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Date:   Tue, 13 Aug 2019 15:36:53 +0300
From:   Laurent Pinchart <laurent.pinchart@ideasonboard.com>
To:     Helen Koike <helen.koike@collabora.com>
Cc:     Shuah Khan <skhan@linuxfoundation.org>,
        =?utf-8?B?QW5kcsOp?= Almeida <andrealmeid@collabora.com>,
        mchehab@kernel.org, hverkuil@xs4all.nl,
        linux-kernel@vger.kernel.org, linux-media@vger.kernel.org,
        kernel@collabora.com
Subject: Re: [PATCH 0/3] Collapse vimc into single monolithic driver
Message-ID: <20190813123653.GE5009@pendragon.ideasonboard.com>
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Hi Helen,

On Tue, Aug 13, 2019 at 09:25:01AM -0300, Helen Koike wrote:
> On 8/13/19 6:56 AM, Laurent Pinchart wrote:
> > On Mon, Aug 12, 2019 at 08:41:33PM -0300, Helen Koike wrote:
> >> On 8/12/19 7:14 PM, Shuah Khan wrote:
> >>> On 8/12/19 1:10 PM, Shuah Khan wrote:
> >>>> On 8/12/19 12:52 PM, André Almeida wrote:
> >>>>> On 8/12/19 11:08 AM, Shuah Khan wrote:
> >>>>>> On 8/9/19 9:51 PM, Helen Koike wrote:
> >>>>>>> On 8/9/19 9:24 PM, André Almeida wrote:
> >>>>>>>> On 8/9/19 9:17 PM, Shuah Khan wrote:
> >>>>>>>>> On 8/9/19 5:52 PM, André Almeida wrote:
> >>>>>>>>>> On 8/9/19 6:45 PM, Shuah Khan wrote:
> >>>>>>>>>>> vimc uses Component API to split the driver into functional
> >>>>>>>>>>> components.
> >>>>>>>>>>> The real hardware resembles a monolith structure than component and
> >>>>>>>>>>> component structure added a level of complexity making it hard to
> >>>>>>>>>>> maintain without adding any real benefit.
> >>>>>>>>>>>        The sensor is one vimc component that would makes sense to be a
> >>>>>>>>>>> separate
> >>>>>>>>>>> module to closely align with the real hardware. It would be easier to
> >>>>>>>>>>> collapse vimc into single monolithic driver first and then split the
> >>>>>>>>>>> sensor off as a separate module.
> >>>>>>>>>>>
> >>>>>>>>>>> This patch series emoves the component API and makes minimal
> >>>>>>>>>>> changes to
> >>>>>>>>>>> the code base preserving the functional division of the code
> >>>>>>>>>>> structure.
> >>>>>>>>>>> Preserving the functional structure allows us to split the sensor off
> >>>>>>>>>>> as a separate module in the future.
> >>>>>>>>>>>
> >>>>>>>>>>> Major design elements in this change are:
> >>>>>>>>>>>        - Use existing struct vimc_ent_config and struct
> >>>>>>>>>>> vimc_pipeline_config
> >>>>>>>>>>>          to drive the initialization of the functional components.
> >>>>>>>>>>>        - Make vimc_ent_config global by moving it to vimc.h
> >>>>>>>>>>>        - Add two new hooks add and rm to initialize and register,
> >>>>>>>>>>> unregister
> >>>>>>>>>>>          and free subdevs.
> >>>>>>>>>>>        - All component API is now gone and bind and unbind hooks are
> >>>>>>>>>>> modified
> >>>>>>>>>>>          to do "add" and "rm" with minimal changes to just add and rm
> >>>>>>>>>>> subdevs.
> >>>>>>>>>>>        - vimc-core's bind and unbind are now register and unregister.
> >>>>>>>>>>>        - vimc-core invokes "add" hooks from its
> >>>>>>>>>>> vimc_register_devices().
> >>>>>>>>>>>          The "add" hooks remain the same and register subdevs. They
> >>>>>>>>>>> don't
> >>>>>>>>>>>          create platform devices of their own and use vimc's
