linux-kernel - Re: [PATCH 2/4] drm/format-helper: Add drm_fb_gray8_to_mono

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Message-ID: <9b8bbc6a-e499-99c7-8fe6-95c4256ef61d@redhat.com>
Date:   Mon, 14 Mar 2022 15:07:43 +0100
From:   Javier Martinez Canillas <javierm@...hat.com>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux Fbdev development list <linux-fbdev@...r.kernel.org>,
        Maxime Ripard <maxime@...no.tech>,
        Daniel Vetter <daniel.vetter@...ll.ch>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        DRI Development <dri-devel@...ts.freedesktop.org>,
        Noralf Trønnes <noralf@...nnes.org>,
        Daniel Vetter <daniel@...ll.ch>,
        David Airlie <airlied@...ux.ie>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>
Subject: Re: [PATCH 2/4] drm/format-helper: Add
 drm_fb_gray8_to_mono_reversed()

Hello Geert,

On 3/14/22 14:40, Geert Uytterhoeven wrote:
> Hi Javier,
> 
> On Mon, Jan 31, 2022 at 9:12 PM Javier Martinez Canillas
> <javierm@...hat.com> wrote:
>> Add support to convert 8-bit grayscale to reversed monochrome for drivers
>> that control monochromatic displays, that only have 1 bit per pixel depth.
>>
>> This helper function was based on repaper_gray8_to_mono_reversed() from
>> the drivers/gpu/drm/tiny/repaper.c driver.
>>
>> Signed-off-by: Javier Martinez Canillas <javierm@...hat.com>
> 
>> --- a/drivers/gpu/drm/drm_format_helper.c
>> +++ b/drivers/gpu/drm/drm_format_helper.c
>> @@ -584,3 +584,38 @@ int drm_fb_blit_toio(void __iomem *dst, unsigned int dst_pitch, uint32_t dst_for
>>         return -EINVAL;
>>  }
>>  EXPORT_SYMBOL(drm_fb_blit_toio);
>> +
>> +/**
>> + * drm_fb_gray8_to_mono_reversed - Convert grayscale to reversed monochrome
>> + * @dst: reversed monochrome destination buffer
> 
> What's the meaning of "reversed"?

Originally I took this helper from the repaper driver and it was called
repaper_gray8_to_mono_reversed(), so the naming just got carried over.

> During the last few days, I've been balancing between (a) "reverse
> video" and (b) "reverse bit order", but none of them seems to be true.
>
> (a) The code maps 0-127 to 0 and 8-255 to 1, which just reduces from
>     256 to 2 grayscale levels, without inversion. The result is also
>     white-on-black on my ssd130x OLED.
> (b) On little-endian, the CFB drawops use little-endian bit order,
>     which is what ends up in "byte" in the code below.
>

As far as I understand is (a) from the options since the repaper display
uses black-on-white. But as you pointed out the ssd130x OLEDs instead do
white-on-black.

I should had probably just name the helper drm_fb_gray8_to_monochrome()
or maybe drm_fb_gray8_to_gray1() ?

-- 
Best regards,

Javier Martinez Canillas
Linux Engineering
Red Hat