linux-kernel - Re: [PATCH] usb: hubs: Decrease IN-endpoint poll interval for Microchip USB491x hub

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Message-ID: <66137982-c6a4-48c8-860f-c87cc6e01101@rowland.harvard.edu>
Date:   Fri, 24 Nov 2023 09:57:14 -0500
From:   Alan Stern <stern@...land.harvard.edu>
To:     Hardik Gajjar <hgajjar@...adit-jv.com>
Cc:     gregkh@...uxfoundation.org, corbet@....net, tj@...nel.org,
        rdunlap@...radead.org, paulmck@...nel.org,
        linux-doc@...r.kernel.org, linux-usb@...r.kernel.org,
        linux-kernel@...r.kernel.org, erosca@...adit-jv.com,
        Martin.Mueller5@...bosch.com
Subject: Re: [PATCH] usb: hubs: Decrease IN-endpoint poll interval for
 Microchip USB491x hub

On Fri, Nov 24, 2023 at 03:50:05PM +0100, Hardik Gajjar wrote:
> On Thu, Nov 23, 2023 at 01:17:03PM -0500, Alan Stern wrote:
> > On Thu, Nov 23, 2023 at 09:19:48AM +0100, Hardik Gajjar wrote:
> > > There is a potential delay in announcing downstream USB bus activity to
> > > Linux USB drivers due to the default interrupt endpoint having a poll
> > > interval of 256ms.
> > > 
> > > Microchip has recommended ignoring the device descriptor and reducing
> > > that value to 32ms, as it was too late to modify it in silicon.
> > > 
> > > This patch aims to speed up the USB enumeration process, facilitating
> > > the successful completion of Apple CarPlay certifications and enhancing
> > > user experience when utilizing USB devices through the Microchip Multihost
> > > Hub.
> > > 
> > > A new quirk, USB_QUIRK_REDUCE_FRAME_INTR_BINTERVAL, accelerates the
> > > notification process by changing the Endpoint interrupt poll interval
> > > from 256ms to 32ms.
> > 
> > But this is meant to apply only to hubs, right?  So shouldn't it be a 
> > HUB_QUIRK_32_MS_INTR_INTERVAL macro, used in hub.c's hub_id_table, 
> > rather than a general USB quirk?
> 
> Thank you, Alan, for the feedback. To confirm my understanding, are you suggesting
> moving all implementations to hub.c, adding the hub-specific quirk, and using the
> same quirk to update the bInterval value parsed by usb_get_configuration() in
> usb_enumerate_device()?"

Basically, yes.  The update should be performed in the hub_probe() 
routine if the quirk flag is set, before hub_configure() is called.  It 
might be necessary to add a usb_set_interface() call as well, in case 
the old bInterval value has already been cached by the host controller 
driver.

Alan Stern