linux-kernel - Re: [PATCH 3/3] drm/msm/dsi: Enable DATABUS

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Message-ID: <mfzmioovf54lcuiuzvk4fuz26elag6iw3ohbdhgym7k3qzirhx@dd7vu7ms6azz>
Date:   Wed, 14 Jun 2023 12:03:39 +0200
From:   Marijn Suijten <marijn.suijten@...ainline.org>
To:     Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
Cc:     Jessica Zhang <quic_jesszhan@...cinc.com>,
        Rob Clark <robdclark@...il.com>, Sean Paul <sean@...rly.run>,
        David Airlie <airlied@...il.com>,
        Daniel Vetter <daniel@...ll.ch>, quic_abhinavk@...cinc.com,
        linux-arm-msm@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        freedreno@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/3] drm/msm/dsi: Enable DATABUS_WIDEN for DSI command
 mode

On 2023-06-14 10:49:31, Dmitry Baryshkov wrote:
> On 14/06/2023 04:57, 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@...cinc.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/
> > 
> >   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
> 
> This is useless, it is already obvious from the code below. Instead 
> there should be something like "wide bus extends that to 6 bytes per 
> pclk cycle"

Yes please.  In general, don't paraphrase the code, but explain _why_ it
is doing what it does.  Saying that the widebus feature doubles the
bandwidth per pclk tick is much more clear than "divide by 6 instead of
3" - we can read that from the code.

Overall comments have been very good so far (such as the original "to
account for the fact that DPU sends 3 bytes per pclk cycle"), though!

> >   		 */
> > -		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);
> > +
> >   		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) {
> > +			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);
> 
> Is widebus applicable only to the CMD mode, or video mode can employ it too?

The patch description states that it was not tested on video-mode yet,
so I assume it will.  But this should also be highlighted with a comment
(e.g. /* XXX: Allow for video-mode once tested/fixed */), _especially_
on the check for MIPI_DSI_MODE_VIDEO above.

If I understand this correctly DSC is not working for video mode at all
on these setups, right?  Or no-one was able to test it?  I'm inclined to
request dropping these artifical guards to have as little friction as
possible when someone starts enabling and testing this - and less
patches removing artificial bounds in the future.

- Marijn