linux-kernel - Re: [PATCH 2/3] drm/msm/dpu: Set DATABUS

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Date:   Tue, 20 Jun 2023 14:38:34 -0700
From:   Jessica Zhang <quic_jesszhan@...cinc.com>
To:     Dmitry Baryshkov <dmitry.baryshkov@...aro.org>,
        Marijn Suijten <marijn.suijten@...ainline.org>,
        Abhinav Kumar <quic_abhinavk@...cinc.com>
CC:     Rob Clark <robdclark@...il.com>, Sean Paul <sean@...rly.run>,
        David Airlie <airlied@...il.com>,
        Daniel Vetter <daniel@...ll.ch>,
        <linux-arm-msm@...r.kernel.org>, <freedreno@...ts.freedesktop.org>,
        <dri-devel@...ts.freedesktop.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/3] drm/msm/dpu: Set DATABUS_WIDEN on command mode
 encoders



On 6/16/2023 5:35 PM, Dmitry Baryshkov wrote:
> On 17/06/2023 00:54, Marijn Suijten wrote:
>> On 2023-06-16 14:18:39, Abhinav Kumar wrote:
>>>
>>>
>>> On 6/14/2023 12:56 AM, Dmitry Baryshkov wrote:
>>>> On 14/06/2023 04:57, Jessica Zhang wrote:
>>>>> Add a DPU INTF op to set the DATABUS_WIDEN register to enable the
>>>>> databus-widen mode datapath.
>>>>>
>>>>> Signed-off-by: Jessica Zhang <quic_jesszhan@...cinc.com>
>>>>> ---
>>>>>    drivers/gpu/drm/msm/disp/dpu1/dpu_encoder_phys_cmd.c |  3 +++
>>>>>    drivers/gpu/drm/msm/disp/dpu1/dpu_hw_intf.c          | 12 
>>>>> ++++++++++++
>>>>>    drivers/gpu/drm/msm/disp/dpu1/dpu_hw_intf.h          |  3 +++
>>>>>    3 files changed, 18 insertions(+)
>>>>>
>>>>> diff --git a/drivers/gpu/drm/msm/disp/dpu1/dpu_encoder_phys_cmd.c
>>>>> b/drivers/gpu/drm/msm/disp/dpu1/dpu_encoder_phys_cmd.c
>>>>> index b856c6286c85..124ba96bebda 100644
>>>>> --- a/drivers/gpu/drm/msm/disp/dpu1/dpu_encoder_phys_cmd.c
>>>>> +++ b/drivers/gpu/drm/msm/disp/dpu1/dpu_encoder_phys_cmd.c
>>>>> @@ -70,6 +70,9 @@ static void _dpu_encoder_phys_cmd_update_intf_cfg(
>>>>>        if (intf_cfg.dsc != 0 && 
>>>>> phys_enc->hw_intf->ops.enable_compression)
>>>>>            
>>>>> phys_enc->hw_intf->ops.enable_compression(phys_enc->hw_intf);
>>>>> +
>>>>> +    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?

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?

Thanks,

Jessica Zhang

> 
>>>
>>> So lets say the function is called intf_cfg2_xxx(..., bool widebus, bool
>>> data_compress)
>>>
>>> Now for the caller who wants to just enable widebus this will be
>>>
>>> intf_cfg2_xxx(....., true, false)
>>>
>>> if we want to do both
>>>
>>> intf_cfg2_xxx(...., true, true)
>>>
>>> the last one where we want to do just data_compress(highly unlikely and
>>> not recommended)
>>>
>>> intf_cfg2_xxx(...., false, true)
>>>
>>> Now someone looking at the code will have to go and find out what each
>>> bool is.
>>>
>>> Whereas with separate ops, its kind of implicit.
>>
>> That's why you never pass bools as function argument (edge-case if it is
>> the only argument, and its meaning becomes clear from the function
>> name).  Use enumerations anywhere else.
>>
>> - Marijn
>>
>>>
>>> For that reason, I dont think this patch is bad at all.
> 
> -- 
> With best wishes
> Dmitry
>