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Message-ID: <d7b93f39-e866-899b-578f-17f7bdd5bdd5@linaro.org>
Date:   Tue, 18 Jul 2023 09:04:19 +0300
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Subject: Re: [PATCH v2] drm/msm/dsi: Enable DATABUS_WIDEN for DSI command mode
Content-Language: en-GB
To:     Jessica Zhang <quic_jesszhan@quicinc.com>
Cc:     freedreno@lists.freedesktop.org, quic_abhinavk@quicinc.com,
        dri-devel@lists.freedesktop.org, linux-kernel@vger.kernel.org,
        linux-arm-msm@vger.kernel.org,
        Marijn Suijten <marijn.suijten@somainline.org>,
        Sean Paul <sean@poorly.run>
References: <20230713-add-widebus-support-v2-1-ad0added17b6@quicinc.com>
 <91bc6348-2030-85dd-1492-1609b392793f@linaro.org>
 <05996344-0e43-7f37-c99a-42c04f91dc83@quicinc.com>
 <CAA8EJppFDcrVdegskSD0TJPOdSzVw_50+Bq+u8LKn26jdKE=tw@mail.gmail.com>
 <19ff3766-a73e-761d-8965-015817fa9c0c@quicinc.com>
 <637e4979-cadc-2167-a14f-25363c9da990@linaro.org>
 <2804b7c7-0ed2-6490-3316-7a38eb1262f8@quicinc.com>
From:   Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
In-Reply-To: <2804b7c7-0ed2-6490-3316-7a38eb1262f8@quicinc.com>
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On 18/07/2023 03:30, Jessica Zhang wrote:
> 
> 
> On 7/14/2023 6:38 PM, Dmitry Baryshkov wrote:
>> On 15/07/2023 03:59, Jessica Zhang wrote:
>>>
>>>
>>> On 7/14/2023 3:30 PM, Dmitry Baryshkov wrote:
>>>> On Fri, 14 Jul 2023 at 22:03, Jessica Zhang 
>>>> <quic_jesszhan@quicinc.com> wrote:
>>>>>
>>>>>
>>>>>
>>>>> On 7/13/2023 6:23 PM, Dmitry Baryshkov wrote:
>>>>>> On 14/07/2023 03:21, Jessica Zhang wrote:
>>>>>>> DSI 6G v2.5.x+ and DPU 7.x+ support a data-bus widen mode that 
>>>>>>> allows DSI
>>>>>>> to send 48 bits of compressed data per pclk instead of 24.
>>>>>>>
>>>>>>> For all chipsets that support this mode, enable it whenever DSC is
>>>>>>> enabled as recommended by the hardware programming guide.
>>>>>>>
>>>>>>> Only enable this for command mode as we are currently unable to 
>>>>>>> validate
>>>>>>> it for video mode.
>>>>>>>
>>>>>>> Signed-off-by: Jessica Zhang <quic_jesszhan@quicinc.com>
>>>>>>> ---
>>>>>>> Note: The dsi.xml.h changes were generated using the headergen2 
>>>>>>> script in
>>>>>>> envytools [2], but the changes to the copyright and rules-ng-ng 
>>>>>>> source
>>>>>>> file
>>>>>>> paths were dropped.
>>>>>>
>>>>>> Separate commit please, so that it can be replaced by headers sync 
>>>>>> with
>>>>>> Mesa3d.
>>>>>
>>>>> Hi Dmitry,
>>>>>
>>>>> Acked.
>>>>>
>>>>>>
>>>>>>>
>>>>>>> [1] https://patchwork.freedesktop.org/series/120580/
>>>>>>> [2] https://github.com/freedreno/envytools/
>>>>>>>
>>>>>>> -- 
>>>>>>> Changes in v2:
>>>>>>> - Rebased on top of "drm/msm/dpu: Re-introduce dpu core revision"
>>>>>>> - Squashed all commits to avoid breaking feature if the series is 
>>>>>>> only
>>>>>>> partially applied
>>>>>>
>>>>>> No. Please unsquash it. Please design the series so that the patches
>>>>>> work even if it is only partially applied.
>>>>>
>>>>> Acked.
