Re: How to resolve an issue in swiotlb environment?

Alan Stern
2019-06-11T14:51:17Z

On Tue, 11 Jun 2019, Christoph Hellwig wrote:

> Hi Alan,
>
> thanks for the explanation. It seems like what usb wants is to:
>
> - set sg_tablesize to 1 for devices that can't handle scatterlist at all

Hmmm. usb-storage (and possible other drivers too) currently handles
such controllers by setting up an SG transfer as a series of separate
URBs, one for each scatterlist entry. But this is not the same thing,
for two reasons:

It has less I/O overhead than setting sg_tablesize to 1 because
it sets up the whole transfer as a single SCSI command, which
requires much less time and traffic on the USB bus than sending
multiple commands.

It has that requirement about each scatterlist element except
the last being a multiple of the maximum packet size in length.
(This is because the USB protocol says that a transfer ends
whenever a less-than-maximum-size packet is encountered.)

We would like to avoid the extra I/O overhead for host controllers that
can't handle SG. In fact, switching to sg_tablesize = 1 would probably
be considered a regression.

> - set the virt boundary as-is for devices supporting "basic" scatterlist,
> although that still assumes they can rejiggle them because for example
> you could still get a smaller than expected first segment ala (assuming
> a 1024 byte packet size and thus 1023 virt_boundary_mask):
>
> | 0 .. 511 | 512 .. 1023 | 1024 .. 1535 |
>
> as the virt_bondary does not guarantee that the first segment is
> the same size as all the mid segments.

But that is exactly the problem we need to solve.

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.

> - do not set any limit on xhci
>
> But that just goes back to the original problem, and that is that with
> swiotlb we are limited in the total dma mapping size, and recent block
> layer changes in the way we handle the virt_boundary mean we now build
> much larger requests by default. For SCSI ULDs to take that into
> account I need to call dma_max_mapping_size() and use that as the
> upper bound for the request size. My plan is to do that in scsi_lib.c,
> but for that we need to expose the actual struct device that the dma
> mapping is perfomed on to the scsi layer. If that device is different
> from the sysfs hierchary struct device, which it is for usb the ULDD
> needs to scsi_add_host_with_dma and pass the dma device as well. How
> do I get at the dma device (aka the HCDs pci_dev or similar) from
> usb-storage/uas?

From usb_stor_probe2(): us->pusb_dev->bus->sysdev.
From uas_probe(): udev->bus->sysdev.

The ->sysdev field points to the device used for DMA mapping. It is
often the same as ->controller, but sometimes it is
->controller->parent because of the peculiarities of some platforms.

Alan Stern