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Message-ID: <b47e3be7-7de1-0f0c-8aa6-054e99dcaab3@linaro.org>
Date: Wed, 5 Oct 2022 01:40:12 +0300
From: Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
To: Marijn Suijten <marijn.suijten@...ainline.org>,
phone-devel@...r.kernel.org, Rob Clark <robdclark@...il.com>,
Vinod Koul <vkoul@...nel.org>,
~postmarketos/upstreaming@...ts.sr.ht,
AngeloGioacchino Del Regno
<angelogioacchino.delregno@...ainline.org>,
Konrad Dybcio <konrad.dybcio@...ainline.org>,
Martin Botka <martin.botka@...ainline.org>,
Jami Kettunen <jami.kettunen@...ainline.org>,
David Airlie <airlied@...ux.ie>,
Daniel Vetter <daniel@...ll.ch>,
Abhinav Kumar <quic_abhinavk@...cinc.com>,
Sean Paul <sean@...rly.run>,
Thomas Zimmermann <tzimmermann@...e.de>,
Javier Martinez Canillas <javierm@...hat.com>,
Alex Deucher <alexander.deucher@....com>,
Douglas Anderson <dianders@...omium.org>,
Vladimir Lypak <vladimir.lypak@...il.com>,
dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
linux-arm-msm@...r.kernel.org, freedreno@...ts.freedesktop.org,
Marek Vasut <marex@...x.de>
Subject: Re: [PATCH 3/5] drm/msm/dsi: Account for DSC's bits_per_pixel having
4 fractional bits
On 05/10/2022 01:35, Marijn Suijten wrote:
> On 2022-10-04 17:45:50, Dmitry Baryshkov wrote:
>> On Sat, 1 Oct 2022 at 22:08, Marijn Suijten
>> <marijn.suijten@...ainline.org> wrote:
>> [..]
>>> - bytes_in_slice = DIV_ROUND_UP(dsc->slice_width * dsc->bits_per_pixel, 8);
>>> + bytes_in_slice = DIV_ROUND_UP(dsc->slice_width * bpp, 8);
>>
>>
>> bytes_in_slice = DIV_ROUND_UP(dsc->slice_width * dsc->bits_per_pixel, 8 * 16); ?
>
> Not necessarily a fan of this, it "hides" the fact that we are dealing
> with 4 fractional bits (1/16th precision, it is correct though); but
> since this is the only use of `bpp` I can change it and document this
> fact wiht a comment on top (including referencing the validation pointed
> out in dsi_populate_dsc_params()).
>
> Alternatively we can inline the `>> 4` here?
No, I don't think so. If we shift by 4 bits, we'd loose the fractional
part. DIV_ROUND_UP( .... , 8 * 16) ensures that we round it up rather
than just dropping it.
>
>>>
>>> dsc->slice_chunk_size = bytes_in_slice;
>>>
>>> @@ -913,6 +918,7 @@ static void dsi_timing_setup(struct msm_dsi_host *msm_host, bool is_bonded_dsi)
>>> u32 va_end = va_start + mode->vdisplay;
>>> u32 hdisplay = mode->hdisplay;
>>> u32 wc;
>>> + int ret;
>>>
>>> DBG("");
>>>
>>> @@ -948,7 +954,9 @@ static void dsi_timing_setup(struct msm_dsi_host *msm_host, bool is_bonded_dsi)
>>> /* we do the calculations for dsc parameters here so that
>>> * panel can use these parameters
>>> */
>>> - dsi_populate_dsc_params(dsc);
>>> + ret = dsi_populate_dsc_params(dsc);
>>> + if (ret)
>>> + return;
>>>
>>> /* Divide the display by 3 but keep back/font porch and
>>> * pulse width same
>>> @@ -1229,6 +1237,10 @@ static int dsi_cmd_dma_add(struct msm_dsi_host *msm_host,
>>> if (packet.size < len)
>>> memset(data + packet.size, 0xff, len - packet.size);
>>>
>>> + if (msg->type == MIPI_DSI_PICTURE_PARAMETER_SET)
>>> + print_hex_dump(KERN_DEBUG, "ALL:", DUMP_PREFIX_NONE,
>>> + 16, 1, data, len, false);
>>> +
>>> if (cfg_hnd->ops->tx_buf_put)