>>>>>
>>>>>>
>>>>>>> - Moved DATABUS_WIDEN bit setting to dsi_ctr_enable() (Marijn)
>>>>>>> - Have DPU check if wide bus is requested by output driver (Dmitry)
>>>>>>> - Introduced bytes_per_pclk variable for dsi_timing_setup() hdisplay
>>>>>>> adjustment (Marijn)
>>>>>>> - Link to v1:
>>>>>>> https://lore.kernel.org/r/20230525-add-widebus-support-v1-0-c7069f2efca1@quicinc.com
>>>>>>> ---
>>>>>>>    drivers/gpu/drm/msm/disp/dpu1/dpu_encoder.c        | 10 
>>>>>>> ++++++----
>>>>>>>    .../gpu/drm/msm/disp/dpu1/dpu_encoder_phys_cmd.c   |  4 +++-
>>>>>>>    drivers/gpu/drm/msm/disp/dpu1/dpu_hw_intf.c        |  3 +++
>>>>>>>    drivers/gpu/drm/msm/disp/dpu1/dpu_hw_intf.h        |  1 +
>>>>>>>    drivers/gpu/drm/msm/dsi/dsi.c                      |  5 +++++
>>>>>>>    drivers/gpu/drm/msm/dsi/dsi.h                      |  1 +
>>>>>>>    drivers/gpu/drm/msm/dsi/dsi.xml.h                  |  1 +
>>>>>>>    drivers/gpu/drm/msm/dsi/dsi_host.c                 | 23
>>>>>>> +++++++++++++++++++++-
>>>>>>>    drivers/gpu/drm/msm/msm_drv.h                      |  6 ++++++
>>>>>>>    9 files changed, 48 insertions(+), 6 deletions(-)
>>>>>>>
>>>>>>> diff --git a/drivers/gpu/drm/msm/disp/dpu1/dpu_encoder.c
>>>>>>> b/drivers/gpu/drm/msm/disp/dpu1/dpu_encoder.c
>>>>>>> index f0a2a1dca741..6aed63c06c1d 100644
>>>>>>> --- a/drivers/gpu/drm/msm/disp/dpu1/dpu_encoder.c
>>>>>>> +++ b/drivers/gpu/drm/msm/disp/dpu1/dpu_encoder.c
>>>>>>> @@ -2411,6 +2411,7 @@ struct drm_encoder *dpu_encoder_init(struct
>>>>>>> drm_device *dev,
>>>>>>>        struct dpu_kms *dpu_kms = to_dpu_kms(priv->kms);
>>>>>>>        struct drm_encoder *drm_enc = NULL;
>>>>>>>        struct dpu_encoder_virt *dpu_enc = NULL;
>>>>>>> +    int index = disp_info->h_tile_instance[0];
>>>>>>>        int ret = 0;
>>>>>>>        dpu_enc = devm_kzalloc(dev->dev, sizeof(*dpu_enc), 
>>>>>>> GFP_KERNEL);
>>>>>>> @@ -2439,13 +2440,14 @@ struct drm_encoder *dpu_encoder_init(struct
>>>>>>> drm_device *dev,
>>>>>>>        timer_setup(&dpu_enc->frame_done_timer,
>>>>>>>                dpu_encoder_frame_done_timeout, 0);
>>>>>>> -    if (disp_info->intf_type == INTF_DSI)
>>>>>>> +    if (disp_info->intf_type == INTF_DSI) {
>>>>>>>            timer_setup(&dpu_enc->vsync_event_timer,
>>>>>>>                    dpu_encoder_vsync_event_handler,
>>>>>>
>>>>>> While you are touching this part, could you please drop
>>>>>> dpu_encoder_vsync_event_handler() and
>>>>>> dpu_encoder_vsync_event_work_handler(), they are useless?
>>>>>
>>>>> Since these calls aren't related to widebus, I don't think I'll 
>>>>> include
>>>>> it in this series. However, I can post this cleanup as a separate 
>>>>> patch
>>>>> and add that as a dependency if that's ok.
>>>>
>>>> Sure, that will work for me. Thank you!
