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

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Message-ID: <Y5rkzAESkLt/u6kP@pendragon.ideasonboard.com>
Date:   Thu, 15 Dec 2022 11:11:40 +0200
From:   Laurent Pinchart <laurent.pinchart@...asonboard.com>
To:     Ricardo Ribalda <ribalda@...omium.org>
Cc:     Sergey Senozhatsky <senozhatsky@...omium.org>,
        Yunke Cao <yunkec@...omium.org>,
        Ming Lei <tom.leiming@...il.com>,
        Jonathan Cameron <Jonathan.Cameron@...wei.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 Ricardo,

On Thu, Dec 15, 2022 at 11:08:05AM +0200, Laurent Pinchart wrote:
> On Thu, Dec 15, 2022 at 08:59:14AM +0100, Ricardo Ribalda wrote:
> > On Thu, 15 Dec 2022 at 02:15, Sergey Senozhatsky 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
> 
> I'd like to get feedback on this from DMA and USB experts. Expanding the
> CC list of the original patch would help (especially including the
> linux-usb mailing list).

Also, do we need the allocation change ? It doesn't seem to simplify the
code that much, neither in terms of lines of code

>  2 files changed, 48 insertions(+), 49 deletions(-)

nor in terms of complexity. Maybe we could keep the union and offsetof
changes, and drop the allocation change ? In any case, those are two
different changes, so I'd split them in two patches at least.

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

-- 
Regards,

Laurent Pinchart