From: Pekka Paalanen <ppaalanen@gmail.com>
To: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>,
Maxime Ripard <mripard@kernel.org>,
Thomas Zimmermann <tzimmermann@suse.de>,
David Airlie <airlied@linux.ie>, Daniel Vetter <daniel@ffwll.ch>,
Javier Martinez Canillas <javierm@redhat.com>,
Sam Ravnborg <sam@ravnborg.org>, Helge Deller <deller@gmx.de>,
Linux Fbdev development list <linux-fbdev@vger.kernel.org>,
Linux/m68k <linux-m68k@vger.kernel.org>,
DRI Development <dri-devel@lists.freedesktop.org>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v2 05/10] drm/fourcc: Add DRM_FORMAT_C[124]
Date: Mon, 14 Mar 2022 17:05:39 +0200 [thread overview]
Message-ID: <20220314170539.17400f93@eldfell> (raw)
In-Reply-To: <CAMuHMdVq19wpA_7nKKTm-G2EmK3cMxxP6nbR_u=vkazqCZ=KhQ@mail.gmail.com>
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On Mon, 14 Mar 2022 14:30:18 +0100
Geert Uytterhoeven <geert@linux-m68k.org> wrote:
> On Mon, Mar 7, 2022 at 9:53 PM Geert Uytterhoeven <geert@linux-m68k.org> wrote:
> > Introduce fourcc codes for color-indexed frame buffer formats with two,
> > four, and sixteen colors, and provide a mapping from bit per pixel and
> > depth to fourcc codes.
> >
> > As the number of bits per pixel is less than eight, these rely on proper
> > block handling for the calculation of bits per pixel and pitch.
> >
> > Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
>
> > --- a/include/uapi/drm/drm_fourcc.h
> > +++ b/include/uapi/drm/drm_fourcc.h
> > @@ -99,7 +99,10 @@ extern "C" {
> > #define DRM_FORMAT_INVALID 0
> >
> > /* color index */
> > -#define DRM_FORMAT_C8 fourcc_code('C', '8', ' ', ' ') /* [7:0] C */
> > +#define DRM_FORMAT_C1 fourcc_code('C', '1', ' ', ' ') /* [7:0] C0:C1:C2:C3:C4:C5:C6:C7 1:1:1:1:1:1:1:1 eight pixels/byte */
> > +#define DRM_FORMAT_C2 fourcc_code('C', '2', ' ', ' ') /* [7:0] C0:C1:C2:C3 2:2:2:2 four pixels/byte */
> > +#define DRM_FORMAT_C4 fourcc_code('C', '4', ' ', ' ') /* [7:0] C0:C1 4:4 two pixels/byte */
> > +#define DRM_FORMAT_C8 fourcc_code('C', '8', ' ', ' ') /* [7:0] C 8 one pixel/byte */
> >
> > /* 8 bpp Red */
> > #define DRM_FORMAT_R8 fourcc_code('R', '8', ' ', ' ') /* [7:0] R */
>
> After replying to Ilia's comment[1], I realized the CFB drawing
> operations use native byte and bit ordering, unless
> FBINFO_FOREIGN_ENDIAN is set.
> While Amiga, Atari, and Sun-3 use big-endian bit ordering,
> e.g. Acorn VIDC[2] uses little endian, and SH7760[3] is configurable
> (sh7760fb configures ordering to match host order).
> BTW, ssd130{7fb,x}_update_rect() both assume little-endian, so I
> guess they are broken on big-endian.
> Fbtest uses big-endian bit ordering, so < 8 bpp is probably broken
> on little-endian.
>
> Hence the above should become:
>
> #define DRM_FORMAT_C1 fourcc_code('C', '1', ' ', ' ') /*
> [7:0] C7:C6:C5:C4:C3:C2:C1:C0 1:1:1:1:1:1:1:1 eight pixels/byte */
> #define DRM_FORMAT_C2 fourcc_code('C', '2', ' ', ' ') /*
> [7:0] C3:C2:C1:C0 2:2:2:2 four pixels/byte */
> #define DRM_FORMAT_C4 fourcc_code('C', '4', ' ', ' ') /*
> [7:0] C1:C0 4:4 two pixels/byte */
>
> The same changes should be made for DRM_FORMAT_[RD][124].
>
> The fbdev emulation code should gain support for these with and without
> DRM_FORMAT_BIG_ENDIAN, the latter perhaps only on big-endian platforms?