>>>>
>>>>>
>>>>>>
>>>>>>>                    0);
>>>>>>> -    else if (disp_info->intf_type == INTF_DP)
>>>>>>> -        dpu_enc->wide_bus_en = msm_dp_wide_bus_available(
>>>>>>> -                priv->dp[disp_info->h_tile_instance[0]]);
>>>>>>> +        dpu_enc->wide_bus_en =
>>>>>>> msm_dsi_is_widebus_enabled(priv->dsi[index]);
>>>>>>> +    } else if (disp_info->intf_type == INTF_DP) {
>>>>>>> +        dpu_enc->wide_bus_en =
>>>>>>> msm_dp_wide_bus_available(priv->dp[index]);
>>>>>>> +    }
>>>>>>>        INIT_DELAYED_WORK(&dpu_enc->delayed_off_work,
>>>>>>>                dpu_encoder_off_work);
>>>>>>> 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 df88358e7037..dace6168be2d 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
>>>>>>> @@ -69,8 +69,10 @@ static void 
>>>>>>> _dpu_encoder_phys_cmd_update_intf_cfg(
>>>>>>>                    phys_enc->hw_intf,
>>>>>>>                    phys_enc->hw_pp->idx);
>>>>>>> -    if (intf_cfg.dsc != 0)
>>>>>>> +    if (intf_cfg.dsc != 0) {
>>>>>>>            cmd_mode_cfg.data_compress = true;
>>>>>>> +        cmd_mode_cfg.wide_bus_en =
>>>>>>> dpu_encoder_is_widebus_enabled(phys_enc->parent);
>>>>>>> +    }
>>>>>>>        if (phys_enc->hw_intf->ops.program_intf_cmd_cfg)
>>>>>>>
>>>>>>> phys_enc->hw_intf->ops.program_intf_cmd_cfg(phys_enc->hw_intf,
>>>>>>> &cmd_mode_cfg);
>>>>>>> diff --git a/drivers/gpu/drm/msm/disp/dpu1/dpu_hw_intf.c
>>>>>>> b/drivers/gpu/drm/msm/disp/dpu1/dpu_hw_intf.c
>>>>>>> index 8ec6505d9e78..dc6f3febb574 100644
>>>>>>> --- a/drivers/gpu/drm/msm/disp/dpu1/dpu_hw_intf.c
>>>>>>> +++ b/drivers/gpu/drm/msm/disp/dpu1/dpu_hw_intf.c
>>>>>>> @@ -521,6 +521,9 @@ static void
>>>>>>> dpu_hw_intf_program_intf_cmd_cfg(struct dpu_hw_intf *ctx,
>>>>>>>        if (cmd_mode_cfg->data_compress)
>>>>>>>            intf_cfg2 |= INTF_CFG2_DCE_DATA_COMPRESS;
>>>>>>> +    if (cmd_mode_cfg->wide_bus_en)
>>>>>>> +        intf_cfg2 |= INTF_CFG2_DATABUS_WIDEN;
>>>>>>> +
>>>>>>>        DPU_REG_WRITE(&ctx->hw, INTF_CONFIG2, intf_cfg2);
>>>>>>>    }
>>>>>>> diff --git a/drivers/gpu/drm/msm/disp/dpu1/dpu_hw_intf.h
>>>>>>> b/drivers/gpu/drm/msm/disp/dpu1/dpu_hw_intf.h
>>>>>>> index 77f80531782b..c539025c418b 100644
>>>>>>> --- a/drivers/gpu/drm/msm/disp/dpu1/dpu_hw_intf.h
>>>>>>> +++ b/drivers/gpu/drm/msm/disp/dpu1/dpu_hw_intf.h
>>>>>>> @@ -50,6 +50,7 @@ struct dpu_hw_intf_status {
>>>>>>>    struct dpu_hw_intf_cmd_mode_cfg {
>>>>>>>        u8 data_compress;    /* enable data compress between dpu 
>>>>>>> and dsi */
>>>>>>> +    u8 wide_bus_en;        /* enable databus widen mode */
>>>>>>>    };
>>>>>>>    /**
>>>>>>> diff --git a/drivers/gpu/drm/msm/dsi/dsi.c
>>>>>>> b/drivers/gpu/drm/msm/dsi/dsi.c
>>>>>>> index baab79ab6e74..e3cc06c94397 100644
>>>>>>> --- a/drivers/gpu/drm/msm/dsi/dsi.c
>>>>>>> +++ b/drivers/gpu/drm/msm/dsi/dsi.c
>>>>>>> @@ -17,6 +17,11 @@ struct drm_dsc_config
>>>>>>> *msm_dsi_get_dsc_config(struct msm_dsi *msm_dsi)
>>>>>>>        return msm_dsi_host_get_dsc_config(msm_dsi->host);
>>>>>>>    }
>>>>>>> +bool msm_dsi_is_widebus_enabled(struct msm_dsi *msm_dsi)
>>>>>>> +{
>>>>>>> +    return msm_dsi_host_is_widebus_supported(msm_dsi->host);
>>>>>>
>>>>>> This is incorrect. It will enable widebus even for non-DSC cases.
