Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Date:   Thu, 15 Dec 2022 10:10:25 +0900
From:   Sergey Senozhatsky <senozhatsky@chromium.org>
To:     Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc:     Ricardo Ribalda <ribalda@chromium.org>,
        Yunke Cao <yunkec@chromium.org>,
        Max Staudt <mstaudt@google.com>,
        Mauro Carvalho Chehab <mchehab@kernel.org>,
        Sergey Senozhatsky <senozhatsky@chromium.org>,
        linux-kernel@vger.kernel.org, linux-media@vger.kernel.org
Subject: Re: [PATCH] media: uvcvideo: Do not alloc dev->status
Message-ID: <Y5p0AfcKaHWxEjGV@google.com>
References: <20221214-uvc-status-alloc-v1-0-a0098ddc7c93@chromium.org>
 <Y5nFap/r4OgqgmeK@pendragon.ideasonboard.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <Y5nFap/r4OgqgmeK@pendragon.ideasonboard.com>
Precedence: bulk

On (22/12/14 14:45), Laurent Pinchart wrote:
> On Wed, Dec 14, 2022 at 12:23:41PM +0100, Ricardo Ribalda wrote:
> > UVC_MAX_STATUS_SIZE is 16, simplify the code by inlining dev->status.
> 
> The reason why uvc_status is allocated dynamically is to ensure cache
> line alignment, as the USB host controller may DMA to the buffer.

Oh, that's a very subtle detail.

> > +struct uvc_status_streaming {
> > +	u8	button;
> > +} __packed;
> > +
> > +struct uvc_status_control {
> > +	u8	bSelector;
> > +	u8	bAttribute;
> > +	u8	bValue[11];
> > +} __packed;
> > +
> > +struct uvc_status {
> > +	u8	bStatusType;
> > +	u8	bOriginator;
> > +	u8	bEvent;
> > +	union {
> > +		struct uvc_status_control control;
> > +		struct uvc_status_streaming streaming;
> > +	};
> > +} __packed;
> > +
> >  struct uvc_device {
> >  	struct usb_device *udev;
> >  	struct usb_interface *intf;
> > @@ -559,7 +579,7 @@ struct uvc_device {
> >  	/* Status Interrupt Endpoint */
> >  	struct usb_host_endpoint *int_ep;
> >  	struct urb *int_urb;
> > -	u8 *status;
> > +	struct uvc_status status;

So we can do something like this maybe?

+	struct uvc_status status ____cacheline_aligned;

And this actually even improves code readability, IMHO, in a
sense that we explicitly say that it has to be cache aligned.