Re: [PATCH v7 2/7] mtk-mdp: add driver to probe mdp components

[houlong wei <houlong.wei@mediatek.com>]

Hi Ezequiel,

Thank you for your attention to this series of patches. I answer partial of your questions below.
Regards,
Houlong

On Sat, 2021-09-04 at 20:34 +0800, Ezequiel Garcia wrote:
> Hi Eizan,
> 
> Sorry for seeing this series so late.
> 
> On Wed, 25 Aug 2021 at 03:35, Eizan Miyamoto <eizan@chromium.org>
> wrote:
> > 
> > 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.
> 
> Why were not being properly configured? What was the problem?
> Why not fixing that instead?
> 
> Does this mean the driver is currently broken and unusable?

This series of patches are supplements to another series, please refer
to  
https://patchwork.kernel.org/project/linux-mediatek/list/?series=515129c
, which add device link between the mtk-iommu consumer and the mtk-larb 
devices. Without that series of patches, the mtk-mdp driver can work
well so far.
But with that series, it seems the device link only can be established
for the device which is registered as a platform driver. That's why
Eizan adds this series of patches to make all mdp components to be
registered as platform drivers.

> 
> > 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.
> > 
> 
> As I have stated in Yunfei, I am not convinced you need an async
> framework
> at all. It seems all these devices could have been linked together
> in the device tree, and then have a master device to tie them.
> 
> I.e. something like
> 
> mdp {
>   mdp_rdma0 {
>   }
>   mdp_rsz0 {
>   }
>   mdp_rsz1 {
>   }
> }
> 

The commit message of the patch below explains that " If the mdp_*
nodes are under an mdp sub-node, their corresponding platform device
does not automatically get its iommu assigned properly."
Please refer to 
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/arch/arm64/boot/dts/mediatek/mt8173.dtsi?h=v5.14.1&id=8127881f741dbbf9a1da9e9bc59133820160b217

> All this async games seem like making the driver really obfuscated,
> which will mean harder to debug and maintain.
> I am not sure we want that burden.
> 
> Even if we are all fully convinced that you absolutely need
> an async framework, then what's wrong with v4l2-async?
> 
> I would start by addressing what is wrong with the IOMMUs
> in the current design.
> 
> Thanks,
> Ezequiel
> 
> > - 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.
> > 
> > Signed-off-by: Eizan Miyamoto <eizan@chromium.org>
> > Reviewed-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
> > Reviewed-by: Houlong Wei <houlong.wei@mediatek.com>
> > ---
> >