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

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Message-ID: <20250519005715.532cf34d@foxbook>
Date: Mon, 19 May 2025 00:57:15 +0200
From: Michał Pecio <michal.pecio@...il.com>
To: Jay Chen <shawn2000100@...il.com>
Cc: mathias.nyman@...el.com, gregkh@...uxfoundation.org,
 linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org,
 jay.chen@...mens.com
Subject: Re: [PATCH v3] usb: xhci: Set avg_trb_len = 8 for EP0 during
 Address Device Command

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@...il.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
>