>>> cfg_hnd->ops->tx_buf_put(msm_host);
>>>
>>> @@ -1786,6 +1798,12 @@ static int dsi_populate_dsc_params(struct drm_dsc_config *dsc)
>>> int data;
>>> int final_value, final_scale;
>>> int i;
>>> + int bpp = dsc->bits_per_pixel >> 4;
>>> +
>>> + if (dsc->bits_per_pixel & 0xf) {
>>> + pr_err("DSI does not support fractional bits_per_pixel\n");
>>> + return -EINVAL;
>>> + }
>>>
>>> dsc->rc_model_size = 8192;
>>> dsc->first_line_bpg_offset = 12;
>>> @@ -1807,7 +1825,7 @@ static int dsi_populate_dsc_params(struct drm_dsc_config *dsc)
>>> }
>>>
>>> dsc->initial_offset = 6144; /* Not bpp 12 */
>>> - if (dsc->bits_per_pixel != 8)
>>> + if (bpp != 8)
>>> dsc->initial_offset = 2048; /* bpp = 12 */
>>>
>>> mux_words_size = 48; /* bpc == 8/10 */
>>> @@ -1830,16 +1848,16 @@ static int dsi_populate_dsc_params(struct drm_dsc_config *dsc)
>>> * params are calculated
>>> */
>>> groups_per_line = DIV_ROUND_UP(dsc->slice_width, 3);
>>> - dsc->slice_chunk_size = dsc->slice_width * dsc->bits_per_pixel / 8;
>>> - if ((dsc->slice_width * dsc->bits_per_pixel) % 8)
>>> + dsc->slice_chunk_size = dsc->slice_width * bpp / 8;
>>> + if ((dsc->slice_width * bpp) % 8)
>>
>> One can use fixed point math here too:
>>
>> dsc->slice_chunk_size = (dsc->slice_width * dsc->bits_per_pixel + 8 *
>> 16 - 1)/ (8 * 16);
>
> Good catch, this is effectively a DIV_ROUND_UP() that we happened to
> call bytes_in_slice above...
>
> Shall I tackle this in the same patch, or insert another cleanup patch?
It's up to you. I usually prefer separate patches, even if just to ease
bisecting between unrelated changes.
>
> In fact dsi_populate_dsc_params() is called first (this comment),
> followed by dsi_update_dsc_timing(), meaning that slice_chunk_size is
> already provided and shouldn't be recomputed.
>
>>> dsc->slice_chunk_size++;
>>>
>>> /* rbs-min */
>>> min_rate_buffer_size = dsc->rc_model_size - dsc->initial_offset +
>>> - dsc->initial_xmit_delay * dsc->bits_per_pixel +
>>> + dsc->initial_xmit_delay * bpp +
>>> groups_per_line * dsc->first_line_bpg_offset;
>>>
>>> - hrd_delay = DIV_ROUND_UP(min_rate_buffer_size, dsc->bits_per_pixel);
>>> + hrd_delay = DIV_ROUND_UP(min_rate_buffer_size, bpp);
>>>
>>> dsc->initial_dec_delay = hrd_delay - dsc->initial_xmit_delay;
>>>
>>> @@ -1862,7 +1880,7 @@ static int dsi_populate_dsc_params(struct drm_dsc_config *dsc)
>>> data = 2048 * (dsc->rc_model_size - dsc->initial_offset + num_extra_mux_bits);
>>> dsc->slice_bpg_offset = DIV_ROUND_UP(data, groups_total);
>>>
>>> - target_bpp_x16 = dsc->bits_per_pixel * 16;
>>> + target_bpp_x16 = bpp * 16;
>>>
>>> data = (dsc->initial_xmit_delay * target_bpp_x16) / 16;
>>
>> It looks like this can be replaced with the direct multiplication
>> instead, maybe with support for overflow/rounding.
>
> Thanks, Abhinav pointed out the same in patch 1/5 which originally
> cleaned up most - but apparently not all! - of the math here. I don't
> think this value should ever overlow, nor does this `* 16 / 16` have any
> effect on rounding (that'd be `/ 16 * 16`).
Ack
>
>>> final_value = dsc->rc_model_size - data + num_extra_mux_bits;
>>> --
>>> 2.37.3
>>>
>>
>>
>> --
>> With best wishes
>> Dmitry
--
With best wishes
Dmitry
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