Re: [PATCH v2 2/2] usb: dwc2: wa for isoc packet tx fifo flush at periodic frame boundary

[Thinh Nguyen <Thinh.Nguyen@synopsys.com>]

On Wed, Nov 04, 2026, Fabrice Gasnier wrote:
> If an isochronous packet is still being transmitted, when the end of
> periodic frame interval elapses, using dma desc on dwc2 controller,
> the device wrongly flushes the pending packets from the tx fifo.
> This can lead to a huge packet is sent on the bus, leading to the host
> finally reset the bus, and re-enumerate it.
> 
> Workaround it by tuning the periodic frame interval limit from 80% to 95%,
> on the affected cores (2.70a to 4.00a). It makes the issue very unlikely
> to occur.
> 
> Fixes: 540ccba08599 ("usb: dwc2: gadget: Start and complete DDMA isoc transfers")
> Cc: stable@vger.kernel.org
> Signed-off-by: Fabrice Gasnier <fabrice.gasnier@foss.st.com>
> ---
> Changes in v2:
> - Thinh review comments:
> - Fix a typo 'flushes'
> - Use DCFG_PERFRINT() macro
> - Add DWC2_HS_IOT_REV_1_00a and DWC2_FS_IOT_REV_1_00a core revisions.
> - Add a Fixes tag, and Cc stable
> ---
>  drivers/usb/dwc2/core.h   |  1 +
>  drivers/usb/dwc2/gadget.c | 15 +++++++++++++++
>  2 files changed, 16 insertions(+)
> 
> diff --git a/drivers/usb/dwc2/core.h b/drivers/usb/dwc2/core.h
> index 015a5fb7842d..54c040da0cae 100644
> --- a/drivers/usb/dwc2/core.h
> +++ b/drivers/usb/dwc2/core.h
> @@ -1108,6 +1108,7 @@ struct dwc2_hsotg {
>  
>  	/* DWC OTG HW Release versions */
>  #define DWC2_CORE_REV_4_30a	0x4f54430a
> +#define DWC2_CORE_REV_2_70a	0x4f54270a
>  #define DWC2_CORE_REV_2_71a	0x4f54271a
>  #define DWC2_CORE_REV_2_72a     0x4f54272a
>  #define DWC2_CORE_REV_2_80a	0x4f54280a
> diff --git a/drivers/usb/dwc2/gadget.c b/drivers/usb/dwc2/gadget.c
> index bdabf3c39da1..b84cb6b2cefa 100644
> --- a/drivers/usb/dwc2/gadget.c
> +++ b/drivers/usb/dwc2/gadget.c
> @@ -3511,6 +3511,21 @@ void dwc2_hsotg_core_init_disconnected(struct dwc2_hsotg *hsotg,
>  	if (hsotg->params.ipg_isoc_en)
>  		dcfg |= DCFG_IPG_ISOC_SUPPORDED;
>  
> +	/*
> +	 * WA for Device Flushes Ongoing ISOC IN Packet at End-of-Periodic-Frame Boundary.
> +	 * Once passed the end of periodic frame, and the controller is transmitting an
> +	 * ISOC IN packet, the device flushes the transmit FIFO. Tune periodic frame
> +	 * interval to 95% instead of 80% to avoid it.
> +	 */
> +	if (using_desc_dma(hsotg) &&
> +	    ((hsotg->hw_params.snpsid >= DWC2_CORE_REV_2_70a &&
> +	      hsotg->hw_params.snpsid <= DWC2_CORE_REV_4_00a) ||
> +	     hsotg->hw_params.snpsid == DWC2_FS_IOT_REV_1_00a ||
> +	     hsotg->hw_params.snpsid == DWC2_HS_IOT_REV_1_00a)) {
> +		dcfg &= ~DCFG_PERFRINT_MASK;
> +		dcfg |= DCFG_PERFRINT(DCFG_PERFRINT_LIMIT);
> +	}
> +
>  	dwc2_writel(hsotg, dcfg,  DCFG);
>  
>  	dwc2_hsotg_init_fifo(hsotg);
> 
> -- 
> 2.43.0
> 

Acked-by: Thinh Nguyen <Thinh.Nguyen@synopsys.com>

Thanks,
Thinh