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From:   Jagan Teki <jagan@amarulasolutions.com>
Date:   Thu, 3 Oct 2019 09:53:54 +0530
Message-ID: <CAMty3ZDW4XHyW+6XL_RSVHqTSk79-r749pa0n5e6VbUzowAsiw@mail.gmail.com>
Subject: Re: [linux-sunxi] [PATCH 3/3] Revert "drm/sun4i: dsi: Rework a bit
 the hblk calculation"
To:     Icenowy Zheng <icenowy@aosc.io>
Cc:     Maxime Ripard <mripard@kernel.org>, Chen-Yu Tsai <wens@csie.org>,
        David Airlie <airlied@linux.ie>,
        Daniel Vetter <daniel@ffwll.ch>,
        dri-devel <dri-devel@lists.freedesktop.org>,
        linux-arm-kernel <linux-arm-kernel@lists.infradead.org>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        linux-sunxi <linux-sunxi@googlegroups.com>
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Hi Wens,

On Tue, Oct 1, 2019 at 1:34 PM Icenowy Zheng <icenowy@aosc.io> wrote:
>
> This reverts commit 62e7511a4f4dcf07f753893d3424decd9466c98b.
>
> This commit, although claimed as a refactor, in fact changed the
> formula.
>
> By expanding the original formula, we can find that the const 10 is not
> substracted, instead it's added to the value (because 10 is negative
> when calculating hsa, and hsa itself is negative when calculating hblk).
> This breaks the similar pattern to other formulas, so restoring the
> original formula is more proper.
>
> Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
> ---
>  drivers/gpu/drm/sun4i/sun6i_mipi_dsi.c | 9 ++-------
>  1 file changed, 2 insertions(+), 7 deletions(-)
>
> diff --git a/drivers/gpu/drm/sun4i/sun6i_mipi_dsi.c b/drivers/gpu/drm/sun4i/sun6i_mipi_dsi.c
> index 2d3e822a7739..cb5fd19c0d0d 100644
> --- a/drivers/gpu/drm/sun4i/sun6i_mipi_dsi.c
> +++ b/drivers/gpu/drm/sun4i/sun6i_mipi_dsi.c
> @@ -577,14 +577,9 @@ static void sun6i_dsi_setup_timings(struct sun6i_dsi *dsi,
>                           (mode->hsync_start - mode->hdisplay) * Bpp - HFP_PACKET_OVERHEAD);
>
>                 /*
> -                * The blanking is set using a sync event (4 bytes)
> -                * and a blanking packet (4 bytes + payload + 2
> -                * bytes). Its minimal size is therefore 10 bytes.
> +                * hblk seems to be the line + porches length.
>                  */
> -#define HBLK_PACKET_OVERHEAD   10
> -               hblk = max((unsigned int)HBLK_PACKET_OVERHEAD,
> -                          (mode->htotal - (mode->hsync_end - mode->hsync_start)) * Bpp -
> -                          HBLK_PACKET_OVERHEAD);
> +               hblk = mode->htotal * Bpp - hsa;

The original formula is correct according to BSP [1] and work with my
panels which I have tested before. May be the horizontal timings on
panels you have leads to negative value.

[1] https://github.com/ayufan-pine64/linux-pine64/blob/my-hacks-1.2-with-drm/drivers/video/sunxi/disp2/disp/de/lowlevel_sun50iw1/de_dsi.c#L919