linux-kernel - Re: [PATCH 1/8] drm/ssd130x: Fix pitch calculation in ssd130x_fb_blit

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Date:   Fri, 14 Jul 2023 11:34:11 +0200
From:   Javier Martinez Canillas <javierm@...hat.com>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        David Airlie <airlied@...il.com>,
        Daniel Vetter <daniel@...ll.ch>
Cc:     Geert Uytterhoeven <geert@...ux-m68k.org>,
        linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org
Subject: Re: [PATCH 1/8] drm/ssd130x: Fix pitch calculation in
 ssd130x_fb_blit_rect()

Geert Uytterhoeven <geert@...ux-m68k.org> writes:

Hello Geert,

Thanks for your patch!

> The page height must be taken into account only for vertical coordinates
> and heights, not for horizontal coordinates and widths.
>
> Fixes: 179a790aaf2a0127 ("drm/ssd130x: Set the page height value in the device info data")
> Signed-off-by: Geert Uytterhoeven <geert@...ux-m68k.org>
> ---
>  drivers/gpu/drm/solomon/ssd130x.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/gpu/drm/solomon/ssd130x.c b/drivers/gpu/drm/solomon/ssd130x.c
> index afb08a8aa9fcdaf2..b4c376962629580b 100644
> --- a/drivers/gpu/drm/solomon/ssd130x.c
> +++ b/drivers/gpu/drm/solomon/ssd130x.c
> @@ -596,7 +596,7 @@ static int ssd130x_fb_blit_rect(struct drm_framebuffer *fb, const struct iosys_m
>  	rect->y1 = round_down(rect->y1, page_height);
>  	rect->y2 = min_t(unsigned int, round_up(rect->y2, page_height), ssd130x->height);
>  
> -	dst_pitch = DIV_ROUND_UP(drm_rect_width(rect), page_height);
> +	dst_pitch = DIV_ROUND_UP(drm_rect_width(rect), 8);
>

That's true for ssd130x controllers that use R1, but when doing that
change one of my goals was to prepare the driver for supporting the
ssd132x family that use a 16-grayscale pixel format (R4).

For those controllers, the pixels are encoded in 4-bit and each page
has two pixels. So for those controllers the dst_pitch will need to
be DIV_ROUND_UP(drm_rect_width(rect), 2) instead since the width is
not 8 in that case.

So I would prefer to skip this patch from your set, because otherwise
we will need to revert it when adding support for SSD132x controllers.

-- 
Best regards,

Javier Martinez Canillas
Core Platforms
Red Hat