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Message-ID: <a4688db2-4230-7485-a688-bc6253d2d4b8@quicinc.com>
Date:   Thu, 22 Jun 2023 15:37:52 -0700
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Subject: Re: [PATCH 2/3] drm/msm/dpu: Set DATABUS_WIDEN on command mode
 encoders
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To:     Dmitry Baryshkov <dmitry.baryshkov@linaro.org>,
        Marijn Suijten <marijn.suijten@somainline.org>,
        Jessica Zhang <quic_jesszhan@quicinc.com>
CC:     Rob Clark <robdclark@gmail.com>, Sean Paul <sean@poorly.run>,
        David Airlie <airlied@gmail.com>,
        Daniel Vetter <daniel@ffwll.ch>,
        <linux-arm-msm@vger.kernel.org>, <freedreno@lists.freedesktop.org>,
        <dri-devel@lists.freedesktop.org>, <linux-kernel@vger.kernel.org>
References: <20230525-add-widebus-support-v1-0-c7069f2efca1@quicinc.com>
 <20230525-add-widebus-support-v1-2-c7069f2efca1@quicinc.com>
 <c74c9e0e-d059-f0e3-4350-03089c37131a@linaro.org>
 <cce68370-3fd9-4c9a-258e-af0d5d057fda@quicinc.com>
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 <81a5e241-ec82-7414-8752-4ce3cb084959@linaro.org>
 <f14f2c31-38c2-0600-3a29-17e83afececf@quicinc.com>
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 <8dcd643f-9644-a4e7-a0d5-eefa28084a88@linaro.org>
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 <b119470d-f656-71d1-8b87-b4b8196f5220@linaro.org>
From:   Abhinav Kumar <quic_abhinavk@quicinc.com>
In-Reply-To: <b119470d-f656-71d1-8b87-b4b8196f5220@linaro.org>
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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.

> Not to mention that it allows us to program required registers directly 
> (by setting values) rather than using RMW cycles and thus depending on 
> the value being previously programmed to these registers.
> 

This will not change. We will still have to use RMW cycles to preserve 
the reset values of some of the fields as those are the right values for 
the registers and shouldnt be touched.

>>
>>>>  Similar to how dpu_hw_intf_setup_timing_engine() programs the
>>>> hw_intf - including widebus! - for video-mode.
>>>>
>>>> Or even more generically, have a struct similar to intf_timing_params
>>>> that says how the intf needs to be configured - without the caller
>>>> knowing about INTF_CONFIG2.
>>>>
>>>> struct dpu_hw_intf_cfg is a very good example of how we can use a 
>>>> single
>>>> struct and a single callback to configure multiple registers at once
>>>> based on some input parameters.
>>>>
>>>>> In addition, it seems that video mode does all its INTF_CONFIG2
>>>>> configuration separately in dpu_hw_intf_setup_timing_engine(). If we
>>>>> have a generic set_intf_config2() op, it might be good to have it as
>>>>> part of a larger cleanup where we have both video and command mode use
>>>>> the generic op. What are your thoughts on this?
>>>>
>>>> Not in that way, but if there is a generic enable_compression() or
>>>> configure_compression() callback (or even more generic, similar to
>>>> setup_intf_cfg in dpu_hw_ctl) that would work for both video-mode and
>>>> command-mode, maybe that is beneficial.
>>>
>>> I'd rather not do this. Let's just 'setup timing enging' vs 'setup 
>>> CMD'. For example, it might also include setting up other INTF 
>>> parameters for CMD mode (if anything is required later on).
>>>
>>
>> Agreed on setup CMD but I dont know whether we need a setup CMD at all.
>> Seems like an overkill.
>>
>>>>
>>>> - Marijn
>>>
>