Re: [PATCH 3/3] drm/msm/dsi: Enable DATABUS_WIDEN for DSI command mode

[Marijn Suijten <marijn.suijten@somainline.org>]

On 2023-06-13 18:57:13, Jessica Zhang wrote:
> DSI 6G v2.5.x+ supports a data-bus widen mode that allows DSI to send
> 48 bits of compressed data per pclk instead of 24.
> 
> For all chipsets that support this mode, enable it whenever DSC is
> enabled as recommend by the hardware programming guide.
> 
> Only enable this for command mode as we are currently unable to validate
> it for video mode.
> 
> Signed-off-by: Jessica Zhang <quic_jesszhan@quicinc.com>
> ---
> 
> Note: The dsi.xml.h changes were generated using the headergen2 script in
> envytools [1], but the changes to the copyright and rules-ng-ng source file
> paths were dropped.
> 
> [1] https://github.com/freedreno/envytools/

More interesting would be a link to the Mesa MR upstreaming this
bitfield to dsi.xml [2] (which I have not found on my own yet).

[2]: https://gitlab.freedesktop.org/mesa/mesa/-/blame/main/src/freedreno/registers/dsi/dsi.xml

>  drivers/gpu/drm/msm/dsi/dsi.xml.h  |  1 +
>  drivers/gpu/drm/msm/dsi/dsi_host.c | 19 ++++++++++++++++++-
>  2 files changed, 19 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/msm/dsi/dsi.xml.h b/drivers/gpu/drm/msm/dsi/dsi.xml.h
> index a4a154601114..2a7d980e12c3 100644
> --- a/drivers/gpu/drm/msm/dsi/dsi.xml.h
> +++ b/drivers/gpu/drm/msm/dsi/dsi.xml.h
> @@ -664,6 +664,7 @@ static inline uint32_t DSI_CMD_MODE_MDP_CTRL2_INPUT_RGB_SWAP(enum dsi_rgb_swap v
>  	return ((val) << DSI_CMD_MODE_MDP_CTRL2_INPUT_RGB_SWAP__SHIFT) & DSI_CMD_MODE_MDP_CTRL2_INPUT_RGB_SWAP__MASK;
>  }
>  #define DSI_CMD_MODE_MDP_CTRL2_BURST_MODE			0x00010000
> +#define DSI_CMD_MODE_MDP_CTRL2_DATABUS_WIDEN			0x00100000
> 
>  #define REG_DSI_CMD_MODE_MDP_STREAM2_CTRL			0x000001b8
>  #define DSI_CMD_MODE_MDP_STREAM2_CTRL_DATA_TYPE__MASK		0x0000003f
> diff --git a/drivers/gpu/drm/msm/dsi/dsi_host.c b/drivers/gpu/drm/msm/dsi/dsi_host.c
> index 5d7b4409e4e9..1da5238e7105 100644
> --- a/drivers/gpu/drm/msm/dsi/dsi_host.c
> +++ b/drivers/gpu/drm/msm/dsi/dsi_host.c
> @@ -927,6 +927,9 @@ static void dsi_timing_setup(struct msm_dsi_host *msm_host, bool is_bonded_dsi)
>  	u32 hdisplay = mode->hdisplay;
>  	u32 wc;
>  	int ret;
> +	bool widebus_supported = msm_host->cfg_hnd->major == MSM_DSI_VER_MAJOR_6G &&
> +			msm_host->cfg_hnd->minor >= MSM_DSI_6G_VER_MINOR_V2_5_0;
> +
> 
>  	DBG("");
> 
> @@ -973,8 +976,15 @@ static void dsi_timing_setup(struct msm_dsi_host *msm_host, bool is_bonded_dsi)
>  		 *
>  		 * hdisplay will be divided by 3 here to account for the fact
>  		 * that DPU sends 3 bytes per pclk cycle to DSI.
> +		 *
> +		 * If widebus is supported, set DATABUS_WIDEN register and divide hdisplay by 6
> +		 * instead of 3

So this should allow us to divide pclk by 2, or have much lower latency?
Otherwise it'll tick enough times to transmit the data twice.

Note that I brought up the exact same concerns when you used the 3:1
ratio from dsi_bpp / dsc_bpp in your pclk reduction patch (instad of the
number of bits/bytes that DPU sends to DSI per pclk), but no-one has
replied to my inquiry yet.

>  		 */
> -		hdisplay = DIV_ROUND_UP(msm_dsc_get_bytes_per_line(msm_host->dsc), 3);
> +		if (!(msm_host->mode_flags & MIPI_DSI_MODE_VIDEO) && widebus_supported)
> +			hdisplay = DIV_ROUND_UP(msm_dsc_get_bytes_per_line(msm_host->dsc), 6);
> +		else
> +			hdisplay = DIV_ROUND_UP(msm_dsc_get_bytes_per_line(msm_host->dsc), 3);

Nit: I wonder if this is more concise when written as:

    u32 bytes_per_pclk;
    ...
    if (!video && widebus)
        bytes_per_pclk = 6;
    else
        bytes_per_pclk = 3;

    hdisplay = DIV_ROUND_UP(msm_dsc_get_bytes_per_line(msm_host->dsc),
                            bytes_per_pclk);

That is less duplication, **and** the value becomes self-documenting!

> +
>  		h_total += hdisplay;
>  		ha_end = ha_start + hdisplay;
>  	}
> @@ -1027,6 +1037,13 @@ static void dsi_timing_setup(struct msm_dsi_host *msm_host, bool is_bonded_dsi)
>  		dsi_write(msm_host, REG_DSI_CMD_MDP_STREAM0_TOTAL,
>  			DSI_CMD_MDP_STREAM0_TOTAL_H_TOTAL(hdisplay) |
>  			DSI_CMD_MDP_STREAM0_TOTAL_V_TOTAL(mode->vdisplay));
> +
> +		if (msm_host->dsc && widebus_supported) {

Can we also support widebus for uncompressed streams (sending 2 pixels
of bpp=24 per pclk), and if so, is that something you want to add in the
future (a comment would be nice)?

> +			u32 mdp_ctrl2 = dsi_read(msm_host, REG_DSI_CMD_MODE_MDP_CTRL2);
> +
> +			mdp_ctrl2 |= DSI_CMD_MODE_MDP_CTRL2_DATABUS_WIDEN;
> +			dsi_write(msm_host, REG_DSI_CMD_MODE_MDP_CTRL2, mdp_ctrl2);
> +		}

Same comment as on your BURST_MODE patch (which this'll conflict with):
does this belong to the timing setup or is it better moved to
dsi_ctrl_config?

- Marijn

>  	}
>  }
> 
> 
> --
> 2.40.1
>