linux-kernel - Re: [PATCH v2] media: uvcvideo: Do not alloc dev->status

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Date:   Thu, 15 Dec 2022 08:59:14 +0100
From:   Ricardo Ribalda <ribalda@...omium.org>
To:     Sergey Senozhatsky <senozhatsky@...omium.org>
Cc:     Yunke Cao <yunkec@...omium.org>, Ming Lei <tom.leiming@...il.com>,
        Jonathan Cameron <Jonathan.Cameron@...wei.com>,
        Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Max Staudt <mstaudt@...gle.com>, linux-kernel@...r.kernel.org,
        linux-media@...r.kernel.org
Subject: Re: [PATCH v2] media: uvcvideo: Do not alloc dev->status

Hi Sergey

Thanks for looking into this

On Thu, 15 Dec 2022 at 02:15, Sergey Senozhatsky
<senozhatsky@...omium.org> wrote:
>
> On (22/12/14 14:37), Ricardo Ribalda wrote:
> [..]
> > +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 input_dev *input;
> >       char input_phys[64];
> >
> > @@ -572,6 +592,12 @@ struct uvc_device {
> >       } async_ctrl;
> >
> >       struct uvc_entity *gpio_unit;
> > +
> > +     /*
> > +      * Ensure that status is aligned, making it safe to use with
> > +      * non-coherent DMA.
> > +      */
> > +     struct uvc_status status __aligned(ARCH_KMALLOC_MINALIGN);
>
>         ____cacheline_aligned ?
>
> I don't see anyone using ARCH_KMALLOC_MINALIGN except for slab.h

Seems like cacheline is not good enough:

https://github.com/torvalds/linux/commit/12c4efe3509b8018e76ea3ebda8227cb53bf5887
https://lore.kernel.org/all/20220405135758.774016-1-catalin.marinas@arm.com/

and ARCH_KMALLOC_MINALIGN is what we have today and is working...

But yeah, the name for that define is not the nicest :)

I added Jonathan Cameron, on cc, as he had to deal with something
similar for iio in case we are missing something


ps: and I thought this was an easy change :P

-- 
Ricardo Ribalda