>
> [1] https://lore.kernel.org/r/CAKb7UvgEdm9U=+RyRwL0TGRfA_Qc7NbhCWoZOft2DKdXggtKYw@mail.gmail.com/
> [2] See p.30 of the VIDC datasheet
> http://chrisacorns.computinghistory.org.uk/docs/Acorn/Misc/Acorn_VIDC_Datasheet.pdf
> [3] See p.1178 of the SH7660 datasheet
> https://datasheet.octopart.com/HD6417760BL200AV-Renesas-datasheet-14105759.pdf
Hi Geert,
why would CPU endianess affect the order of bits in a byte?
Do you mean that bit 0 one machine is (1 << 0), and on another machine
bit 0 is (1 << 7)?
In C, we have only one way to address bits of a byte and that is with
arithmetic. You cannot take the address of a bit any other way, can you?
Can we standardise on "bit n of a byte is addressed as (1 << n)"?
I don't mind in which order the pixels are inside a byte, as long as it
doesn't change by CPU endianess. If you need both directions, then use
two different drm_fourcc codes that do not change their meaning by CPU
endianess. Just like we have XRGB and BGRX formats.
I would not like to see DRM_FORMAT_BIG_ENDIAN used for this, it would
conflate a whole new concept into the mess that is little- vs.
big-endian.
Thanks,
pq
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next prev parent reply other threads:[~2022-03-14 15:06 UTC|newest]
Thread overview: 34+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-03-07 20:52 [PATCH v2 00/10] drm: Add support for low-color frame buffer formats Geert Uytterhoeven
2022-03-07 20:52 ` [PATCH v2 01/10] drm/fourcc: Add drm_format_info_bpp() helper Geert Uytterhoeven
2022-03-09 12:46 ` Javier Martinez Canillas
2022-03-07 20:52 ` [PATCH v2 02/10] drm/fourcc: Add drm_format_info.is_color_indexed flag Geert Uytterhoeven
2022-03-09 12:50 ` Javier Martinez Canillas
2022-03-07 20:52 ` [PATCH v2 03/10] drm/client: Use actual bpp when allocating frame buffers Geert Uytterhoeven
2022-03-09 12:51 ` Javier Martinez Canillas
2022-03-07 20:52 ` [PATCH v2 04/10] drm/framebuffer: Use actual bpp for DRM_IOCTL_MODE_GETFB Geert Uytterhoeven
2022-03-09 12:53 ` Javier Martinez Canillas
2022-03-07 20:52 ` [PATCH v2 05/10] drm/fourcc: Add DRM_FORMAT_C[124] Geert Uytterhoeven
2022-03-08 9:04 ` Pekka Paalanen
2022-03-09 12:57 ` Javier Martinez Canillas
2022-03-14 13:30 ` Geert Uytterhoeven
2022-03-14 15:05 ` Pekka Paalanen [this message]
2022-03-14 19:01 ` Geert Uytterhoeven
2022-03-14 22:15 ` Finn Thain
2022-03-15 7:32 ` Pekka Paalanen
2022-03-15 7:51 ` Geert Uytterhoeven
2022-03-15 7:52 ` Geert Uytterhoeven
2022-03-15 8:45 ` Pekka Paalanen
2022-03-15 8:57 ` Geert Uytterhoeven
2022-03-15 10:48 ` Pekka Paalanen
2022-03-07 20:52 ` [PATCH v2 06/10] drm/fb-helper: Add support for DRM_FORMAT_C[124] Geert Uytterhoeven
2022-03-09 13:10 ` Javier Martinez Canillas
2022-03-09 13:14 ` Geert Uytterhoeven
2022-03-07 20:52 ` [PATCH v2 RFC 07/10] drm/gem-fb-helper: Use actual bpp for size calculations Geert Uytterhoeven
2022-03-09 13:16 ` Javier Martinez Canillas
2022-03-07 20:52 ` [PATCH v2 RFC 08/10] drm/fourcc: Document that single-channel "red" can be any color Geert Uytterhoeven
2022-03-08 9:06 ` Pekka Paalanen
2022-03-09 13:33 ` Javier Martinez Canillas
2022-03-07 20:52 ` [PATCH v2 RFC 09/10] drm/fourcc: Add DRM_FORMAT_R[124] Geert Uytterhoeven
2022-03-09 13:39 ` Javier Martinez Canillas
2022-03-07 20:52 ` [PATCH v2 RFC 10/10] drm/fourcc: Add DRM_FORMAT_D[1248] Geert Uytterhoeven
2022-03-09 13:41 ` Javier Martinez Canillas
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