>>>>>
>>>>> FWIW, all calls for msm_dsi_is_widebus_enabled() and
>>>>> msm_dsi_host_is_widebus_supported() are guarded by a DSC check.
>>>>>
>>>>> That being said, I also see your point that 
>>>>> msm_dsi_is_widebus_enabled()
>>>>> is an incorrect name since this will only check if widebus is 
>>>>> supported.
>>>>>
>>>>> Maybe a better change would be to change msm_dsi_is_widebus_enabled to
>>>>> *_is_widebus_supported(), move the setting of dpu_enc->wide_bus_en for
>>>>> both DP and DSI to dpu_encoder_virt_atomic_enable(), then for DSI set
>>>>> wide_bus_en = dpu_enc->dsc && dsi_is_widebus_supported().
>>>>
>>>> I think we should change msm_dp_wide_bus_available() to
>>>> msm_dp_wide_bus_enabled(). We don't have a way to tell DP (or DSI) if
>>>> widebus really should be enabled or not. So it would be better to make
>>>> DP and DSI drivers provide is_widebus_enabled function.
>>>
>>> I think making changes to the DP API is outside the scope of this 
>>> series since these changes are focused on supporting widebus for DSI 
>>> specifically.
>>
>> Sure, the DP can be handled separately, if we agree on the API.
>>
>>>
>>>>
>>>>>
>>>>>>
>>>>>>> +}
>>>>>>> +
>>>>>>>    static int dsi_get_phy(struct msm_dsi *msm_dsi)
>>>>>>>    {
>>>>>>>        struct platform_device *pdev = msm_dsi->pdev;
>>>>>>> diff --git a/drivers/gpu/drm/msm/dsi/dsi.h
>>>>>>> b/drivers/gpu/drm/msm/dsi/dsi.h
>>>>>>> index bd3763a5d723..219a9f756759 100644
>>>>>>> --- a/drivers/gpu/drm/msm/dsi/dsi.h
>>>>>>> +++ b/drivers/gpu/drm/msm/dsi/dsi.h
>>>>>>> @@ -134,6 +134,7 @@ int dsi_calc_clk_rate_6g(struct msm_dsi_host
>>>>>>> *msm_host, bool is_bonded_dsi);
>>>>>>>    void msm_dsi_host_snapshot(struct msm_disp_state *disp_state, 
>>>>>>> struct
>>>>>>> mipi_dsi_host *host);
>>>>>>>    void msm_dsi_host_test_pattern_en(struct mipi_dsi_host *host);
>>>>>>>    struct drm_dsc_config *msm_dsi_host_get_dsc_config(struct
>>>>>>> mipi_dsi_host *host);
>>>>>>> +bool msm_dsi_host_is_widebus_supported(struct mipi_dsi_host *host);
>>>>>>>    /* dsi phy */
>>>>>>>    struct msm_dsi_phy;
>>>>>>> diff --git a/drivers/gpu/drm/msm/dsi/dsi.xml.h
>>>>>>> b/drivers/gpu/drm/msm/dsi/dsi.xml.h
>>>>>>> index a4a154601114..2a7d980e12c3 100644
>>>>>>> --- a/drivers/gpu/drm/msm/dsi/dsi.xml.h
>>>>>>> +++ b/drivers/gpu/drm/msm/dsi/dsi.xml.h
>>>>>>> @@ -664,6 +664,7 @@ static inline uint32_t
>>>>>>> DSI_CMD_MODE_MDP_CTRL2_INPUT_RGB_SWAP(enum dsi_rgb_swap v
>>>>>>>        return ((val) << 
>>>>>>> DSI_CMD_MODE_MDP_CTRL2_INPUT_RGB_SWAP__SHIFT) &
>>>>>>> DSI_CMD_MODE_MDP_CTRL2_INPUT_RGB_SWAP__MASK;
>>>>>>>    }
>>>>>>>    #define DSI_CMD_MODE_MDP_CTRL2_BURST_MODE            0x00010000
>>>>>>> +#define DSI_CMD_MODE_MDP_CTRL2_DATABUS_WIDEN            0x00100000
>>>>>>>    #define REG_DSI_CMD_MODE_MDP_STREAM2_CTRL            0x000001b8
>>>>>>>    #define DSI_CMD_MODE_MDP_STREAM2_CTRL_DATA_TYPE__MASK 0x0000003f
>>>>>>> diff --git a/drivers/gpu/drm/msm/dsi/dsi_host.c
>>>>>>> b/drivers/gpu/drm/msm/dsi/dsi_host.c
>>>>>>> index 645927214871..6ea3476acf0d 100644
>>>>>>> --- a/drivers/gpu/drm/msm/dsi/dsi_host.c
