Re: [PATCH v6] drm/omap: dsi: avoid sending bta sync all the time in writes

Hi,

On 4.08.26 г. 11:37 ч., Andreas Kemnade wrote:
> Some chips need configuration commands to be sent first, before they can
> send data. TC358762 for example needs PPI_LPTXTIMECNT configured
> and PPI_STARTPPI set to 1 to be able to transmit anything. To be able to
> configure such chips, do not send bta sync during writes if no acks are
> requested. Instead just wait for the packet to be sent to avoid FIFO
> overflows. There might be more to do about acks, but there seem to be
> virtually no users of that flag.
> 
> This came to light when fiddling with the Epson Moverio BT-200 display
> which consists of 2 TC358762 bridges with SPI funneled through
> to the unknown display chip. With that patch the bridge can be accessed,
> Reading back registers works, when the above-mentioned registers are set.
> 
> In Command-Mode update, there was a nop sent, apparently the most
> relevant part was the bta sync to actually force low power mode.
> 
> Video mode panel at OMAP4 (BT-200) and video mode at OMAP5 was tested.
> 
> Fixes: e70965386353e ("drm/omap: dsi: simplify write function")
> Signed-off-by: Andreas Kemnade <[email protected]>


Tested on motorolla droid4 (command mode), no visible issues so far.

