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Date:   Fri, 6 Jan 2023 20:36:00 +0200
From:   Laurent Pinchart <laurent.pinchart@ideasonboard.com>
To:     Dave Stevenson <dave.stevenson@raspberrypi.com>
Cc:     Stefan Wahren <stefan.wahren@i2se.com>,
        Umang Jain <umang.jain@ideasonboard.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        linux-staging@lists.linux.dev,
        linux-rpi-kernel@lists.infradead.org,
        linux-arm-kernel@lists.infradead.org, linux-media@vger.kernel.org,
        linux-kernel@vger.kernel.org,
        Florian Fainelli <f.fainelli@gmail.com>,
        Adrien Thierry <athierry@redhat.com>,
        Dan Carpenter <error27@gmail.com>,
        Nicolas Saenz Julienne <nsaenz@kernel.org>,
        Phil Elwell <phil@raspberrypi.com>,
        Kieran Bingham <kieran.bingham@ideasonboard.com>
Subject: Re: [PATCH] staging: vc04_services: vchiq_arm: Create
 platform_device per device
Message-ID: <Y7hqEDtV99lcIf8g@pendragon.ideasonboard.com>
References: <20221220084404.19280-1-umang.jain@ideasonboard.com>
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Hi Dave,

On Fri, Jan 06, 2023 at 05:04:29PM +0000, Dave Stevenson wrote:
> Hi Laurent, Greg, Stefan, and Umang
> 
> Sorry, still catching up from the holiday period.

No need to apologize, I know the feeling :-)