>>>>>>> +++ b/drivers/gpu/drm/msm/dsi/dsi_host.c
>>>>>>> @@ -710,6 +710,14 @@ static void dsi_ctrl_disable(struct 
>>>>>>> msm_dsi_host
>>>>>>> *msm_host)
>>>>>>>        dsi_write(msm_host, REG_DSI_CTRL, 0);
>>>>>>>    }
>>>>>>> +bool msm_dsi_host_is_widebus_supported(struct mipi_dsi_host *host)
>>>>>>> +{
>>>>>>> +    struct msm_dsi_host *msm_host = to_msm_dsi_host(host);
>>>>>>> +
>>>>>>> +    return msm_host->cfg_hnd->major == MSM_DSI_VER_MAJOR_6G &&
>>>>>>> +            msm_host->cfg_hnd->minor >= 
>>>>>>> MSM_DSI_6G_VER_MINOR_V2_5_0;
>>>>>>
>>>>>> Would it be better to push it to the config data, like we did for DP?
>>>>>
>>>>> I don't think so -- I think adding it to the config data as a feature
>>>>> flag would bloat dsi_cfg.c. It would be simpler and cleaner (IMO) to
>>>>> keep this as a version check.
>>>>
>>>> Ok.
>>>>
>>>>>
>>>>>>
>>>>>>> +}
>>>>>>> +
>>>>>>>    static void dsi_ctrl_enable(struct msm_dsi_host *msm_host,
>>>>>>>                struct msm_dsi_phy_shared_timings 
>>>>>>> *phy_shared_timings,
>>>>>>> struct msm_dsi_phy *phy)
>>>>>>>    {
>>>>>>> @@ -757,6 +765,11 @@ static void dsi_ctrl_enable(struct msm_dsi_host
>>>>>>> *msm_host,
>>>>>>>                msm_host->cfg_hnd->minor >= 
>>>>>>> MSM_DSI_6G_VER_MINOR_V1_3) {
>>>>>>>                data = dsi_read(msm_host, 
>>>>>>> REG_DSI_CMD_MODE_MDP_CTRL2);
>>>>>>>                data |= DSI_CMD_MODE_MDP_CTRL2_BURST_MODE;
>>>>>>> +
>>>>>>> +            /* TODO: Allow for video-mode support once 
>>>>>>> tested/fixed */
>>>>>>> +            if (msm_host->cfg_hnd->minor >=
>>>>>>> MSM_DSI_6G_VER_MINOR_V2_5_0 && msm_host->dsc)
>>>>>>
>>>>>> msm_dsi_is_widebus_enabled() && msm_host->dsc
>>>>>
>>>>> *_is_widebus_enabled() also checks for major version >= 6G, so calling
>>>>> it here would be a bit redundant as we're already checking for that 
>>>>> earlier.
>>>>
>>>> However now you have different checks in two different places.
>>>>
>>>> My suggestion is to have the function msm_dsi_is_widebus_supported(),
>>>> which the rest of the code uses to check if wide bus is actually
>>>> enabled in the HW. If at some point DSI 2.11 drops wide bus support,
>>>> I'd like to see a change at a single place, rather than having changes
>>>> all over the code.
>>>
>>> Ok, I think this is a fair point. The main reason I just had the 
>>> minor version check here was to avoid the redundant major version 
>>> check from the BURST_MODE conditional [1].
>>>
>>> What if we just move the MDP_CTRL2 configuration to a separate 
>>> function and do the BURST_MODE and DATABUS_WIDEN checks separately?
>>
>> I think that a function that just writes MDP_CTRL2 is an overkill. 
>> Moving video / CMD setup to separate functions to be called from 
>> dsi_ctrl_enable sounds like a better idea.
> 
> Got it -- will have a separate mdp_ctrl2_config_cmd_mode() helper.

No. First, it's not MDP, it's DSI. And then I especially poited out that
'a function that just writes MDP_CTRL2' is an overkill.

So, if you want a separate helper, `dsi_ctrl_enable_cmd_mode()` sounds 
better.

