Re: [PATCH v4 3/4] media: raspberrypi: Add support for RP1-CFE

[Sakari Ailus]

Hi Jacopo,

On Tue, Sep 10, 2024 at 11:19:57AM +0200, Jacopo Mondi wrote:
> Hi Sakari, Tomi
> 
> On Mon, Sep 09, 2024 at 03:52:42PM GMT, Sakari Ailus wrote:
> > Hi Laurent,
> >
> > On Mon, Sep 09, 2024 at 04:45:16PM +0300, Laurent Pinchart wrote:
> > > On Mon, Sep 09, 2024 at 03:29:30PM +0200, Jacopo Mondi wrote:
> > > > On Mon, Sep 09, 2024 at 01:04:35PM GMT, Tomi Valkeinen wrote:
> > > > > On 09/09/2024 12:13, Jacopo Mondi wrote:
> > > > > > On Mon, Sep 09, 2024 at 08:22:59AM GMT, Tomi Valkeinen wrote:
> > > > > > > On 09/09/2024 08:08, Tomi Valkeinen wrote:
> > > > > > > > On 05/09/2024 14:11, Laurent Pinchart wrote:
> > > > > > > > > On Thu, Sep 05, 2024 at 06:50:48PM +0800, kernel test robot wrote:
> > > > > > > > > > Hi Tomi,
> > > > > > > > > >
> > > > > > > > > > kernel test robot noticed the following build warnings:
> > > > > > > > > >
> > > > > > > > > > [auto build test WARNING on 431c1646e1f86b949fa3685efc50b660a364c2b6]
> > > > > > > > > >
> > > > > > > > > > url:    https://github.com/intel-lab-lkp/linux/commits/Tomi-
> > > > > > > > > > Valkeinen/media-uapi-Add-meta-formats-for-PiSP-FE-config-and-
> > > > > > > > > > stats/20240904-192729
> > > > > > > > > > base:   431c1646e1f86b949fa3685efc50b660a364c2b6
> > > > > > > > > > patch link:    https://lore.kernel.org/r/20240904-rp1-cfe-v4-3-
> > > > > > > > > > f1b5b3d69c81%40ideasonboard.com
> > > > > > > > > > patch subject: [PATCH v4 3/4] media: raspberrypi: Add support
> > > > > > > > > > for RP1-CFE
> > > > > > > > > > config: m68k-allmodconfig (https://download.01.org/0day-ci/
> > > > > > > > > > archive/20240905/202409051822.ZzUGw3XQ-lkp@xxxxxxxxx/config)
> > > > > > > > > > compiler: m68k-linux-gcc (GCC) 14.1.0
> > > > > > > > > > reproduce (this is a W=1 build):
> > > > > > > > > > (https://download.01.org/0day-ci/
> > > > > > > > > > archive/20240905/202409051822.ZzUGw3XQ-lkp@xxxxxxxxx/reproduce)
> > > > > > > > > >
> > > > > > > > > > If you fix the issue in a separate patch/commit (i.e. not just a
> > > > > > > > > > new version of
> > > > > > > > > > the same patch/commit), kindly add following tags
> > > > > > > > > > | Reported-by: kernel test robot <lkp@xxxxxxxxx>
> > > > > > > > > > | Closes: https://lore.kernel.org/oe-kbuild-
> > > > > > > > > > all/202409051822.ZzUGw3XQ-lkp@xxxxxxxxx/
> > > > > > > > > >
> > > > > > > > > > All warnings (new ones prefixed by >>):
> > > > > > > > > >
> > > > > > > > > > > > drivers/media/platform/raspberrypi/rp1-cfe/cfe.c:2445:12:
> > > > > > > > > > > > warning: 'cfe_runtime_resume' defined but not used
> > > > > > > > > > > > [-Wunused-function]
> > > > > > > > > >       2445 | static int cfe_runtime_resume(struct device *dev)
> > > > > > > > > >            |            ^~~~~~~~~~~~~~~~~~
> > > > > > > > > > > > drivers/media/platform/raspberrypi/rp1-cfe/cfe.c:2435:12:
> > > > > > > > > > > > warning: 'cfe_runtime_suspend' defined but not used
> > > > > > > > > > > > [-Wunused-function]
> > > > > > > > > >       2435 | static int cfe_runtime_suspend(struct device *dev)
> > > > > > > > > >            |            ^~~~~~~~~~~~~~~~~~~
> > > > > > > > > > vim +/cfe_runtime_resume +2445
> > > > > > > > > > drivers/media/platform/raspberrypi/ rp1-cfe/cfe.c
> > > > > > > > >
> > > > > > > > > The recommended way to fix this is to switch from SET_RUNTIME_PM_OPS()
> > > > > > > > > to RUNTIME_PM_OPS() and use pm_ptr() to set .driver.pm. This being said,
> > > > > > > > > the driver won't work on a kernel with !CONFIG_PM given how you
> > > > > > > > > implemented probe() and remove().
> > > > > > > > >
> > > > > > > > > The pisp-be driver suffered from the same issue and Jacopo fixed it in
> > > > > > > > > the last version. You can have a look at implement something similar.
> > > > > > > >
> > > > > > > > I can't right away think of any reason to not just depend on CONFIG_PM
> > > > > > > > and be done with it without any tricks. Do you know if there's a reason?
> > > > > >
> > > > > > We had the same discussion, and even if I would be fine depending on
> > > > > > CONFIG_PM, supporting !CONFIG_PM is not that much work, I kept it as
> > > > > > an optional dependency (it was suggested during the review as well)
> > > > > >
> > > > > > >
> > > > > > > Also, I don't think pisp-be is correct. It just calls
> > > > > > > pispbe_runtime_resume() in probe() to wake the IP up (which only enables
> > > > > > > pisp clock), without telling the runtime PM about it. This means the parent
> > > > > > > device and the suppliers may not be powered up.
> > > > > >
> > > > > > Are you referring to the code currently in the tree or to this patch ?
> > > > > > https://patchwork.linuxtv.org/project/linux-media/patch/20240904095836.344833-5-jacopo.mondi@xxxxxxxxxxxxxxxx/
> > > > >
> > > > > Ah, I missed that one.
> > > > >
> > > > > I don't think it fixes the issue I mentioned. If we have PM enabled, and the
> > > > > parent device requires powering up for the child device (BE) to be
> > > > > accessible, the driver will crash when calling pispbe_hw_init(). I think you
> > > > > should call pm_runtime_set_active() before calling pispbe_runtime_resume().
> > > >
> > > > As discussed, this is not a problem currently for BE, but indeed you
> > > > have a point.
> 
> I admit the runtime_pm intrinsics are obscure to me, but Laurent just
> made me notice something.
> 
> Consider the following scenario
> 
> *) Kernel compiled with CONFIG_PM
> *) i2c sensor driver that supports both CONFIG_PM and !CONFIG_PM by:
>   *) Manually power up the sensor during probe
>   *) Call pm_runtime_enable() and pm_runtime_set_active() at the end
>      of the probe routine after having accessed the chip over i2c
>      (like most, if not all the i2c drivers in mainline do including
>      ccs)
> 
> All these drivers work, and during the probe routine before accessing
> the HW, they don't need to power up the parent i2c controller.