> ---
> Changes in v6:
> - clear errors on every write
> - remove unneeded forward declaration
> 
> Changes in v5:
> - send bta sync on VC_CMD again
> - Link to v4: https://patch.msgid.link/[email protected]
> 
> Changes in v4:
> - wait on completition on all packets (was limited to long packets only,
>    because I had the wrong impression that there is no confirmation on
>    these)
> - Link to v3: https://patch.msgid.link/[email protected]
> 
> Changes in v3:
> 
> - Link to v2: https://patch.msgid.link/[email protected]
> - fix things mentioned by claude here:
>    https://lore.gitlab.freedesktop.org/drm-ai-reviews/[email protected]/
>    - fix typos
>    - register ISR before sending packet
>    - check for RX_FIFO_NOT_EMPTY also in for short packets
> 
> Changes in v2:
> - fix commandmode update, need bta sync there
> - do not wait on short packets
> - Link to v1: https://patch.msgid.link/[email protected]
> 
> To: Tomi Valkeinen <[email protected]>
> To: Maarten Lankhorst <[email protected]>
> To: Maxime Ripard <[email protected]>
> To: Thomas Zimmermann <[email protected]>
> To: David Airlie <[email protected]>
> To: Simona Vetter <[email protected]>
> To: Laurent Pinchart <[email protected]>
> To: Sebastian Reichel <[email protected]>
> Cc: Tomi Valkeinen <[email protected]>
> Cc: [email protected]
> Cc: [email protected]
> ---
>   drivers/gpu/drm/omapdrm/dss/dsi.c | 63 +++++++++++++++++++--------------------
>   1 file changed, 31 insertions(+), 32 deletions(-)
> 
> diff --git a/drivers/gpu/drm/omapdrm/dss/dsi.c b/drivers/gpu/drm/omapdrm/dss/dsi.c
> index 27fe7bca9e2c..27bf9bbe0697 100644
> --- a/drivers/gpu/drm/omapdrm/dss/dsi.c
> +++ b/drivers/gpu/drm/omapdrm/dss/dsi.c
> @@ -58,9 +58,6 @@ static void dsi_uninit_dispc(struct dsi_data *dsi);
>   
>   static int dsi_vc_send_null(struct dsi_data *dsi, int vc, int channel);
>   
> -static ssize_t _omap_dsi_host_transfer(struct dsi_data *dsi, int vc,
> -				       const struct mipi_dsi_msg *msg);
> -
>   #ifdef DSI_PERF_MEASURE
>   static bool dsi_perf;
>   module_param(dsi_perf, bool, 0644);
> @@ -2194,28 +2191,45 @@ static int dsi_vc_send_null(struct dsi_data *dsi, int vc, int channel)
>   static int dsi_vc_write_common(struct omap_dss_device *dssdev, int vc,
>   			       const struct mipi_dsi_msg *msg)
>   {
> +	DECLARE_COMPLETION_ONSTACK(completion);
>   	struct dsi_data *dsi = to_dsi_data(dssdev);
> +	u32 err;
>   	int r;
>   
> +	/* wait for IRQ for packet transmission confirmation */
> +	r = dsi_register_isr_vc(dsi, vc, dsi_completion_handler,
> +				&completion, DSI_VC_IRQ_PACKET_SENT);
> +	if (r)
> +		return r;
> +
>   	if (mipi_dsi_packet_format_is_short(msg->type))
>   		r = dsi_vc_send_short(dsi, vc, msg);
>   	else
>   		r = dsi_vc_send_long(dsi, vc, msg);
>   
> -	if (r < 0)
> +	if ((!r) && wait_for_completion_timeout(&completion,
> +				msecs_to_jiffies(500)) == 0)
> +		r = -EIO;
> +
> +	dsi_unregister_isr_vc(dsi, vc, dsi_completion_handler,
> +			      &completion, DSI_VC_IRQ_PACKET_SENT);
> +	if (r)
>   		return r;
>   
> -	/*
> -	 * TODO: we do not always have to do the BTA sync, for example
> -	 * we can improve performance by setting the update window
> -	 * information without sending BTA sync between the commands.
> -	 * In that case we can return early.
> -	 */
> +	/* TODO: find out if more needs to be done for MIPI_DSI_MSG_REQ_ACK */
>   
> -	r = dsi_vc_send_bta_sync(dssdev, vc);
> -	if (r) {
> -		DSSERR("bta sync failed\n");
> -		return r;
> +	if (msg->flags & MIPI_DSI_MSG_REQ_ACK) {
> +		r = dsi_vc_send_bta_sync(dssdev, vc);
> +		if (r) {
> +			DSSERR("bta sync failed\n");
> +			return r;
> +		}
> +	} else {
> +		err = dsi_get_errors(dsi);
> +		if (err) {
> +			DSSERR("Error while sending: %x\n", err);
> +			return -EIO;
> +		}
>   	}
>   
>   	/* RX_FIFO_NOT_EMPTY */
> @@ -3233,21 +3247,6 @@ static int _dsi_update(struct dsi_data *dsi)
>   	return 0;
>   }
>   
> -static int _dsi_send_nop(struct dsi_data *dsi, int vc, int channel)
> -{
> -	const u8 payload[] = { MIPI_DCS_NOP };
> -	const struct mipi_dsi_msg msg = {
> -		.channel = channel,
> -		.type = MIPI_DSI_DCS_SHORT_WRITE,
> -		.tx_len = 1,
> -		.tx_buf = payload,
> -	};
> -
> -	WARN_ON(!dsi_bus_is_locked(dsi));
> -
> -	return _omap_dsi_host_transfer(dsi, vc, &msg);
> -}
> -
>   static int dsi_update_channel(struct omap_dss_device *dssdev, int vc)
>   {
>   	struct dsi_data *dsi = to_dsi_data(dssdev);
> @@ -3268,13 +3267,13 @@ static int dsi_update_channel(struct omap_dss_device *dssdev, int vc)
>   	DSSDBG("dsi_update_channel: %d", vc);
>   
>   	/*
> -	 * Send NOP between the frames. If we don't send something here, the
> +	 * Transition to LP here. If we don't send something here, the
>   	 * updates stop working. This is probably related to DSI spec stating
>   	 * that the DSI host should transition to LP at least once per frame.
>   	 */
> -	r = _dsi_send_nop(dsi, VC_CMD, dsi->dsidev->channel);
> +	r = dsi_vc_send_bta_sync(dssdev, VC_CMD);
>   	if (r < 0) {
> -		DSSWARN("failed to send nop between frames: %d\n", r);
> +		DSSWARN("failed to send bta sync between frames: %d\n", r);
>   		goto err;
>   	}
>   
> 
> ---
> base-commit: dc59e4fea9d83f03bad6bddf3fa2e52491777482
> change-id: 20260528-vm-upstr-c8e7634ebf56
> 
> Best regards,
> --
> Andreas Kemnade <[email protected]>
>