Re: [PATCH] usb: dwc3: gadget: Prevent core from processing stale TRBs

[Pavan Kondeti]

On Mon, Feb 07, 2022 at 09:55:58AM +0530, Udipto Goswami wrote:
> With CPU re-ordering on write instructions, there might
> be a chance that the HWO is set before the TRB is updated
> with the new mapped buffer address.
> And in the case where core is processing a list of TRBs
> it is possible that it fetched the TRBs when the HWO is set
> but before the buffer address is updated.
> Prevent this by adding a memory barrier before the HWO
> is updated to ensure that the core always process the
> updated TRBs.
> 
> Fixes: f6bafc6a1c9 ("usb: dwc3: convert TRBs into bitshifts")
> Signed-off-by: Udipto Goswami <quic_ugoswami@xxxxxxxxxxx>
> ---
> v1: For an ep the trbs can be reused, and if cpu re-ordering also
> takes place, there is a change that the HWO will get set even before
> the trb bpl/bph are updated which will lead controller to access a
> stale buffer address from the previous transactions.
> 
>  drivers/usb/dwc3/gadget.c | 13 +++++++++++++
>  1 file changed, 13 insertions(+)
> 
> diff --git a/drivers/usb/dwc3/gadget.c b/drivers/usb/dwc3/gadget.c
> index 520031b..183b909 100644
> --- a/drivers/usb/dwc3/gadget.c
> +++ b/drivers/usb/dwc3/gadget.c
> @@ -1291,6 +1291,19 @@ static void __dwc3_prepare_one_trb(struct dwc3_ep *dep, struct dwc3_trb *trb,
>  	if (usb_endpoint_xfer_bulk(dep->endpoint.desc) && dep->stream_capable)
>  		trb->ctrl |= DWC3_TRB_CTRL_SID_SOFN(stream_id);
>  
> +	/*
> +	 * As per data book 4.2.3.2TRB Control Bit Rules section
> +	 *
> +	 * The controller autonomously checks the HWO field of a TRB to determine if the
> +	 * entire TRB is valid. Therefore, software must ensure that the rest of the TRB
> +	 * is valid before setting the HWO field to '1'. In most systems, this means that
> +	 * software must update the fourth DWORD of a TRB last.
> +	 *
> +	 * However there is a possibility of CPU re-ordering here which can cause
> +	 * controller to observe the HWO bit set prematurely.
> +	 * Add a write memory barrier to prevent CPU re-ordering.
> +	 */
> +	wmb();
>  	trb->ctrl |= DWC3_TRB_CTRL_HWO;
>  

Looks good to me. FWIW,

Reviewed-by: Pavankumar Kondeti <quic_pkondeti@xxxxxxxxxxx>

It is very similar to what we have in xHC during TRB preparation.

commit 576667bad341516edc4e18eb85acb0a2b4c9c9d9
Author: Mathias Nyman <mathias.nyman@xxxxxxxxxxxxxxx>
Date:   Fri Jan 15 18:19:06 2021 +0200

xhci: make sure TRB is fully written before giving it to the controller

Thanks,
Pavan