This isn't done explicitly by the I²C drivers, indeed but...

> 
> Might it be that during probe() the parent is guaranteed to be enabled ?

...yes.

> 
> I add a look in the driver-core and pm Documentation/ but found
> nothing.
> 
> A quick stroll in driver/base/ got me to __device_attach() and it
> seems parents are powered up before attaching a driver to a device
> (which in my understanding should be what ends up calling probe()).
> Clearly I've no real understanding of what I'm talking about when it
> comes to driver-core, so take this with a grain of salt.

This only works with runtime PM enabled.

> 
> > > >
> > > > Sad note: most of all the occurrences of "grep set_active" in
> > > > drivers/media/platform/ show that set_active() is used as I've done in
> > > > my patch
> > > >
> > > > > However, you said above that "supporting !CONFIG_PM is not that much work".
> > > > > Maybe not. But how much work is it to get it right (for both PM and !PM),
> > > > > and make sure it stays right? =).
> > > > >
> > > > > Just my opinion, but if there are zero use cases for the !PM, I would just
> > > > > go with "depends on PM" to keep the driver simpler, less bug-prone, and
> > > > > easier to maintain.
> > >
> > > I'm fine with that, and for platform drivers, that's my preferred
> > > option. Sakari ?
> >
> > I'm concerned with your (?) recent finding that many architectures don't
> > have support for CONFIG_PM. In this case the device is very unlikely to be
> > used outside ARM(64) so I guess it's fine.
> >
> 
> Also, this IP is RPi specific, and the !CONFIG_PM case is not used or
> tested on Pi.
> 
> However, I think this current patch is correct (assuming the above
> reasoning on i2c sensor drivers is correct) and doesn't require
> CONFIG_PM, so I would be tempted to keep this version.

I understand the current patch does depend on CONFIG_PM: it requires
runtime PM to be operational to start the clock, for instance.

> 
> > >
> > > > I don't see a use case for !PM and we confirmed with RPi they don't
> > > > need to support it. During the review of a previous version of the BE
> > > > driver iirc I've been asked to support !PM but I'm not sure I recall
> > > > the reasons.
> > >
> > > I hope it wasn't me :-)
> >
> > Me neither. Although it'd be nice: CONFIG_PM isn't a hardware specific
> > option as such. As one part of the kernel requires !CONFIG_PM and another
> > CONFIG_PM, we can expect problems, at least in principle.
> >
> > Ideally all architectures would support it so CONFIG_PM could be removed
> > and we could say the problem has been solved. :-)

-- 
Kidnregards,

Sakari Ailus