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Message-ID: <f11fc3c1-4b19-3186-9c56-b1bac221cf83@quicinc.com>
Date:   Thu, 8 Jun 2023 18:09:57 -0700
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Subject: Re: [Freedreno] [PATCH v5 2/5] drm/msm/dsi: Adjust pclk rate for
 compression
Content-Language: en-US
To:     Jessica Zhang <quic_jesszhan@quicinc.com>,
        Marijn Suijten <marijn.suijten@somainline.org>
CC:     <freedreno@lists.freedesktop.org>, Sean Paul <sean@poorly.run>,
        <dri-devel@lists.freedesktop.org>, <linux-kernel@vger.kernel.org>,
        "Konrad Dybcio" <konrad.dybcio@linaro.org>,
        Rob Clark <robdclark@gmail.com>,
        "Daniel Vetter" <daniel@ffwll.ch>, <linux-arm-msm@vger.kernel.org>,
        Dmitry Baryshkov <dmitry.baryshkov@linaro.org>,
        David Airlie <airlied@gmail.com>
References: <20230405-add-dsc-support-v5-0-028c10850491@quicinc.com>
 <20230405-add-dsc-support-v5-2-028c10850491@quicinc.com>
 <js3mcglahq53mcyxa6deltjlu4xpc2pnafwz2rbk3dl4ovws2o@5xw2wzvfaj2v>
 <f63435ca-933a-52be-d879-1d9cc9441107@quicinc.com>
From:   Abhinav Kumar <quic_abhinavk@quicinc.com>
In-Reply-To: <f63435ca-933a-52be-d879-1d9cc9441107@quicinc.com>
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On 6/8/2023 5:56 PM, Jessica Zhang wrote:
> 
> 
> On 6/8/2023 1:36 PM, Marijn Suijten wrote:
>> Same title suggestion as earlier: s/adjust/reduce
> 
> Hi Marijn,
> 
> Acked.
> 
>>
>> On 2023-05-22 18:08:56, Jessica Zhang wrote:
>>> Adjust the pclk rate to divide hdisplay by the compression ratio when 
>>> DSC
>>> is enabled.
>>>
>>> Signed-off-by: Jessica Zhang <quic_jesszhan@quicinc.com>
>>> ---
>>>   drivers/gpu/drm/msm/dsi/dsi_host.c | 21 ++++++++++++++++++---
>>>   1 file changed, 18 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/drivers/gpu/drm/msm/dsi/dsi_host.c 
>>> b/drivers/gpu/drm/msm/dsi/dsi_host.c
>>> index a448931af804..88f370dd2ea1 100644
>>> --- a/drivers/gpu/drm/msm/dsi/dsi_host.c
>>> +++ b/drivers/gpu/drm/msm/dsi/dsi_host.c
>>> @@ -561,7 +561,18 @@ void dsi_link_clk_disable_v2(struct msm_dsi_host 
>>> *msm_host)
>>>       clk_disable_unprepare(msm_host->byte_clk);
>>>   }
>>> -static unsigned long dsi_get_pclk_rate(const struct drm_display_mode 
>>> *mode, bool is_bonded_dsi)
>>> +static unsigned long dsi_adjust_compressed_pclk(const struct 
>>> drm_display_mode *mode,
>>
>> Nit: adjust_pclk_for_compression
> 
> Acked.
> 
>>
>> As discussed before we realized that this change is more-or-less a hack,
>> since downstream calculates pclk quite differently - at least for
>> command-mode panels.  Do you still intend to land this patch this way,
>> or go the proper route by introducing the right math from the get-go?
>> Or is the math at least correct for video-mode panels?
> 
> Sorry but can you please clarify what exactly is incorrect or different 
> about this math when compared to downstream? And, if you think that this 
> math is incorrect, what exactly has to be changed to make it the "right 
> math"?
> 

Agree with Jessica, just calling the math a hack without explaining why 
you think it is so is not justified especially when a great detail of 
explanation was given on the bug. Sorry but its a bit harsh on the 
developers.

> We've already shown step-by-step [1] not only how the resulting pclk 
> from the downstream code matches out upstream calculations, but also how 
> the inclusion of porches in the upstream math would make up for the fact 
> that upstream has no concept of transfer time [2].
> 
> If the lack of transfer time in the upstream math is the issue, I 
> believe that that's not within the scope of this series, as transfer 
> time is not something specific to DSC.
> 
> Moreover, as stated in an earlier revision [3], there is no way to 
> validate DSC over DSI for video mode. As far as I know, we do not have a 
> way to validate video mode datapath for DSI in general.
> 
> [1] https://gitlab.freedesktop.org/drm/msm/-/issues/24#note_1936144
> [2] https://gitlab.freedesktop.org/drm/msm/-/issues/24#note_1945792
> [3] 
> https://patchwork.freedesktop.org/patch/535117/?series=117219&rev=1#comment_970492
> 
>>
>> This function requires a documentation comment to explain that all.
