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

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Date:   Wed, 21 Jun 2023 17:17:29 +0200
From:   Marijn Suijten <marijn.suijten@...ainline.org>
To:     Jessica Zhang <quic_jesszhan@...cinc.com>
Cc:     Dmitry Baryshkov <dmitry.baryshkov@...aro.org>,
        Abhinav Kumar <quic_abhinavk@...cinc.com>,
        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 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).  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.

- Marijn