> >>>>>>>>>>> pdev.dev as
> >>>>>>>>>>>          their reference device. The "add" hooks save their
> >>>>>>>>>>> vimc_ent_device(s)
> >>>>>>>>>>>          in the corresponding vimc_ent_config.
> >>>>>>>>>>>        - vimc-core invokes "rm" hooks from its unregister to
> >>>>>>>>>>> unregister
> >>>>>>>>>>> subdevs
> >>>>>>>>>>>          and cleanup.
> >>>>>>>>>>>        - vimc-core invokes "add" and "rm" hooks with pointer to struct
> >>>>>>>>>>> vimc_device
> >>>>>>>>>>>          and the corresponding struct vimc_ent_config pointer.
> >>>>>>>>>>>        The following configure and stream test works on all devices.
> >>>>>>>>>>>             media-ctl -d platform:vimc -V '"Sensor
> >>>>>>>>>>> A":0[fmt:SBGGR8_1X8/640x480]'
> >>>>>>>>>>>        media-ctl -d platform:vimc -V '"Debayer
> >>>>>>>>>>> A":0[fmt:SBGGR8_1X8/640x480]'
> >>>>>>>>>>>        media-ctl -d platform:vimc -V '"Sensor
> >>>>>>>>>>> B":0[fmt:SBGGR8_1X8/640x480]'
> >>>>>>>>>>>        media-ctl -d platform:vimc -V '"Debayer
> >>>>>>>>>>> B":0[fmt:SBGGR8_1X8/640x480]'
> >>>>>>>>>>>             v4l2-ctl -z platform:vimc -d "RGB/YUV Capture" -v
> >>>>>>>>>>> width=1920,height=1440
> >>>>>>>>>>>        v4l2-ctl -z platform:vimc -d "Raw Capture 0" -v
> >>>>>>>>>>> pixelformat=BA81
> >>>>>>>>>>>        v4l2-ctl -z platform:vimc -d "Raw Capture 1" -v
> >>>>>>>>>>> pixelformat=BA81
> >>>>>>>>>>>             v4l2-ctl --stream-mmap --stream-count=100 -d /dev/video1
> >>>>>>>>>>>        v4l2-ctl --stream-mmap --stream-count=100 -d /dev/video2
> >>>>>>>>>>>        v4l2-ctl --stream-mmap --stream-count=100 -d /dev/video3
> >>>>>>>>>>>
> >>>>>>>>>>> The third patch in the series fixes a general protection fault found
> >>>>>>>>>>> when rmmod is done while stream is active.
> >>>>>>>>>>
> >>>>>>>>>> I applied your patch on top of media_tree/master and I did some
> >>>>>>>>>> testing.
> >>>>>>>>>> Not sure if I did something wrong, but just adding and removing the
> >>>>>>>>>> module generated a kernel panic:
> >>>>>>>>>
> >>>>>>>>> Thanks for testing.
> >>>>>>>>>
> >>>>>>>>> Odd. I tested modprobe and rmmod both.I was working on Linux 5.3-rc2.
> >>>>>>>>> I will apply these to media latest and work from there. I have to
> >>>>>>>>> rebase these on top of the reverts from Lucas and Helen
> >>>>>>>>
> >>>>>>>> Ok, please let me know if I succeeded to reproduce.
> >>>>>>>>
> >>>>>>>>>>
> >>>>>>>>>> ~# modprobe vimc
> >>>>>>>>>> ~# rmmod vimc
> >>>>>>>>>> [   16.452974] stack segment: 0000 [#1] SMP PTI
> >>>>>>>>>> [   16.453688] CPU: 0 PID: 2038 Comm: rmmod Not tainted 5.3.0-rc2+ #36
> >>>>>>>>>> [   16.454678] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996),
> >>>>>>>>>> BIOS 1.12.0-20181126_142135-anatol 04/01/2014
> >>>>>>>>>> [   16.456191] RIP: 0010:kfree+0x4d/0x240
> >>>>>>>>>>
> >>>>>>>>>> <registers values...>
> >>>>>>>>>>
> >>>>>>>>>> [   16.469188] Call Trace:
> >>>>>>>>>> [   16.469666]  vimc_remove+0x35/0x90 [vimc]
> >>>>>>>>>> [   16.470436]  platform_drv_remove+0x1f/0x40
> >>>>>>>>>> [   16.471233]  device_release_driver_internal+0xd3/0x1b0
> >>>>>>>>>> [   16.472184]  driver_detach+0x37/0x6b
> >>>>>>>>>> [   16.472882]  bus_remove_driver+0x50/0xc1
> >>>>>>>>>> [   16.473569]  vimc_exit+0xc/0xca0 [vimc]
> >>>>>>>>>> [   16.474231]  __x64_sys_delete_module+0x18d/0x240