>>
>>> +        const struct drm_dsc_config *dsc)
>>> +{
>>> +    int new_hdisplay = DIV_ROUND_UP(mode->hdisplay * 
>>> drm_dsc_get_bpp_int(dsc),
>>
>> This sounds like a prime candidate for msm_dsc_get_bytes_per_line(), if
>> bits_per_component==8 is assumed.  In fact, it then becomes identical
>> to the following line in dsi_host.c which you added previously:
>>
>>     hdisplay = DIV_ROUND_UP(msm_dsc_get_bytes_per_line(msm_host->dsc), 
>> 3);
>>
>> If not, what is the difference between these two calculations?  Maybe
>> they both need to be in a properly-named helper.
> 
> While the math technically matches up (assuming, also, that 
> mode->hdisplay == slice_width * slice_count for all cases), there are 
> conceptual differences between the pclk and hdisplay calculations.
> 
> Just to reiterate what was already said on IRC:
> 
> In the pclk calculation, we're multiplying pclk by the compression ratio 
> (which would be target_bpp / src_bpp) -- please note that here, we 
> calculate src_bpp by doing bpc * 3.
> 
> In the hdisplay calculation, we calculate the bytes per line and divide 
> by 3 (bytes) to account for the fact that we can only process 3 bytes 
> per pclk cycle.
> 
> So while I understand why you would want to put this behind a shared 
> helper, I think abstracting the pclk and hdisplay math away would 
> obfuscate the conceptual difference between the 2 calculations.
> 
>>
>>> +            dsc->bits_per_component * 3);
>>
>> As we established in the drm/msm issue [2] there is currently a
>> confusion whether this /3 (and the /3 in dsi_timing_setup) come from the
>> ratio between dsi_get_bpp() and dsc->bpp or something else.  Can you
>> clarify that with constants and comments?
> 
> Sure, we are planning to add a patch to the end of this series 
> documenting the math.
> 
>>
>> [2]: https://gitlab.freedesktop.org/drm/msm/-/issues/24
>>
>>> +
>>> +    return (new_hdisplay + (mode->htotal - mode->hdisplay))
>>> +            * mode->vtotal * drm_mode_vrefresh(mode);
>>
>> As clarified in [1] I was not necessarily suggesting to move this math
>> to a separate helper, but to also use a few more properly-named
>> intermediate variables to not have multi-line math and self-documenting
>> code.  These lines could be split to be much more clear.
> 
> Acked.
> 
>>
>> [1]: 
>> https://lore.kernel.org/linux-arm-msm/u4x2vldkzsokfcpbkz3dtwcllbdk4ljcz6kzuaxt5frx6g76o5@uku6abewvye7/
>>
>>> +}
>>> +
>>> +static unsigned long dsi_get_pclk_rate(const struct drm_display_mode 
>>> *mode,
>>> +        const struct drm_dsc_config *dsc, bool is_bonded_dsi)
>>>   {
>>>       unsigned long pclk_rate;
>>> @@ -576,6 +587,10 @@ static unsigned long dsi_get_pclk_rate(const 
>>> struct drm_display_mode *mode, bool
>>>       if (is_bonded_dsi)
>>>           pclk_rate /= 2;
>>> +    /* If DSC is enabled, divide hdisplay by compression ratio */
>>> +    if (dsc)
>>> +        pclk_rate = dsi_adjust_compressed_pclk(mode, dsc);
>>
>> The confusion with this comment (and the reason the aforementioned
>> discussion [2] carried on so long) stems from the fact a division makes
>> sense for a bit/byte clock, but not for a pixel clock: we still intend
>> to send the same number of pixels, just spending less bytes on them.  So
>> as you clarify the /3 above, can you also clarify that here or drop this
>> comment completely when the function is correctly documented instead?
> 
> Acked.
> 
> Thanks,
> 
> Jessica Zhang
> 
>>
>> - Marijn
>>
>>> +
>>>       return pclk_rate;
>>>   }
>>> @@ -585,7 +600,7 @@ unsigned long dsi_byte_clk_get_rate(struct 
>>> mipi_dsi_host *host, bool is_bonded_d
>>>       struct msm_dsi_host *msm_host = to_msm_dsi_host(host);
>>>       u8 lanes = msm_host->lanes;
>>>       u32 bpp = dsi_get_bpp(msm_host->format);
>>> -    unsigned long pclk_rate = dsi_get_pclk_rate(mode, is_bonded_dsi);
>>> +    unsigned long pclk_rate = dsi_get_pclk_rate(mode, msm_host->dsc, 
>>> is_bonded_dsi);
>>>       unsigned long pclk_bpp;
>>>       if (lanes == 0) {
>>> @@ -604,7 +619,7 @@ unsigned long dsi_byte_clk_get_rate(struct 
>>> mipi_dsi_host *host, bool is_bonded_d
>>>   static void dsi_calc_pclk(struct msm_dsi_host *msm_host, bool 
>>> is_bonded_dsi)
>>>   {
>>> -    msm_host->pixel_clk_rate = dsi_get_pclk_rate(msm_host->mode, 
>>> is_bonded_dsi);
>>> +    msm_host->pixel_clk_rate = dsi_get_pclk_rate(msm_host->mode, 
>>> msm_host->dsc, is_bonded_dsi);
>>>       msm_host->byte_clk_rate = 
>>> dsi_byte_clk_get_rate(&msm_host->base, is_bonded_dsi,
>>>                               msm_host->mode);
>>>
>>> -- 
>>> 2.40.1
>>>