linux-kernel - Re: [PATCH 2/2] USB: xhci: use urb hcpriv mempool for private data

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Message-ID: <875848ec-f92b-48a9-b5c2-792bb0121824@suse.com>
Date: Tue, 13 May 2025 10:21:06 +0200
From: Oliver Neukum <oneukum@...e.com>
To: David Wang <00107082@....com>, mathias.nyman@...el.com,
 gregkh@...uxfoundation.org
Cc: stern@...land.harvard.edu, linux-usb@...r.kernel.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] USB: xhci: use urb hcpriv mempool for private data

Hi,

On 13.05.25 07:55, David Wang wrote:
> xhci keeps alloc/free private data for each enqueue/dequeue cycles,
> when using a USB webcam, the memory allocation frequency could reach
> about 1k/s.
> 
> URB objects have longer lifecycle than private data, hand over ownership
> of private data to urb can save lots of memory allocations over time.

I am afraid I need to make a comment about a principal issue.

This patch set overlooks a fundamental issue. You cannot guarantee
that an URB is reused by the same HC. For example you cannot rule
out that the next time your URB is resubmitted, it will land with
XHCI again.

That means you cannot touch just how one HCD handles private data.
Or you need to record which HC the URB was last used for.

	Regards
		Oliver