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

[houlong wei]

Sorry, I fix a link error below.

On Mon, 2021-09-06 at 00:23 +0800, houlong wei wrote:
> 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@xxxxxxxxxxxx>
> > 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

Fix the link address: 
https://patchwork.kernel.org/project/linux-mediatek/list/?series=523771

> , 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@xxxxxxxxxxxx>
> > > Reviewed-by: Enric Balletbo i Serra <enric.balletbo@xxxxxxxxxxxxx
> > > >
> > > Reviewed-by: Houlong Wei <houlong.wei@xxxxxxxxxxxx>
> > > ---
> > > 
> 
>