> >>>>>>>>>> [   16.475036]  do_syscall_64+0x43/0x110
> >>>>>>>>>> [   16.475656]  entry_SYSCALL_64_after_hwframe+0x44/0xa9
> >>>>>>>>>> [   16.476504] RIP: 0033:0x7fceb8dafa4b
> >>>>>>>>>>
> >>>>>>>>>> <registers values...>
> >>>>>>>>>>
> >>>>>>>>>> [   16.484853] Modules linked in: vimc(-) videobuf2_vmalloc
> >>>>>>>>>> videobuf2_memops v4l2_tpg videobuf2_v4l2 videobuf2_common
> >>>>>>>>>> [   16.486187] ---[ end trace 91e5e0894e254d49 ]---
> >>>>>>>>>> [   16.486758] RIP: 0010:kfree+0x4d/0x240
> >>>>>>>>>>
> >>>>>>>>>> <registers values...>
> >>>>>>>>>>
> >>>>>>>>>> fish: “rmmod vimc” terminated by signal SIGSEGV (Address boundary
> >>>>>>>>>> error)
> >>>>>>>>>>
> >>>>>>>>>> I just added the module after booting, no other action was made.
> >>>>>>>>>> Here is
> >>>>>>>>>> how my `git log --oneline` looks like:
> >>>>>>>>>>
> >>>>>>>>>> 897d708e922b media: vimc: Fix gpf in rmmod path when stream is active
> >>>>>>>>>> 2e4a5ad8ad6d media: vimc: Collapse component structure into a single
> >>>>>>>>>> monolithic driver
> >>>>>>>>>> 7c8da1687e92 media: vimc: move private defines to a common header
> >>>>>>>>>> 97299a303532 media: Remove dev_err() usage after platform_get_irq()
> >>>>>>>>>> 25a3d6bac6b9 media: adv7511/cobalt: rename driver name to adv7511-v4l2
> >>>>>>>
> >>>>>>> I couldn't reproduce the error, my tree looks the same:
> >>>>>>>
> >>>>>>> [I] koike@floko ~/m/o/linux> git log --oneline
> >>>>>>> e3345155c8ed (HEAD) media: vimc: Fix gpf in rmmod path when stream is
> >>>>>>> active
> >>>>>>> 43e9e2fe761f media: vimc: Collapse component structure into a single
> >>>>>>> monolithic driver
> >>>>>>> 8a6d0b9adde0 media: vimc: move private defines to a common header
> >>>>>>> 97299a303532 (media/master) media: Remove dev_err() usage after
> >>>>>>> platform_get_irq()
> >>>>>>> 25a3d6bac6b9 media: adv7511/cobalt: rename driver name to adv7511-v4l2
> >>>>>>
> >>>>>> Thanks Helen for trying to reproduce and sharing the result.
> >>>>>
> >>>>> Me and Helen found out what is the problem. If you follow this call trace:
> >>>>>
> >>>>> vimc_ent_sd_unregister()
> >>>>> v4l2_device_unregister_subdev()
> >>>>> v4l2_subdev_release()
> >>>>>
> >>>>> You'll notice that this last function calls the `release` callback
> >>>>> implementation of the subdevice. For instance, the `release` of
> >>>>> vimc-sensor is this one:
> >>>>>
> >>>>> static void vimc_sen_release(struct v4l2_subdev *sd)
> >>>>> {
> >>>>>     struct vimc_sen_device *vsen =
> >>>>>                 container_of(sd, struct vimc_sen_device, sd);
> >>>>>
> >>>>>     v4l2_ctrl_handler_free(&vsen->hdl);
> >>>>>     tpg_free(&vsen->tpg);
> >>>>>     kfree(vsen);
> >>>>> }
> >>>>>
> >>>>> And then you can see that `vsen` has been freed. Back to
> >>>>> vimc_ent_sd_unregister(), after v4l2_device_unregister_subdev(), the
> >>>>> function will call vimc_pads_cleanup(). This is basically a
> >>>>> kfree(ved->pads), but `ved` has just been freed at
> >>>>> v4l2_subdev_release(), producing a memory fault.
> >>>>>
> >>>>> To fix that, we found two options:
> >>>>>
> >>>>> - place the kfree(ved->pads) inside the release callback of each
> >>>>> subdevice and removing vimc_pads_cleanup() from
> >>>>> vimc_ent_sd_unregister()
> >>>>> - use a auxiliary variable to hold the address of the pads, for instance:
