Re: Maximum data size in a single transfer for MS driver

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On Wed, 24 Feb 2010, Ramya Desai wrote:

> I did exactly. I removed the following statement from
> store_max_sectors (drivers/usb/storage/scsiglue.c) function.
> " && ms <= SCSI_DEFAULT_MAX_SECTORS". I also changed the .max_sectors
> = 2048 in the host template.
> 
> Then I restarted the system and build the usb-storage without any issue.
> 
> After inserting my device, I found 1024 in
> /sys/block/sdb/device/max_sectors. So, I tried to increase the value
> through echo 2048 > /sys/block/sdb/device/max_sectors. When I do this,
> it invoked the store_max_sectors() in the scsiglue.c file and able to
> see ms (the variable in the store_max_sectors()) is 2048. But, the
> value in the /sys/block/sdb/device/max_sectors is still 1024 only.

That's right.  Writing 2048 to that file sets max_hw_sectors to 2048 
while leaving max_sectors set at 1024.  The value displayed in the file 
is max_sectors -- it really ought to be max_hw_sectors.  You can fix 
this by editing the show_max_sectors() function in scsiglue.c: change 
"queue_max_sectors" to "queue_max_hw_sectors".

> When I looked into the /sys/block/sdb/queue/max_hw_sectors_kb, it
> shows 1024, which is 2048 sectors.

As it should be.

>  Then, I tried to increase the value
> in /sys/block/sdb/queue/max_sectors_kb through echo 1024 >
> /sys/block/sdb/queue/max_sectors_kb. After this, I saw 1024 in the
> /sys/block/sdb/queue/max_sectors_kb. When I see 1024 in
> /sys/block/sdb/queue/max_sectors_kb then I found 2048 in
> /sys/block/sdb/device/max_sectors.

Right, because that file displays max_sectors instead of 
max_hw_sectors.

>  However, this time, still my device
> received 512 KB (1024) data transfer buffers in scsi_command.

At this point everything was set up correctly.  I don't know why the 
transfer lengths are still 512 KB.  Maybe someone else has an idea.

Alan Stern

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