> On Mon, 26 Dec 2022 at 10:04, Laurent Pinchart wrote:
> > On Fri, Dec 23, 2022 at 12:24:22PM +0100, Stefan Wahren wrote:
> > > Am 22.12.22 um 18:35 schrieb Laurent Pinchart:
> > > > On Thu, Dec 22, 2022 at 01:59:28PM +0530, Umang Jain wrote:
> > > >> On 12/21/22 6:40 PM, Greg Kroah-Hartman wrote:
> > > >>> On Wed, Dec 21, 2022 at 01:14:59PM +0200, Laurent Pinchart wrote:
> > > >>>> On Tue, Dec 20, 2022 at 02:14:04PM +0530, Umang Jain wrote:
> > > >>>>> Create a proper per device platorm_device structure for all the child
> > > >>>>> devices that needs to be registered by vchiq platform driver. Replace
> > > >>>>> the vchiq_register_child() with platform_add_devices() to register the
> > > >>>>> child devices.
> > > >>>>
> > > >>>> This explains what the patch does, but not why.
> > > >>>>
> > > >>>>> This is part of an effort to address TODO item "Get rid of all non
> > > >>>>> essential global structures and create a proper per device structure"
> > > >>>>
> > > >>>> And this explains part of the reason only. Could you please expand the
> > > >>>> commit message with the reasoning behind this change ? It's not clear
> > > >>>> from the change below why this is needed and good.
> > > >>
> > > >> Ok, I thought the TODO reference was sufficient but I'll expand on it.
> > > >>
> > > >>>>> Signed-off-by: Umang Jain <umang.jain@ideasonboard.com>
> > > >>>>> ---
> > > >>>>>    .../interface/vchiq_arm/vchiq_arm.c           | 59 ++++++++++---------
> > > >>>>>    1 file changed, 31 insertions(+), 28 deletions(-)
> > > >>>>>
> > > >>>>> diff --git a/drivers/staging/vc04_services/interface/vchiq_arm/vchiq_arm.c b/drivers/staging/vc04_services/interface/vchiq_arm/vchiq_arm.c
> > > >>>>> index 22de23f3af02..fa42ea3791a7 100644
> > > >>>>> --- a/drivers/staging/vc04_services/interface/vchiq_arm/vchiq_arm.c
> > > >>>>> +++ b/drivers/staging/vc04_services/interface/vchiq_arm/vchiq_arm.c
> > > >>>>> @@ -65,8 +65,29 @@ int vchiq_susp_log_level = VCHIQ_LOG_ERROR;
> > > >>>>>    DEFINE_SPINLOCK(msg_queue_spinlock);
> > > >>>>>    struct vchiq_state g_state;
> > > >>>>>
> > > >>>>> -static struct platform_device *bcm2835_camera;
> > > >>>>> -static struct platform_device *bcm2835_audio;
> > > >>>>> +static u64 vchiq_device_dmamask = DMA_BIT_MASK(32);
> > > >>>>
> > > >>>> The fact that this isn't const and is used by two different
> > > >>>> platform_device instances is worrying. Either it can be made const, or
> > > >>>> it's wrong.
> > > >>
> > > >> ack.
> > > >>
> > > >>>>> +
> > > >>>>> +static struct platform_device bcm2835_camera = {
> > > >>>>> +       .name           = "bcm2835-camera",
> > > >>>>> +       .id             = PLATFORM_DEVID_NONE,
> > > >>>>> +       .dev            = {
> > > >>>>> +               .dma_mask       = &vchiq_device_dmamask,
> > > >>>>> +       }
> > > >>>>> +};
> > > >>>>> +
> > > >>>>> +static struct platform_device bcm2835_audio = {
> > > >>>>> +       .name           = "bcm2835_audio",
> > > >>>>> +       .id             = PLATFORM_DEVID_NONE,
> > > >>>>> +       .dev            = {
> > > >>>>> +               .dma_mask       = &vchiq_device_dmamask,
> > > >>>>> +       }
> > > >>>>> +
> > > >>>>
> > > >>>> Extra blank line.
> > > >>
> > > >> oops, checkpatch.pl didn't catch this :-/
> > > >>
> > > >>>>> +};
> > > >>>>> +
> > > >>>>> +static struct platform_device *vchiq_devices[] __initdata = {
> > > >>>>
> > > >>>> Make it const.
> > > >>>>
> > > >>>>> +       &bcm2835_camera,
> > > >>>>> +       &bcm2835_audio,
> > > >>>>> +};
> > > >>>>>
> > > >>>>>    struct vchiq_drvdata {
> > > >>>>>         const unsigned int cache_line_size;
> > > >>>>> @@ -1763,28 +1784,6 @@ static const struct of_device_id vchiq_of_match[] = {
> > > >>>>>    };
> > > >>>>>    MODULE_DEVICE_TABLE(of, vchiq_of_match);
> > > >>>>>
> > > >>>>> -static struct platform_device *
> > > >>>>> -vchiq_register_child(struct platform_device *pdev, const char *name)
> > > >>>>> -{
> > > >>>>> -       struct platform_device_info pdevinfo;
> > > >>>>> -       struct platform_device *child;
> > > >>>>> -
> > > >>>>> -       memset(&pdevinfo, 0, sizeof(pdevinfo));
> > > >>>>> -
> > > >>>>> -       pdevinfo.parent = &pdev->dev;
> > > >>>>> -       pdevinfo.name = name;
> > > >>>>> -       pdevinfo.id = PLATFORM_DEVID_NONE;
> > > >>>>> -       pdevinfo.dma_mask = DMA_BIT_MASK(32);
> > > >>>>> -
> > > >>>>> -       child = platform_device_register_full(&pdevinfo);
> > > >>>>> -       if (IS_ERR(child)) {
