Re: [PATCH 3/5] drm/msm/dsi: Account for DSC's bits_per_pixel having 4 fractional bits

[Marijn Suijten]

Doing some self-review as these patches accrued some bit-rot while
waiting to be sent.

On 2022-10-01 21:08:05, Marijn Suijten wrote:
> drm_dsc_config's bits_per_pixel field holds a fractional value with 4
> bits, which all panel drivers should adhere to for
> drm_dsc_pps_payload_pack() to generate a valid payload.  All code in the
> DSI driver here seems to assume that this field doesn't contain any
> fractional bits, hence resulting in the wrong values being computed.
> Since none of the calculations leave any room for fractional bits or
> seem to indicate any possible area of support, disallow such values
> altogether.
> 
> Fixes: b9080324d6ca ("drm/msm/dsi: add support for dsc data")
> Signed-off-by: Marijn Suijten <marijn.suijten@xxxxxxxxxxxxxx>
> ---
>  drivers/gpu/drm/msm/dsi/dsi_host.c | 34 +++++++++++++++++++++++-------
>  1 file changed, 26 insertions(+), 8 deletions(-)
> 
> diff --git a/drivers/gpu/drm/msm/dsi/dsi_host.c b/drivers/gpu/drm/msm/dsi/dsi_host.c
> index cb6f2fa11f58..42a5c9776f52 100644
> --- a/drivers/gpu/drm/msm/dsi/dsi_host.c
> +++ b/drivers/gpu/drm/msm/dsi/dsi_host.c
> @@ -847,6 +847,11 @@ static void dsi_update_dsc_timing(struct msm_dsi_host *msm_host, bool is_cmd_mod
>  	u32 pkt_per_line;
>  	u32 bytes_in_slice;
>  	u32 eol_byte_num;
> +	int bpp = dsc->bits_per_pixel >> 4;

This should have been u16 instead of int.

> +
> +	if (dsc->bits_per_pixel & 0xf)

Should there be a define for this mask?

> +		/* dsi_populate_dsc_params() already caught this case */
> +		pr_err("DSI does not support fractional bits_per_pixel\n");

This file mostly uses pr_err(), but it's probably cleaner to use
DRM_DEV_ERROR(&msm_host->pdev->dev, ...) even though it's not prevalent
yet?

>  
>  	/* first calculate dsc parameters and then program
>  	 * compress mode registers
> @@ -860,7 +865,7 @@ static void dsi_update_dsc_timing(struct msm_dsi_host *msm_host, bool is_cmd_mod
>  	if (slice_per_intf > dsc->slice_count)
>  		dsc->slice_count = 1;
>  
> -	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);
>  
>  	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;

Same u16 here.

- Marijn

> +
> +	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)
>  		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;
>  	final_value =  dsc->rc_model_size - data + num_extra_mux_bits;
> -- 
> 2.37.3
>