Hi Alan,
On 6/18/19 9:28 AM, shuah wrote:
On 6/14/19 8:44 AM, Alan Stern wrote:
On Thu, 13 Jun 2019, shuah wrote:
Great! So all we have to do is fix vhci-hcd. Then we can remove all
the virt_boundary_mask stuff from usb-storage and uas entirely.
(I'm assuming wireless USB isn't a genuine issue. As far as I know, it
is pretty much abandoned at this point.)
Valentina and Shua: Adding SG support to vhci-hcd shouldn't be too
hard. It ought to be possible even without changing the network
protocol.
I will start taking a look at this. Is there a target release in plan
to drop virt_boundary_mask stuff?
Not yet. But since it doesn't do what we want anyway, this should be
fixed quickly.
I missed a lot of the thread info. and went looking for it and found the
following summary of the problem:
==================
The issue which prompted the commit this thread is about arose in a
situation where the block layer set up a scatterlist containing buffer
sizes something like:
4096 4096 1536 1024
and the maximum packet size was 1024. The situation was a little
unusual, because it involved vhci-hcd (a virtual HCD). This doesn't
matter much in normal practice because:
Block devices normally have a block size of 512 bytes or more.
Smaller values are very uncommon. So scatterlist element sizes
are always divisible by 512.
xHCI is the only USB host controller type with a maximum packet
size larger than 512, and xHCI hardware can do full
scatter-gather so it doesn't care what the buffer sizes are.
So another approach would be to fix vhci-hcd and then trust that the
problem won't arise again, for the reasons above. We would be okay so
long as nobody tried to use a USB-SCSI device with a block size of 256
bytes or less.
===================
Out of the summary, the following gives me pause:
"xHCI hardware can do full scatter-gather so it doesn't care what the
buffer sizes are."
vhci-hcd won't be able to count on hardware being able to do full
scatter-gather. It has to deal with a variety of hardware with
varying speeds.
"We would be okay so long as nobody tried to use a USB-SCSI device with
a block size of 256 bytes or less."
At least a USB Storage device, I test with says 512 block size. Can we
count on not seeing a device with block size <= 256 bytes?
In any case, I am looking into adding SG support vhci-hci at the moment.
Looks like the following is the repo, I should be working with?
git://git.infradead.org/users/hch/misc.git
thanks,
-- Shuah
