Date:   Fri, 9 Jun 2023 10:24:27 -0700
From:   Jessica Zhang <quic_jesszhan@...cinc.com>
To:     Dmitry Baryshkov <dmitry.baryshkov@...aro.org>,
        Marijn Suijten <marijn.suijten@...ainline.org>
CC:     Rob Clark <robdclark@...il.com>,
        Abhinav Kumar <quic_abhinavk@...cinc.com>,
        Sean Paul <sean@...rly.run>, David Airlie <airlied@...il.com>,
        Daniel Vetter <daniel@...ll.ch>,
        Konrad Dybcio <konrad.dybcio@...aro.org>,
        <linux-arm-msm@...r.kernel.org>, <dri-devel@...ts.freedesktop.org>,
        <freedreno@...ts.freedesktop.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v5 2/5] drm/msm/dsi: Adjust pclk rate for compression



On 6/9/2023 9:58 AM, Dmitry Baryshkov wrote:
> On 08/06/2023 23:36, Marijn Suijten wrote:
>> Same title suggestion as earlier: s/adjust/reduce
>>
>> 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@...cinc.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
>>
>> 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?
> 
> Can we please postpone the cmd-vs-video discussion? Otherwise I will 
> reserve myself a right to push a patch dropping CMD mode support until 
> somebody comes with a proper way to handle CMD clock calculation.
> 
> 
> It is off-topic for the sake of DSC 1.2 support. Yes, all CMD panel 
> timings are a kind of a hack and should be improved. No, we can not do 
> this as a part of this series. I think everybody agrees that with the 
> current way of calculating CMD panel timings, this function does some 
> kind of a trick.
> 
>>
>> 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);
> 
> This would imply a simple /3, but as far as I understand it is not 
> correct here.
> 
>>
>> If not, what is the difference between these two calculations?  Maybe
>> they both need to be in a properly-named helper.
>>
>>> +            dsc->bits_per_component * 3);
> 
> I hope to see a documentation patch to be posted, telling that this 
> scales hdisplay and thus pclk by the factor of compressed_bpp / 
> uncompressed_bpp.
> 
> This is not how it is usually done, but I would accept a separate 
> documentation patch going over the calculation here and in 
> dsi_timing_setup (and maybe other unobvious cases, if there is anything 
> left).
> 
>>
>> 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?
>>
>> [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.
> 
> I think it's fine more or less. One pair of parenthesis is unnecessary, 
> but that's mostly it. Maybe `new_htotal' variable would make some sense.
> 
> Also, please excuse me if this was discussed somewhere. This calculation 
> means that only the displayed data is compressed, but porches are not 
> touched. Correct?

Hi Dmitry,

Correct, we will apply the compression ratio to only the hdisplay but 
keep the porches as is.

> 
>>
>> [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);
> 
> Looking for the perfection, I'd also move the pclk adjustment to come 
> before the is_bonded_dsi check.

Acked.

Thanks,

Jessica Zhang

> 
>>
>> 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?
>>
>> - 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
>>>
> 
> -- 
> With best wishes
> Dmitry
> 

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