Re: [PATCH v3] usb: xhci: Set avg_trb_len = 8 for EP0 during Address Device Command

Michał Pecio
2025-05-18T22:57:22Z

On Fri, 16 May 2025 11:39:05 +0800, Jay Chen wrote:
> According to the xHCI 1.2 spec (Section 6.2.3, p.454), the Average
> TRB Length (avg_trb_len) for control endpoints should be set to 8.
>
> Currently, during the Address Device Command,

Not only during this command, but forever.

> EP0's avg_trb_len remains 0, which may cause some xHCI hardware to
> reject the Input Context, resulting in device enumeration failures.
> In extreme cases, using a zero avg_trb_len in calculations may lead
> to division-by-zero errors and unexpected system crashes.

Only the xHC internal firmware could crash or misbeahve from that.

> This patch sets avg_trb_len to 8 for EP0 in
> xhci_setup_addressable_virt_dev(), ensuring compliance with the spec
> and improving compatibility across various host controller
> implementations.

Any particular problems which motivated this effort?

I tried an equivalent change (with = instead of |=) on several HCs
and found no difference, including no elimination of known HW bugs.

> v3:
> - Corrected author name in commit metadata and added changelog.
>
> v2:
> - Fixed malformed patch formatting issue pointed out by maintainer.
>
> Link: https://bugzilla.kernel.org/show_bug.cgi?id=220033
> Signed-off-by: Jay Chen <shawn2000100@gmail.com>
> ---
> drivers/usb/host/xhci-mem.c | 2 ++
> 1 file changed, 2 insertions(+)
>
> diff --git a/drivers/usb/host/xhci-mem.c b/drivers/usb/host/xhci-mem.c
> index d698095fc88d..fed9e9d1990c 100644
> --- a/drivers/usb/host/xhci-mem.c
> +++ b/drivers/usb/host/xhci-mem.c
> @@ -1166,6 +1166,8 @@ int xhci_setup_addressable_virt_dev(struct xhci_hcd *xhci, struct usb_device *ud
> ep0_ctx->deq = cpu_to_le64(dev->eps[0].ring->first_seg->dma |
> dev->eps[0].ring->cycle_state);
>
> + ep0_ctx->tx_info |= cpu_to_le32(EP_AVG_TRB_LENGTH(8));

The rest of ep0 tx_info is zero, so this could be = instead of |=.

> +
> trace_xhci_setup_addressable_virt_device(dev);
>
> /* Steps 7 and 8 were done in xhci_alloc_virt_device() */
> --
> 2.43.5
>