil Pemberton <philpem@xxxxxxxxxxxxx>
> ---
>   drivers/ata/libata-scsi.c | 32 ++++++++++++++++++++++++++++++++
>   1 file changed, 32 insertions(+)
>
> diff --git a/drivers/ata/libata-scsi.c b/drivers/ata/libata-scsi.c
> index 7c3d31dc49a1..2d714efc855f 100644
> --- a/drivers/ata/libata-scsi.c
> +++ b/drivers/ata/libata-scsi.c
> @@ -2953,6 +2953,15 @@ static unsigned int atapi_xlat(struct ata_queued_cmd *qc)
>   	memset(qc->cdb, 0, dev->cdb_len);
>   	memcpy(qc->cdb, scmd->cmnd, scmd->cmd_len);
>   
> +	/*
> +	 * SCSI-2 CDB LUN encoding: bits 7:5 of byte 1 (3-bit field).
> +	 * The SCSI layer caps the LUN at shost->max_lun (<= ATAPI_MAX_LUN),
> +	 * so this should never trip; warn and reject if it does.
> +	 */
> +	if (WARN_ON_ONCE(scmd->device->lun >= dev->nr_luns))
> +		return AC_ERR_INVALID;

cf my comment to the previous patch.
I'd prefer to have 'scmd->device->host->max_lun' as the upper limit.
And we should check the that value does not exceed ATAPI_MAX_LUN.

Cheers,

Hannes
--
Dr. Hannes Reinecke                  Kernel Storage Architect
hare@xxxxxxx                                +49 911 74053 688
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