> > > >>>>> -               dev_warn(&pdev->dev, "%s not registered\n", name);
> > > >>>>> -               child = NULL;
> > > >>>>> -       }
> > > >>>>> -
> > > >>>>> -       return child;
> > > >>>>> -}
> > > >>>>> -
> > > >>>>>    static int vchiq_probe(struct platform_device *pdev)
> > > >>>>>    {
> > > >>>>>         struct device_node *fw_node;
> > > >>>>> @@ -1832,8 +1831,11 @@ static int vchiq_probe(struct platform_device *pdev)
> > > >>>>>                 goto error_exit;
> > > >>>>>         }
> > > >>>>>
> > > >>>>> -       bcm2835_camera = vchiq_register_child(pdev, "bcm2835-camera");
> > > >>>>> -       bcm2835_audio = vchiq_register_child(pdev, "bcm2835_audio");
> > > >>>>> +       err = platform_add_devices(vchiq_devices, ARRAY_SIZE(vchiq_devices));
> > > >>>>> +       if (err) {
> > > >>>>> +               dev_warn(&pdev->dev, "Failed to add vchiq child devices");
> > > >>>>> +               goto error_exit;
> > > >>>>> +       }
> > > >>>>
> > > >>>> If you unbind and rebind this driver, the platform_device instances
> > > >>>> defined as global variables will be reused, and I'm pretty sure that
> > > >>>> will cause issues, for instance with the kobj->state_initialized check
> > > >>>> in kobject_init() (called from device_initialize(), itself called from
> > > >>>> platform_device_register(), from platform_add_devices()). I'm not sure
> > > >>>> static instances of platform_device are a very good idea in general.
> > > >>>
> > > >>> static instances of any device are a horrible idea, but it seems that
> > > >>> many drivers do this and abuse platform devices this way :(
> > > >>
> > > >> It seems  I have been a victim of the abuse usage while looking for
> > > >> platform_device references in the codebase. I'm working on a new
> > > >> approach for this.
> > > >>
> > > >> Currently (as per the linux-next branch), the vchiq driver will happily
> > > >> carry on if any of the child  platform device registration fails. That
> > > >> means if bcm2835-audio fails to register, bcm2835-camera will  still
> > > >> kept registered I suppose.
> > > >>
> > > >> However with usage of platform_add_devices() in this patch, I introduced
> > > >> a functionality change (I'm realizing this now) - any failure of child
> > > >> platform device registeration will -unregister- all the other platform
> > > >> devices i.e. if bcm2835-audio fails, bcm2835-camera will also get
> > > >> unregistered.
> > > >>
> > > >> Should I be working towards the status-quo behavior ? Or it's sane to
> > > >> unregistered other platform devices if one of the fails like
> > > >> platform_add_devices() does ? (This affects my new approach as well,
> > > >> hence the question)
> > > >
> > > > If it doesn't cause too much extra complexity, it would be nice to skip
> > > > devices that can't be registered successfully, and still support the
> > > > other ones. I don't expect registration failures to be a occuring
> > > > normally, so if this causes too much completely, I think it would still
> > > > be fine to fail more harshly.
> > > >
> > > >>> Ideally this should be done properly, with the correct devices created
> > > >>> automatically based on the device tree structure, NOT hard-coded into a
> > > >>> .c file like this.
> > > >>>
> > > >>> So I too really do not like this change, why are these not being created
> > > >>> by the firware layer automatically?
> > > >>
> > > >> Not sure if this is a helpful comment, but as far I know, there can be
> > > >> vchiq child platform devices which probably don't have a Device tree
> > > >> entry. like the bcm2835-isp [1] I posted earlier.
> > > >>
> > > >> [1] https://lore.kernel.org/lkml/20221121214722.22563-1-umang.jain@ideasonboard.com/
> > > >
> > > > Those devices are implemented and exposed by the firmware running on the
> > > > VC4. The device tree describes the VC4 itself with the resources
> > > > required to communicate with it through a mailbox interface. I was going
> > > > to say that the platform devices are then created based on what the
> > > > firmware exposes, but that's not right, they're indeed hardcoded in the
> > > > vchiq driver. Adding corresponding DT nodes (as children of the vchiq DT
> > > > node) could make sense. Dave, do you have any opinion on this ?
> > >
> > > i vaguely remember the discussion how to represent audio and camera
> > > interface in the device tree. Representing as child nodes of the VC4 has
> > > been rejected on the device tree mailing some years ago, because this
