Re: [PATCH v2 1/2] [media] mtk-mdp: add driver to probe mdp components

[Chun-Kuang Hu <chunkuang.hu@kernel.org>]

Hi, Eizan:

Eizan Miyamoto <eizan@chromium.org> 於 2020年5月6日 週三 下午4:41寫道：
>
> Broadly, this patch (1) adds a driver for various MTK MDP components to
> go alongside the main MTK MDP driver, and (2) hooks them all together
> using the component framework.
>
> (1) Up until now, the MTK MDP driver controls 8 devices in the device
> tree on its own. When running tests for the hardware video decoder, we
> found that the iommus and LARBs were not being properly configured. To
> configure them, a driver for each be added to mtk_mdp_comp so that
> mtk_iommu_add_device() can (eventually) be called from dma_configure()
> inside really_probe().
>
> (2) The integration into the component framework allows us to defer the
> registration with the v4l2 subsystem until all the MDP-related devices
> have been probed, so that the relevant device node does not become
> available until initialization of all the components is complete.
>
> Some notes about how the component framework has been integrated:
>
> - The driver for the rdma0 component serves double duty as the "master"
>   (aggregate) driver as well as a component driver. This is a non-ideal
>   compromise until a better solution is developed. This device is
>   differentiated from the rest by checking for a "mediatek,vpu" property
>   in the device node.
>
> - The list of mdp components remains hard-coded as mtk_mdp_comp_dt_ids[]
>   in mtk_mdp_core.c, and as mtk_mdp_comp_driver_dt_match[] in
>   mtk_mdp_comp.c. This unfortunate duplication of information is
>   addressed in a following patch in this series.
>
> - The component driver calls component_add() for each device that is
>   probed.
>
> - In mtk_mdp_probe (the "master" device), we scan the device tree for
>   any matching nodes against mtk_mdp_comp_dt_ids, and add component
>   matches for them. The match criteria is a matching device node
>   pointer.
>
> - When the set of components devices that have been probed corresponds
>   with the list that is generated by the "master", the callback to
>   mtk_mdp_master_bind() is made, which then calls the component bind
>   functions.
>
> - Inside mtk_mdp_master_bind(), once all the component bind functions
>   have been called, we can then register our device to the v4l2
>   subsystem.
>
> - The call to pm_runtime_enable() in the master device is called after
>   all the components have been registered by their bind() functions
>   called by mtk_mtp_master_bind(). As a result, the list of components
>   will not change while power management callbacks mtk_mdp_suspend()/
>   resume() are accessing the list of components.

For the component binding problem, MDP is similar to DRM driver [1],
and DRM is probed by mmsys driver [2] (mmsys is a device which control
display clock, display routing, mdp clock, mdp routing). Maybe you
could refer to what DRM does.

[1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/gpu/drm/mediatek/mtk_drm_drv.c?h=v5.8-rc6
[2] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?h=v5.8-rc6&id=667c769246b01c53ad0925d603d2a2531abd3ef2

Regards,
Chun-Kuang.

>
> Signed-off-by: Eizan Miyamoto <eizan@chromium.org>