> >>>>>
> >>>>> void vimc_ent_sd_unregister(...)
> >>>>> {
> >>>>>      struct media_pad *pads = ved->pads;
> >>>>>      ...
> >>>>>      vimc_pads_cleanup(pads);
> >>>>> }
> >>>>>
> >>>>>
> >>>>
> >>>> I fixed a problem in the thirds patch. vimc-capture uses the first
> >>>> approach - placing the kfree(ved->pads) inside the release callback.
> >>>>
> >>>> I am debugging another such problem in unbind path while streaming.
> >>>> I am working on v2 and I will look for the rmmod problem and fix it.
> >>>>
> >>>> thanks again for testing and finding the root cause.
> >>>> -- Shuah
> >>>
> >>> Hi Andre,
> >>>
> >>> Here is what's happening.
> >>>
> >>> Before this change, you can't really do rmmod vimc, because vimc is in
> >>> use by other component drivers. With the collapse, now you can actually
> >>> do rmmod on vimc and this problem in vimc_ent_sd_unregister() that frees
> >>> pads first and the does v4l2_device_unregister_subdev().
> >>>
> >>> I fixed this in the 3/3 patch. I can reproduce the problem with patches 1 and 2, and patch 3 fixes it.
> >>>
> >>> Did you test with the third patch in this series?
> >>
> >> yes, we tested with 3/3, but the new problem now is when doing the following
> >> in this order:
> >>
> >>     v4l2_device_unregister_subdev(sd);
> >>     vimc_pads_cleanup(ved->pads);
> >>
> >>
> >> v4l2_device_unregister_subdev() calls the release function of the subdevice that
> >> frees the ved object, so ved->pads is not valid anymore. That is why André suggested
> >> a temporary variable to hold ved->pads and to be able to free it later:
> >>
> >>     struct media_pad *pads = ved->pads;
> >>
> >>     v4l2_device_unregister_subdev(sd);
> >>     vimc_pads_cleanup(pads); // So we don't use the ved object here anymore.
> > 
> > Can't you simply call vimc_pads_cleanup() in the release function of the
> > subdevice before freeing the ved object ?
> 
> Yes we can, that is the other option Andre suggested.
> The  advantage of doing it inside vimc_ent_sd_unregister() is that
> who allocated the memory in the first place was vimc_ent_sd_register(), and also,
> this is a common code to all subdevs, so letting it in vimc_ent_sd_unregister()
> will make sure no subdevs ever forget to free this memory.
> But saving the pointer to a variable to free it later is not that pretty
> either.

The release function of a subdevice is meant to be used as a destructor,
it should free all the resources allocated for the subdevice. Your
subdevices (such as vimc_sca_device) are essentially objects derived
from a common vimc subdevice object (vimc_ent_device), I thus suggest
providing four functions to deal with vimc_ent_device:

- vimc_ent_sd_init() to initialise the base vimc_ent_device object (most
  of the code is currently in vimc_ent_sd_register()), to be called by
  the init function of each subdevice type (the bind functions in the
  current code)

- vimc_ent_sd_register() to only register the vimc_ent_device once it
  has been fully initialised

- vimc_end_sd_unregister() to unregister the vimc_ent_device

- vimc_end_sd_cleanup() to free the resources allocated by
  vimc_ent_sd_init() (including calling vimc_pads_cleanup()), to be
  called by the subdevice release handler

-- 
Regards,

Laurent Pinchart