Re: [PATCH] scsi: Fix scsi_get/set_resid() interface

From: Bart Van Assche <bvanassche@acm.org>
To: Damien Le Moal <damien.lemoal@wdc.com>,
	linux-scsi@vger.kernel.org,
	"Martin K . Petersen" <martin.petersen@oracle.com>,
	linux-usb@vger.kernel.org, usb-storage@lists.one-eyed-alien.net,
	Alan Stern <stern@rowland.harvard.edu>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Justin Piszcz <jpiszcz@lucidpixels.com>
Subject: Re: [PATCH] scsi: Fix scsi_get/set_resid() interface
Date: Mon, 28 Oct 2019 13:38:01 -0700	[thread overview]
Message-ID: <eb8f6e3e-0350-9688-58c8-9d777ba93298@acm.org> (raw)
In-Reply-To: <20191028105732.29913-1-damien.lemoal@wdc.com>

On 10/28/19 3:57 AM, Damien Le Moal wrote:
> struct scsi_cmnd cmd->req.resid_len which is returned and set
> respectively by the helper functions scsi_get_resid() and
> scsi_set_resid() is an unsigned int. Reflect this fact in the interface
> of these helper functions.
> [ ... ]
> -static inline void scsi_set_resid(struct scsi_cmnd *cmd, int resid)
> +static inline void scsi_set_resid(struct scsi_cmnd *cmd, unsigned int resid)
>   {
>   	cmd->req.resid_len = resid;
>   }
>   
> -static inline int scsi_get_resid(struct scsi_cmnd *cmd)
> +static inline unsigned int scsi_get_resid(struct scsi_cmnd *cmd)
>   {
>   	return cmd->req.resid_len;
>   }

 From the iSCSI RFC:

    SCSI-Presented Data Transfer Length (SPDTL) is the term this document
    uses (see Section 1.1 for definition) to represent the aggregate data
    length that the target SCSI layer attempts to transfer using the
    local iSCSI layer for a task.  Expected Data Transfer Length (EDTL)
    is the iSCSI term that represents the length of data that the iSCSI
    layer expects to transfer for a task.  EDTL is specified in the SCSI
    Command PDU.

    When SPDTL = EDTL for a task, the target iSCSI layer completes the
    task with no residuals.  Whenever SPDTL differs from EDTL for a task,
    that task is said to have a residual.

    If SPDTL > EDTL for a task, iSCSI Overflow MUST be signaled in the
    SCSI Response PDU as specified in [RFC3720].  The Residual Count MUST
    be set to the numerical value of (SPDTL - EDTL).

    If SPDTL < EDTL for a task, iSCSI Underflow MUST be signaled in the
    SCSI Response PDU as specified in [RFC3720].  The Residual Count MUST
    be set to the numerical value of (EDTL - SPDTL).

    Note that the Overflow and Underflow scenarios are independent of
    Data-In and Data-Out.  Either scenario is logically possible in
    either direction of data transfer.

If the residual is changed from signed into unsigned, how is a SCSI LLD 
expected to report the difference between residual overflow and residual 
underflow to the SCSI core?

Thanks,

Bart.