On 14-12-30 10:34 AM, Alan Stern wrote:
On Tue, 30 Dec 2014, Christoph Hellwig wrote:
The only limits usb-storage imposes on max_sectors are those needed to
work around bugs in the devices' USB bridges. (Okay, there's also
something for tape drive devices, but it probably doesn't belong in
usb-storage -- it should be handled by the SCSI tape driver.)
If the ATA layer needs to set a limit on max_sectors, why doesn't it
simply go ahead and do so?
Because the ATA layer doesn't control the device, the bridge does.
And it seems like it doesn't communicate the maximum transfer size
properly.
_Is_ there any way to communicate the maximum transfer size? I'm not
aware of any SCSI command for it. It isn't part of the USB
mass-storage spec.
In SCSI, the VPD page 0xb0 (Block limits) has a "Maximum transfer
length" field (32 bits long). Its units are logical blocks. Useful
if >0 as 0 means "unreported".
USB to SATA bridges should comply with SAT. However SAT and SAT-2
don't even mention that VPD page. SAT-3 and SAT-4 mention that page
but have "unspecified" next to that field. Basically useless.
IMO about the best SATL is in the MPT SAS-2 and SAS-3 HBAs and
here is that page for a SAS expander attached SATA disk:
# sg_vpd -p bl /dev/sg3
Block limits VPD page (SBC):
Write same non-zero (WSNZ): 0
Maximum compare and write length: 0 blocks
Optimal transfer length granularity: 0 blocks
Maximum transfer length: 0 blocks
Optimal transfer length: 0 blocks
Maximum prefetch length: 0 blocks
Maximum unmap LBA count: 0
Maximum unmap block descriptor count: 0
Optimal unmap granularity: 0
Unmap granularity alignment valid: 0
Unmap granularity alignment: 0
Maximum write same length: 0x0 blocks
Maximum atomic transfer length: 0
Atomic alignment: 0
Atomic transfer length granularity: 0
Maximum atomic transfer length with atomic boundary: 0
Maximum atomic boundary size: 0
Not sure why LSI/Avago even bothered even implementing
that page ...
Doug Gilbert
BTW here is the output of that page from a SAS SSD:
# sg_vpd -p bl /dev/sg5
Block limits VPD page (SBC):
Write same non-zero (WSNZ): 0
Maximum compare and write length: 0 blocks
Optimal transfer length granularity: 8 blocks
Maximum transfer length: 65535 blocks
Optimal transfer length: 0 blocks
Maximum prefetch length: 0 blocks
Maximum unmap LBA count: 393216
Maximum unmap block descriptor count: 512
Optimal unmap granularity: 8
Unmap granularity alignment valid: 0
Unmap granularity alignment: 0
Maximum write same length: 0x0 blocks
Maximum atomic transfer length: 0
Atomic alignment: 0
Atomic transfer length granularity: 0
Maximum atomic transfer length with atomic boundary: 0
Maximum atomic boundary size: 0
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