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    SubjectRe: [PATCH] staging: vc04_services: vchiq_arm: Create platform_device per device
    On Fri, Dec 23, 2022 at 12:24:22PM +0100, Stefan Wahren wrote:
    > Hi Laurent,
    > hi Umang,
    >
    > Am 22.12.22 um 18:35 schrieb Laurent Pinchart:
    > > Hi Umang,
    > >
    > > 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.
    >
    > 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?
    >
    > For more inspiration take a look at this old thread [1]

    Yes, that's the proper way to do this please! This should be a bus and
    dynamically add the devices when found, it is NOT a platform device
    anymore.

    thanks,

    greg k-h

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