[PATCH] drm: bridge: samsung-dsim: Fix waiting for empty cmd transfer FIFO on older Exynos

[Marek Vasut]

On 7/18/23 15:18, Marek Szyprowski wrote:
> Samsung DSIM used in older Exynos SoCs (like Exynos 4210, 4x12, 3250)
> doesn't report empty level of packer header FIFO. In case of those SoCs,
> use the old way of waiting for empty command tranfsfer FIFO, removed
> recently by commit 14806c641582 ("Drain command transfer FIFO before
> transfer").
> 
> Fixes: 14806c641582 ("Drain command transfer FIFO before transfer")
> Signed-off-by: Marek Szyprowski <m.szyprowski at samsung.com>
> ---
> If I remember right, the problem with waiting for the empty command
> transfer FIFO was there from the begging of the Exynos DSI driver and
> Tomash Figa and Andrzej Hajda used a workaround based on waiting until
> the DSIM_SFR_HEADER_FULL bit gets cleared. So far it worked well enough
> on the older Exynos SoCs, but indeed there is no point using it on the
> properly working hardware, like Exynos 5433 or IMX.
> 
> Marek Szyprowski
> ---
>   drivers/gpu/drm/bridge/samsung-dsim.c | 11 +++++++++--
>   include/drm/bridge/samsung-dsim.h     |  1 +
>   2 files changed, 10 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/bridge/samsung-dsim.c b/drivers/gpu/drm/bridge/samsung-dsim.c
> index 9b7a00bafeaa..80eb268d5868 100644
> --- a/drivers/gpu/drm/bridge/samsung-dsim.c
> +++ b/drivers/gpu/drm/bridge/samsung-dsim.c
> @@ -412,6 +412,7 @@ static const struct samsung_dsim_driver_data exynos3_dsi_driver_data = {
>   	.m_min = 41,
>   	.m_max = 125,
>   	.min_freq = 500,
> +	.has_broken_fifoctrl_emptyhdr = 1,
>   };
>   
>   static const struct samsung_dsim_driver_data exynos4_dsi_driver_data = {
> @@ -428,6 +429,7 @@ static const struct samsung_dsim_driver_data exynos4_dsi_driver_data = {
>   	.m_min = 41,
>   	.m_max = 125,
>   	.min_freq = 500,
> +	.has_broken_fifoctrl_emptyhdr = 1,
>   };
>   
>   static const struct samsung_dsim_driver_data exynos5_dsi_driver_data = {
> @@ -1009,8 +1011,13 @@ static int samsung_dsim_wait_for_hdr_fifo(struct samsung_dsim *dsi)
>   	do {
>   		u32 reg = samsung_dsim_read(dsi, DSIM_FIFOCTRL_REG);
>   
> -		if (reg & DSIM_SFR_HEADER_EMPTY)
> -			return 0;
> +		if (!dsi->driver_data->has_broken_fifoctrl_emptyhdr) {
> +			if (reg & DSIM_SFR_HEADER_EMPTY)
> +				return 0;
> +		} else {
> +			if (!(reg & DSIM_SFR_HEADER_FULL))
> +				return 0;

Would it make sense to at least wait a little bit on the old hardware , 
so the data can actually leave the FIFO and be put on the line ?

I mean, imagine you have e.g. a byte of the FIFO space available (i.e. 
FIFO is not full), and you attempt to put in two bytes. The FIFO would 
overflow. If you wait a bit, then there is a chance this overflow 
condition is avoided.