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

[Hans Verkuil <hverkuil@xs4all.nl>]

On 9/5/21 6:23 PM, 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@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.

Hold on, so this means that if that iommu device-link series is merged,
then the mtk-mdp driver breaks? I posted a PR for that iommu series, but
I've just withdrawn that PR until this issue is clarified.

Is it only mtk-mdp that is affected by this iommu device-link series, or
others as well?

Regards,

	Hans

> 
>>
>>> 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>
>>> ---
>>>
>