On 10-12-24 12:46 PM, Randy Dunlap wrote:
On Thu, 23 Dec 2010 21:40:37 -0500 Douglas Gilbert wrote:
As discussed in a thread on this list titled:
"RFC: short reads on block devices"
this patch adds recommendations for LLDs to set resid
when there might be uncertainty about how much data
has been returned by a device.
This patch inline and attached] is against scsi-misc-2.6.git
Signed-off-by: Douglas Gilbert<dgilbert@xxxxxxxxxxxx>
diff --git a/Documentation/scsi/scsi_mid_low_api.txt
b/Documentation/scsi/scsi_mid_low_api.txt
index df322c1..7bcdea7 100644
--- a/Documentation/scsi/scsi_mid_low_api.txt
+++ b/Documentation/scsi/scsi_mid_low_api.txt
@@ -1351,6 +1351,18 @@ Members of interest:
report a DID_ERROR. Better for an LLD to implement
'resid'.
+It is recommended that a LLD set 'resid' on data transfers from a SCSI
+target device (e.g. READs). It is especially important that 'resid' is set
+when such data transfers have sense keys of MEDIUM ERROR and HARDWARE ERROR
+(and possibly RECOVERED ERROR). In these cases if a LLD is in doubt how much
+data has been received then the safest approach is to indicate no bytes have
+been received. For example: to indicate that no valid data has been received
+a LLD might use these helpers:
+ scsi_set_resid(SCpnt, scsi_bufflen(SCpnt));
+where 'SCpnt' is a pointer to a scsi_cmnd object. To indicate only three 512
+bytes blocks has been received 'resid' could be set like this:
+ scsi_set_resid(SCpnt, scsi_bufflen(SCpnt) - (3 * 512));
+
The scsi_cmnd structure is defined in include/scsi/scsi_cmnd.h
Hi Doug,
Using "an LLD" instead of "a LLD" would be more consistent with the rest
of this txt file (and read better to me).
Randy,
There were other examples of "a LLD" which I
cleaned up as well.
Doug Gilbert
diff --git a/Documentation/scsi/scsi_mid_low_api.txt
b/Documentation/scsi/scsi_mid_low_api.txt
index df322c1..10d41ba 100644
--- a/Documentation/scsi/scsi_mid_low_api.txt
+++ b/Documentation/scsi/scsi_mid_low_api.txt
@@ -185,7 +185,7 @@ scsi_remove_host() ---------+
scsi_host_put()
------------------------------------------------------------
-It may be useful for a LLD to keep track of struct Scsi_Host instances
+It may be useful for an LLD to keep track of struct Scsi_Host instances
(a pointer is returned by scsi_host_alloc()). Such instances are "owned"
by the mid-level. struct Scsi_Host instances are freed from
scsi_host_put() when the reference count hits zero.
@@ -749,7 +749,7 @@ void scsi_unblock_requests(struct Scsi_Host * shost)
*
* Notes: Should not be invoked if the "hotplug initialization
* model" is being used. Called internally by exit_this_scsi_driver()
- * in the "passive initialization model". Hence a LLD has no need to
+ * in the "passive initialization model". Hence an LLD has no need to
* call this function directly.
*
* Defined in: drivers/scsi/hosts.c .
@@ -1343,7 +1343,7 @@ Members of interest:
underruns (overruns should be rare). If possible an LLD
should set 'resid' prior to invoking 'done'. The most
interesting case is data transfers from a SCSI target
- device device (i.e. READs) that underrun.
+ device (e.g. READs) that underrun.
underflow - LLD should place (DID_ERROR << 16) in 'result' if
actual number of bytes transferred is less than this
figure. Not many LLDs implement this check and some that
@@ -1351,6 +1351,18 @@ Members of interest:
report a DID_ERROR. Better for an LLD to implement
'resid'.
+It is recommended that an LLD set 'resid' on data transfers from a SCSI
+target device (e.g. READs). It is especially important that 'resid' is set
+when such data transfers have sense keys of MEDIUM ERROR and HARDWARE ERROR
+(and possibly RECOVERED ERROR). In these cases if an LLD is in doubt how much
+data has been received then the safest approach is to indicate no bytes have
+been received. For example: to indicate that no valid data has been received
+an LLD might use these helpers:
+ scsi_set_resid(SCpnt, scsi_bufflen(SCpnt));
+where 'SCpnt' is a pointer to a scsi_cmnd object. To indicate only three 512
+bytes blocks has been received 'resid' could be set like this:
+ scsi_set_resid(SCpnt, scsi_bufflen(SCpnt) - (3 * 512));
+
The scsi_cmnd structure is defined in include/scsi/scsi_cmnd.h
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