Re: [PATCH v4] usb: core: hub: Re-guess ep0 maxpacket size after reset

[sashiko-bot@kernel.org]

> Per USB 2.0 spec section 5.5.3, the ep0 maxpacket size is fixed for
> all device speeds except full-speed devices, where it can be 8, 16,
> 32, or 64 bytes and can only be determined by reading the device
> descriptor.  During initial enumeration the USB core guesses 64 bytes
> for full-speed devices; after reading the descriptor, the real value
> is stored in udev->ep0.desc.wMaxPacketSize and applied to hardware by
> usb_ep0_reinit().
> 
> Commit ff33299ec8bb ("USB: core: Fix race by not overwriting
> udev->descriptor in hub_port_init()") guarded the ep0 maxpacket guess
> block with "if (initial)", so on the reset path the guess is skipped
> and the host keeps using the value from the previous enumeration.  If
> a buggy full-speed device responds with one 8-byte packet and NAKs
> all further polling, the HC doesn't recognize a short transfer but
> keeps polling for the rest of the descriptor till unlink, and the
> [ ... ]
> data stage.
> 
> Fixes: ff33299ec8bb ("USB: core: Fix race by not overwriting udev->descriptor in hub_port_init()")
> Cc: stable@vger.kernel.org
> Signed-off-by: Pengfei Lv <lvpengfei20@huawei.com>

Sashiko has reviewed this patch and found no issues.

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