> 
>>
>>>
>>> So, something like:
>>>
>>> ```
>>> bool burst_supported()
>>> {
>>>      return major >= 6g && minor >= 1.3;
>>> }
>>>
>>>
>>> bool widebus_supported()
>>> {
>>>      return major >= 6g && minor >= 2.5;
>>> }
>>>
>>>
>>> void program_cmd_mdp_ctrl2()
>>> {
>>>      if (major < 6g)
>>>          return;
>>>
>>>      data = read(mdp_ctrl2);
>>>      if (burst_supported)
>>>          data |= burst;
>>>      if (widebus_supported)
>>
>> Two issues with this piece of code.
>> First, you are splitting the 6g and minor checks. Please don't do 
>> this. If might be obvious now, but it is better to keep full condition 
>> in one place, even if it looks redundant to you. The compiler will do 
>> its jobs and make sure that all expressions are evaluated only once. 
>> And this is especially true for widebus, since the driver checks this 
>> condition several times.
> 
> The reason the major and minor checks are split here is because 
> MDP_CTRL2 is only present in DSI 6G.
> 
>>
>> Second problem (which is quite specific to widebus): in this fragment 
>> of code you say that if widebus is supported, it should be enabled. 
>> But it is not true: we should not be enabling widebus if this is not a 
>> DSC mode. So, I will be insisting on is_enabled() kind of function.
> 
> I can add a DSC check here, but have some concerns about the 
> *_is_enabled() function. Please refer to my response below.
> 
>>
>>>          data |= widebus;
>>>
>>>      write(mdp_ctrl2, data);
>>> }
>>>
>>> void dsi_ctrl_enable()
>>> {
>>>
>>>      ...
>>>
>>>      program_cmd_mdp_ctrl2();
>>>
>>>      ...
>>> }
>>>
>>> ```
>>>
>>>>
>>>> Likewise I'd like to have the function msm_dsi_is_widebus_enabled(),
>>>> which is used by the rest of the code to check if the widebus should
>>>> be actually enabled.
>>>>
>>>> Actually I think we can even drop the is_supported at all() and use
>>>> the following code:
>>>>
>>>> bool msm_dsi_is_widebus_enabled()
>>>> {
>>>>     if (major < 6G || minor < V2_5_0)
>>>>       return false;
>>>>
>>>>     return !!msm_host->dsc;
>>>
>>> I'd rather keep this API as *_is_widebus_supported() or 
>>> *_is_widebus_available() (if you'd like it to match DP) and have the 
>>> DSC check outside.
>>>
>>> This is because we already guard for DSC in DSI for the 
>>> dsi_timing_setup() hdisplay adjustments and for the DPU hw_intf 
>>> widebus register configuration. The only place where a DSC check 
>>> needed to be added was for the MDP_CTRL2 configuration.
>>>
>>> So, since most places can already use preexisting DSC checks, having 
>>> another DSC check within msm_dsi_is_widebus_enabled() would either be 
>>> redundant or require extensive refactoring to justify adding the 
>>> check here.
>>
>> As I wrote, I'd prefer using a single is_enabled() function all over 
>> the code to optimizing / removing redundancy in C expressions. If for 
>> some reason the HPG suggests enabling widebus for non-DSC modes (or 
>> e.g. disabling it for yuv4:2:0 DSC), I do not want to have this check 
>> scattered between different places (with a high likelyhood of missing 
>> the most obvious place).
>>
>> This is the place where DSI differs from DP. For DP and widebus it was 
>> really simple: available => enabled.
> 
> Sorry if I didn't make this clear enough earlier, but I think the issue 
> with having a single *_is_widebus_enabled() function is that it will 
> cause redundant checks -- specifically in dsi_timing_setup() where the 
> DSC hdisplay adjustment (that depends on widebus) is already wrapped in 
> an `if (msm_host->dsc)` check. We'd either have to find a workaround to 
> avoid having a redundant `if (dsc)` check, or do a minor version check 
> for this instance.

Again, I don't have an issue with such kind of checks. Please don't try 
to do the job of the compiler.

> 
> In addition, *_is_widebus_enabled() would only work under the assumption 
> that DSC (if supported) will always be enabled during bootup for DSI. If 
> there ends up being support for a panel that allows for DSC to be 
> enabled on the fly after bootup, the proposed *_is_widebus_enabled() 
> check would break since the assumption that DSC is always enabled on 
> bootup would not be true anymore.

Why is that going to break?

-- 
With best wishes
Dmitry