linux-kernel - Re: [PATCH v8 05/14] media: rkisp1: add Rockchip ISP1 subdev driver

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Message-ID: <20190815104515.GO6133@paasikivi.fi.intel.com>
Date:   Thu, 15 Aug 2019 13:45:15 +0300
From:   Sakari Ailus <sakari.ailus@...ux.intel.com>
To:     Tomasz Figa <tfiga@...omium.org>
Cc:     Helen Koike <helen.koike@...labora.com>,
        "open list:ARM/Rockchip SoC..." <linux-rockchip@...ts.infradead.org>,
        devicetree@...r.kernel.org, Eddie Cai <eddie.cai.linux@...il.com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Heiko Stübner <heiko@...ech.de>,
        Chen Jacob <jacob2.chen@...k-chips.com>,
        Jeffy <jeffy.chen@...k-chips.com>,
        钟以崇 <zyc@...k-chips.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Hans Verkuil <hans.verkuil@...co.com>,
        Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        kernel@...labora.com, Ezequiel Garcia <ezequiel@...labora.com>,
        Linux Media Mailing List <linux-media@...r.kernel.org>,
        "list@....net:IOMMU DRIVERS <iommu@...ts.linux-foundation.org>, Joerg
        Roedel <joro@...tes.org>," <linux-arm-kernel@...ts.infradead.org>,
        Shunqian Zheng <zhengsq@...k-chips.com>,
        Jacob Chen <cc@...k-chips.com>,
        Allon Huang <allon.huang@...k-chips.com>
Subject: Re: [PATCH v8 05/14] media: rkisp1: add Rockchip ISP1 subdev driver

On Thu, Aug 15, 2019 at 07:29:59PM +0900, Tomasz Figa wrote:
> On Thu, Aug 15, 2019 at 5:25 PM Sakari Ailus
> <sakari.ailus@...ux.intel.com> wrote:
> >
> > Hi Helen,
> >
> > On Wed, Aug 14, 2019 at 09:58:05PM -0300, Helen Koike wrote:
> >
> > ...
> >
> > > >> +static int rkisp1_isp_sd_set_fmt(struct v4l2_subdev *sd,
> > > >> +                           struct v4l2_subdev_pad_config *cfg,
> > > >> +                           struct v4l2_subdev_format *fmt)
> > > >> +{
> > > >> +  struct rkisp1_device *isp_dev = sd_to_isp_dev(sd);
> > > >> +  struct rkisp1_isp_subdev *isp_sd = &isp_dev->isp_sdev;
> > > >> +  struct v4l2_mbus_framefmt *mf = &fmt->format;
> > > >> +
> > > >
> > > > Note that for sub-device nodes, the driver is itself responsible for
> > > > serialising the access to its data structures.
> > >
> > > But looking at subdev_do_ioctl_lock(), it seems that it serializes the
> > > ioctl calls for subdevs, no? Or I'm misunderstanding something (which is
> > > most probably) ?
> >
> > Good question. I had missed this change --- subdev_do_ioctl_lock() is
> > relatively new. But setting that lock is still not possible as the struct
> > is allocated in the framework and the device is registered before the
> > driver gets hold of it. It's a good idea to provide the same serialisation
> > for subdevs as well.
> >
> > I'll get back to this later.
> >
> > ...
> >
> > > >> +static int rkisp1_isp_sd_s_power(struct v4l2_subdev *sd, int on)
> > > >
> > > > If you support runtime PM, you shouldn't implement the s_power op.
> > >
> > > Is is ok to completly remove the usage of runtime PM then?
> > > Like this http://ix.io/1RJb ?
> >
> > Please use runtime PM instead. In the long run we should get rid of the
> > s_power op. Drivers themselves know better when the hardware they control
> > should be powered on or off.
> >
> 
> One also needs to use runtime PM to handle power domains and power
> dependencies on auxiliary devices, e.g. IOMMU.
> 
> > >
> > > tbh I'm not that familar with runtime PM and I'm not sure what is the
> > > difference of it and using s_power op (and Documentation/power/runtime_pm.rst
> > > is not being that helpful tbh).
> >
> > You can find a simple example e.g. in
> > drivers/media/platform/atmel/atmel-isi.c .
> >
> > >
> > > >
> > > > You'll still need to call s_power on external subdevs though.
> > > >
> > > >> +{
> > > >> +  struct rkisp1_device *isp_dev = sd_to_isp_dev(sd);
> > > >> +  int ret;
> > > >> +
> > > >> +  v4l2_dbg(1, rkisp1_debug, &isp_dev->v4l2_dev, "s_power: %d\n", on);
> > > >> +
> > > >> +  if (on) {
> > > >> +          ret = pm_runtime_get_sync(isp_dev->dev);
> > >
> > > If this is not ok to remove suport for runtime PM, then where should I put
> > > the call to pm_runtime_get_sync() if not in this s_power op ?
> >
> > Basically the runtime_resume and runtime_suspend callbacks are where the
> > device power state changes are implemented, and pm_runtime_get_sync and
> > pm_runtime_put are how the driver controls the power state.
> >
> > So you no longer need the s_power() op at all. The op needs to be called on
> > the pipeline however, as there are drivers that still use it.
> >
> 
> For this driver, I suppose we would _get_sync() when we start
> streaming (in the hardware, i.e. we want the ISP to start capturing
> frames) and _put() when we stop and the driver shouldn't perform any
> access to the hardware when the streaming is not active.

Agreed.

-- 
Sakari Ailus
sakari.ailus@...ux.intel.com