Re: [PATCH 4/4] usb: gadget: dummy_hcd: set no_sg_constraint on the host controller

[Alan Stern]

On Fri, Aug 07, 2026 at 09:54:38AM +0200, Igor Skalkin wrote:
> dummy_hcd copies data in software and has no hardware DMA alignment
> requirement.  Set no_sg_constraint = 1 so the USB core does not reject
> SG URBs whose non-final segments are not a multiple of the endpoint
> maxpacket size.

It is true that dummy_hcd's software implementation has no inherent DMA 
alignment requirement.  But dummy_hcd is meant as a testing tool; it 
emulates real controllers some of which _do_ have alignment 
requirements.

> For SuperSpeed bulk endpoints maxpacket is 1024 bytes; usbtest generates
> SG lists with varying segment sizes (e.g. 512 bytes) that are valid
> transfers but not maxpacket-aligned, causing usb_submit_urb to return
> -EINVAL before the URB reaches the host controller.

This is the appropriate response when emulating, for example, an EHCI 
controller.

A better solution to the problem would be to set no_sg_constraint at 
runtime, according to whether the connection speed is SuperSpeed or 
faster.

Alan Stern

> Assisted-by: OpenCode:claude-sonnet-5
> Signed-off-by: Igor Skalkin <igor.skalkin@xxxxxxxxxxxxxxxx>
> ---
>  drivers/usb/gadget/udc/dummy_hcd.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/drivers/usb/gadget/udc/dummy_hcd.c b/drivers/usb/gadget/udc/dummy_hcd.c
> index 7597c9e9fa66..2cfb1d035b34 100644
> --- a/drivers/usb/gadget/udc/dummy_hcd.c
> +++ b/drivers/usb/gadget/udc/dummy_hcd.c
> @@ -2550,6 +2550,7 @@ static int dummy_setup(struct usb_hcd *hcd)
>  
>  	dum = *((void **)dev_get_platdata(hcd->self.controller));
>  	hcd->self.sg_tablesize = ~0;
> +	hcd->self.no_sg_constraint = 1;
>  	if (usb_hcd_is_primary_hcd(hcd)) {
>  		dum->hs_hcd = hcd_to_dummy_hcd(hcd);
>  		dum->hs_hcd->dum = dum;
> -- 
> 2.49.0
>