> > > doesn't represent the physical (hardware) wiring. It's still possible to
> > > access e.g. the camera interface from the ARM.
> >
> > For the camera, things have changed a lot since the mail thread you've
> > linked. The CSI-2 receiver (and camera sensors) are now described in DT
> > and controlled from the ARM side. I believe the firmware still supports
> > controlling that hardware from the VC4 side (limited to a very small set
> > of camera sensors), but I think we can ignore that from a mainline point
> > of view.
> >
> > The devices that are still controlled from the VC4 side are the camera
> > ISP, the video codec and the audio interface. As far as I can tell,
> > there's no plan to change this in neither the short term or long term
> > future. Based on my limited understanding, this architecture makes sense
> > for the ISP and codec as they share resources in a way that is best
> > handled by the VC4 firmware. I have no idea about the audio side. Dave,
> > please correct me if this is incorrect.
> 
> Audio is only the analogue audio interface. HDMI should now be handled
> under the KMS driver.
> 
> ISP and codec hardware are blocks we haven't got permission from
> Broadcom to open source, therefore they will remain under the
> firmware.
> Analogue audio is doing processing in the firmware that the ARM1176 of
> Pi0/1 hasn't got the grunt to do.
> 
> > > The whole approach with using a separate binding for all the firmware
> > > stuff lead to a lot of trouble on the Raspberry Pi platform (ugly
> > > dependencies between firmware, DT and kernel). So i would like to avoid
> > > this here. In case the current implementation is a no go, how about
> > > letting the ARM core discover the available interfaces e.g. via mailbox
> > > interface?
> >
> > I don't know if this is possible with existing firmware, and, if not, if
> > it could be implemented (the firmware isn't open-source). If not, we
> > will need to handle the current situation in the best possible way,
> > which would require creating devices either in the VCHIQ driver, or
> > through DT. I agree the former is probably best, there would still be a
> > dependency between the kernel and firmware, but DT would at least be out
> > of the picture. A custom bus seems fine to me.
> 
> There is currently no way to enumerate the VCHIQ services that are
> available from the firmware. They are normally all present, but
> configuring the firmware for minimum memory usage does remove all
> except audio.
> The vchiq_open_service call will fail if the relevant service isn't
> running, which is currently handled from the probe of each of the
> drivers.
> 
> There's not a straight 1:1 mapping between the VCHIQ service 4cc and
> the kernel driver. bcm2835-camera, bcm2835-codec, and bcm2835-isp are
> all using the 'mmal' service 4cc, as that then has further selection
> for which MMAL components are present. Just advertising the vchiq
> service therefore isn't sufficient, and you'd be needing a sub-bus for
> the MMAL components and which kernel drivers those spawn.
> 
> Even that is non-trivial as bcm2835-codec supports multiple 1-in,
> 1-out components (video encode, video decode, JPEG encode,
> deinterlace, and a simple usage of the ISP for image conversion), so
> we now need to be passing info into bcm2835-codec as to which
> component to instantiate.
> 
> 
> If there really is a desire to be able to enumerate the VCHIQ services
> running, then it may be possible to add it to the firmware. My gut
> feeling is that it would be more sensible to implement it as a VCHIQ
> query rather than adding a dependency on the mailbox service.
> Doing so is going to be an issue for backwards compatibility, as a new
> kernel running on old firmware will end up with no services (including
> audio and camera that they currently would get).
> 
> Getting a full list of MMAL components could also be done once the
> MMAL service had been opened, but it gets a touch ugly. Again there's
> going to be an issue with backwards compatibility if running an old
> firmware on a new kernel.

I'm not too concerned about backward compatibility, but from your
explanation, it seems like dynamic enumeration isn't worth the trouble.

> > > For more inspiration take a look at this old thread [1]
> > >
> > > But i agree DT binding for vchiq itself is also a TODO
> > >
> > > and any help is appreciated.
> > >
> > > [1] - http://lists.infradead.org/pipermail/linux-rpi-kernel/2017-February/005541.html

-- 
Regards,

Laurent Pinchart