linux-kernel - Re: [PATCH v3 5/5] drm/msm/dsi: Remove incorrect references to slice

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Date:   Fri, 19 May 2023 23:24:26 +0200
From:   Marijn Suijten <marijn.suijten@...ainline.org>
To:     Jessica Zhang <quic_jesszhan@...cinc.com>
Cc:     Rob Clark <robdclark@...il.com>,
        Abhinav Kumar <quic_abhinavk@...cinc.com>,
        Dmitry Baryshkov <dmitry.baryshkov@...aro.org>,
        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 v3 5/5] drm/msm/dsi: Remove incorrect references to
 slice_count

On 2023-05-19 14:17:30, Jessica Zhang wrote:
> Currently, slice_count is being used to calculate word count and
> pkt_per_line. In downstream, these values are calculated using slice per
> packet, which is not the same as slice_count.
> 
> Slice count represents the number of soft slices per interface, and its
> value will not always match that of slice per packet. For example, it is
> possible to have cases where there are multiple soft slices per interface
> but the panel specifies only one slice per packet.
> 
> Thus, use the default value of one slice per packet and remove slice_count
> from the aforementioned calculations.
> 
> Fixes: 08802f515c3c ("drm/msm/dsi: Add support for DSC configuration")
> Fixes: bc6b6ff8135c ("drm/msm/dsi: Use DSC slice(s) packet size to compute word count")
> Signed-off-by: Jessica Zhang <quic_jesszhan@...cinc.com>
> ---
>  drivers/gpu/drm/msm/dsi/dsi_host.c | 24 ++++++++++++++----------
>  1 file changed, 14 insertions(+), 10 deletions(-)
> 
> diff --git a/drivers/gpu/drm/msm/dsi/dsi_host.c b/drivers/gpu/drm/msm/dsi/dsi_host.c
> index d04f8bbd707d..8c8858ee59ec 100644
> --- a/drivers/gpu/drm/msm/dsi/dsi_host.c
> +++ b/drivers/gpu/drm/msm/dsi/dsi_host.c
> @@ -866,18 +866,15 @@ static void dsi_update_dsc_timing(struct msm_dsi_host *msm_host, bool is_cmd_mod
>  	 */
>  	slice_per_intf = msm_dsc_get_slices_per_intf(dsc, hdisplay);
>  
> -	/*
> -	 * If slice_count is greater than slice_per_intf
> -	 * then default to 1. This can happen during partial
> -	 * update.
> -	 */
> -	if (dsc->slice_count > slice_per_intf)
> -		dsc->slice_count = 1;
> -
>  	total_bytes_per_intf = dsc->slice_chunk_size * slice_per_intf;
>  
>  	eol_byte_num = total_bytes_per_intf % 3;
> -	pkt_per_line = slice_per_intf / dsc->slice_count;
> +
> +	/*
> +	 * Default to 1 slice_per_pkt, so pkt_per_line will be equal to
> +	 * slice per intf.
> +	 */
> +	pkt_per_line = slice_per_intf;

Same as the comment below, perhaps it is a good idea to clarify in the
comment here that the original value wa *multiplied by* slice_per_pkt?
Otherwise this default of 1 is "invisible".

Alternatively we could have a `const slice_per_pkt = 1;` at the top, and
have the comment (the elaborate one from below) there?

- Marijn

>  
>  	if (is_cmd_mode) /* packet data type */
>  		reg = DSI_COMMAND_COMPRESSION_MODE_CTRL_STREAM0_DATATYPE(MIPI_DSI_DCS_LONG_WRITE);
> @@ -1001,7 +998,14 @@ static void dsi_timing_setup(struct msm_dsi_host *msm_host, bool is_bonded_dsi)
>  		if (!msm_host->dsc)
>  			wc = hdisplay * dsi_get_bpp(msm_host->format) / 8 + 1;
>  		else
> -			wc = msm_host->dsc->slice_chunk_size * msm_host->dsc->slice_count + 1;
> +			/*
> +			 * When DSC is enabled, WC = slice_chunk_size * slice_per_packet + 1.
> +			 * Currently, the driver only supports default value of slice_per_packet = 1
> +			 *
> +			 * TODO: Expand mipi_dsi_device struct to hold slice_per_packet info
> +			 *       and adjust DSC math to account for slice_per_packet.
> +			 */
> +			wc = msm_host->dsc->slice_chunk_size + 1;
>  
>  		dsi_write(msm_host, REG_DSI_CMD_MDP_STREAM0_CTRL,
>  			DSI_CMD_MDP_STREAM0_CTRL_WORD_COUNT(wc) |
> 
> -- 
> 2.40.1
>