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Date:   Thu, 29 Jun 2023 10:26:31 -0700
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Subject: Re: [Freedreno] [PATCH 2/3] drm/msm/dpu: Set DATABUS_WIDEN on command
 mode encoders
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From:   Abhinav Kumar <quic_abhinavk@quicinc.com>
To:     Dmitry Baryshkov <dmitry.baryshkov@linaro.org>,
        Marijn Suijten <marijn.suijten@somainline.org>,
        Jessica Zhang <quic_jesszhan@quicinc.com>
CC:     Sean Paul <sean@poorly.run>, <linux-kernel@vger.kernel.org>,
        <dri-devel@lists.freedesktop.org>, Rob Clark <robdclark@gmail.com>,
        "Daniel Vetter" <daniel@ffwll.ch>, <linux-arm-msm@vger.kernel.org>,
        David Airlie <airlied@gmail.com>,
        <freedreno@lists.freedesktop.org>
References: <20230525-add-widebus-support-v1-0-c7069f2efca1@quicinc.com>
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On 6/22/2023 4:37 PM, Abhinav Kumar wrote:
> 
> 
> On 6/22/2023 4:14 PM, Dmitry Baryshkov wrote:
>> On 23/06/2023 01:37, Abhinav Kumar wrote:
>>>
>>>
>>> On 6/21/2023 4:46 PM, Dmitry Baryshkov wrote:
>>>> On 22/06/2023 02:01, Abhinav Kumar wrote:
>>>>>
>>>>>
>>>>> On 6/21/2023 9:36 AM, Dmitry Baryshkov wrote:
>>>>>> On 21/06/2023 18:17, Marijn Suijten wrote:
>>>>>>> On 2023-06-20 14:38:34, Jessica Zhang wrote:
>>>>>>> <snip>
>>>>>>>>>>>>> +    if (phys_enc->hw_intf->ops.enable_widebus)
>>>>>>>>>>>>> + phys_enc->hw_intf->ops.enable_widebus(phys_enc->hw_intf);
>>>>>>>>>>>>
>>>>>>>>>>>> No. Please provide a single function which takes necessary
>>>>>>>>>>>> configuration, including compression and wide_bus_enable.
>>>>>>>>>>>>
>>>>>>>>>>>
>>>>>>>>>>> There are two ways to look at this. Your point is coming from 
>>>>>>>>>>> the
>>>>>>>>>>> perspective that its programming the same register but just a 
>>>>>>>>>>> different
>>>>>>>>>>> bit. But that will also make it a bit confusing.
>>>>>>>>>
>>>>>>>>> My point is to have a high-level function that configures the 
>>>>>>>>> INTF for
>>>>>>>>> the CMD mode. This way it can take a structure with necessary
>>>>>>>>> configuration bits.
>>>>>>>>
>>>>>>>> Hi Dmitry,
>>>>>>>>
>>>>>>>> After discussing this approach with Abhinav, we still have a few
>>>>>>>> questions about it:
>>>>>>>>
>>>>>>>> Currently, only 3 of the 32 bits for INTF_CONFIG2 are being used 
>>>>>>>> (the
>>>>>>>> rest are reserved with no plans of being programmed in the 
>>>>>>>> future). Does
>>>>>>>> this still justify the use of a struct to pass in the necessary
>>>>>>>> configuration?
>>>>>>>
>>>>>>> No.  The point Dmitry is making is **not** about this concidentally
>>>>>>> using the same register, but about adding a common codepath to 
>>>>>>> enable
>>>>>>> compression on this hw_intf (regardless of the registers it needs to
>>>>>>> touch).
>>>>>>
>>>>>> Actually to setup INTF for CMD stream (which is equal to setting 
>>>>>> up compression at this point).
>>>>>>
>>>>>
>>>>> Yes it should be setup intf for cmd and not enable compression.
>>>>>
>>>>> Widebus and compression are different features and we should be 
>>>>> able to control them independently.
>>>>>
>>>>> We just enable them together for DSI. So a separation is necessary.
>>>>>
>>>>> But I am still not totally convinced we even need to go down the 
>>>>> path for having an op called setup_intf_cmd() which takes in a 
>>>>> struct like
>>>>>
>>>>> struct dpu_cmd_intf_cfg {
>>>>>      bool data_compress;
>>>>>      bool widebus_en;
>>>>> };
>>>>>
>>>>> As we have agreed that we will not touch the video mode timing 
>>>>> engine path, it leaves us with only two bits.
>>>>>
>>>>> And like I said, its not that these two bits always go together. We 
>>>>> want to be able to control them independently which means that its 
>>>>> not necessary both bits program the same register one by one. We 
>>>>> might just end up programming one of them if we just use widebus.
>>>>>
>>>>> Thats why I am still leaning on keeping this approach.
>>>>
>>>> I do not like the idea of having small functions being called 
>>>> between modules. So, yes there will a config of two booleans, but it 
>>>> is preferable (and more scalable) compared to separate callbacks.
>>>>
>>>
>>> I definitely agree with the scalable part and I even cross checked 
>>> that the number of usable bitfields of this register is going up from 
>>> one chipset to the other although once again that depends on whether 
>>> we will use those features.
>>>
>>> For that reason I am not opposed to the struct idea.
>>>
>>> But there is also another pattern i am seeing which worries me. 
>>> Usable bitfields sometimes even reduce. For those cases, if we go 
>>> with a pre-defined struct it ends up with redundant members as those 
>>> bitfields go away.
>>>
>>> With the function op based approach, we just call the op if the 
>>> feature bit / core revision.
>>>
>>> So I wanted to check once more about the fact that we should consider 
>>> not just expansion but also reduction.
>>
>> As we have to support all generations, there is no actual reduction. 
>> Newer SoCs do not have particular feature/bit, but older ones do. By 
>> having multiple optional ops we just move this knowledge from 
>> ops->complex_callback() to _setup_block_ops(). But more importantly 
>> the caller gets more complicated. Instead of just calling 
>> ops->set_me_up(), it has to check all the optional callbacks and call 
>> the one by one.
>>
> 
> Alright, I am thinking that perhaps because this register is kind of 
> unique that its actually controlling a specific setting in the datapath, 
> downstream also has separate ops for this.
> 
> But thats fine, we can go ahead with the struct based approach.
> 

As data_compress has already landed, let me introduced the struct along 
with the core_revision based approach in the core_revision series and 
this series will expand that struct to include widebus too.