[RFC,1/2] usb: storage: scsiglue: further describe our 240 sector limit

Message ID 1446670112-29088-1-git-send-email-balbi@ti.com
State New
Headers show

Commit Message

Felipe Balbi Nov. 4, 2015, 8:48 p.m.
Just so we have some sort of documentation as to why
we limit our Mass Storage transfers to 240 sectors,
let's update the comment to make clearer that
devices were found that would choke with larger
transfers.

While at that, also make sure to clarify that other
operating systems have similar, albeit different,
limits on mass storage transfers.

Signed-off-by: Felipe Balbi <balbi@ti.com>

---

Guys, how about this version ? First we update comments then,
as a follow up patch, we increase USB3 only to 2048 sectors.

 drivers/usb/storage/scsiglue.c | 18 +++++++++++++++++-
 1 file changed, 17 insertions(+), 1 deletion(-)

-- 
2.6.2

--
To unsubscribe from this list: send the line "unsubscribe linux-usb" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html

Comments

Felipe Balbi Nov. 4, 2015, 10:07 p.m. | #1
Hi,

Sergei Shtylyov <sergei.shtylyov@cogentembedded.com> writes:
> Hello.

>

> On 11/4/2015 11:48 PM, Felipe Balbi wrote:

>

>> Just so we have some sort of documentation as to why

>> we limit our Mass Storage transfers to 240 sectors,

>> let's update the comment to make clearer that

>> devices were found that would choke with larger

>> transfers.

>>

>> While at that, also make sure to clarify that other

>> operating systems have similar, albeit different,

>> limits on mass storage transfers.

>>

>> Signed-off-by: Felipe Balbi <balbi@ti.com>

>> ---

>>

>> Guys, how about this version ? First we update comments then,

>> as a follow up patch, we increase USB3 only to 2048 sectors.

>>

>>   drivers/usb/storage/scsiglue.c | 18 +++++++++++++++++-

>>   1 file changed, 17 insertions(+), 1 deletion(-)

>>

>> diff --git a/drivers/usb/storage/scsiglue.c b/drivers/usb/storage/scsiglue.c

>> index dba51362d2e2..00aadf3a3857 100644

>> --- a/drivers/usb/storage/scsiglue.c

>> +++ b/drivers/usb/storage/scsiglue.c

>> @@ -565,7 +565,23 @@ static const struct scsi_host_template usb_stor_host_template = {

>>   	/* lots of sg segments can be handled */

>>   	.sg_tablesize =			SCSI_MAX_SG_CHAIN_SEGMENTS,

>>

>> -	/* limit the total size of a transfer to 120 KB */

>> +	/*

>> +	 * Limit the total size of a transfer to 120 KB.

>> +	 *

>> +	 * Some devices are known to choke with anything larger. It seems like

>> +	 * the problem stems from the fact that original IDE controllers had

>> +	 * only an 8-bit register to hold the number of sectors in one transfer

>> +	 * and even those couldn't handle a full 256 sectors.

>> +	 *

>> +	 * Because we want to make sure we interoperate with as many devices as

>> +	 * possible, we will maintain a 240 sector transfer size limit for USB

>> +	 * Mass Storage devices.

>> +	 *

>> +	 * Tests show that other operating have similar limits with Microsoft

>

>     Operating systems?


I'll fix this one.

>> +	 * Windows™ 7 limitting transfers to 128 sectors for both USB2 and USB3

>> +	 * and Apple Mac OS X™ 10.11 limitting transfers to 256 sectors for USB2

>

>     Limiting as already noticed.


had fixed this one before, thanks.

-- 
balbi

Patch

diff --git a/drivers/usb/storage/scsiglue.c b/drivers/usb/storage/scsiglue.c
index dba51362d2e2..00aadf3a3857 100644
--- a/drivers/usb/storage/scsiglue.c
+++ b/drivers/usb/storage/scsiglue.c
@@ -565,7 +565,23 @@  static const struct scsi_host_template usb_stor_host_template = {
 	/* lots of sg segments can be handled */
 	.sg_tablesize =			SCSI_MAX_SG_CHAIN_SEGMENTS,
 
-	/* limit the total size of a transfer to 120 KB */
+	/*
+	 * Limit the total size of a transfer to 120 KB.
+	 *
+	 * Some devices are known to choke with anything larger. It seems like
+	 * the problem stems from the fact that original IDE controllers had
+	 * only an 8-bit register to hold the number of sectors in one transfer
+	 * and even those couldn't handle a full 256 sectors.
+	 *
+	 * Because we want to make sure we interoperate with as many devices as
+	 * possible, we will maintain a 240 sector transfer size limit for USB
+	 * Mass Storage devices.
+	 *
+	 * Tests show that other operating have similar limits with Microsoft
+	 * Windows™ 7 limitting transfers to 128 sectors for both USB2 and USB3
+	 * and Apple Mac OS X™ 10.11 limitting transfers to 256 sectors for USB2
+	 * and 2048 for USB3 devices.
+	 */
 	.max_sectors =                  240,
 
 	/* merge commands... this seems to help performance, but