linux-kernel - Re: [PATCH] firmware/sysfb: Fix wrong stride when bits-per-pixel is calculated

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Message-ID: <40edb0fdb0eaff434f4872dd677923a6.squirrel@mail.panix.com>
Date:   Wed, 12 Apr 2023 17:53:25 -0400
From:   "Pierre Asselin" <pa@...ix.com>
To:     "Javier Martinez Canillas" <javierm@...hat.com>
Cc:     "Pierre Asselin" <pa@...ix.com>,
        "Jocelyn Falempe" <jfalempe@...hat.com>,
        "Daniel Vetter" <daniel.vetter@...ll.ch>,
        linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        "Hans de Goede" <hdegoede@...hat.com>,
        "Thomas Zimmermann" <tzimmermann@...e.de>,
        "Ard Biesheuvel" <ardb@...nel.org>
Subject: Re: [PATCH] firmware/sysfb: Fix wrong stride when bits-per-pixel is
 calculated

> Javier Martinez Canillas <javierm@...hat.com> writes:
>
> I still don't understand why this particular configuration didn't work...
>
> The framebuffer starts at 0xd8000000 and has a size of 0x240000 bytes, so

Says who ?  It's the same grub, same video mode as before the regression,
so the size is probably 0x300000 like it always was.

> a r8g8b8 pixel format with resolution 1024x768 should be correct. Since is
> 1024 * 768 * (24 / 8) = 2359296 = 0x240000.

That is internally consistent, but at variance with the video mode
set up by grub.

It is better to sqeeze bits by 4:3 on each line (regression) than to
scatter 4 logical lines across 3 physical lines (regression, patched) !

> Could you please apply the following diff that will print all the relevant
> fields from the screen_info that are used to calculate the bpp and stride.

YES !  I can't peer into that struct screen_info and I don't know to
write the printk's.  (Hm, doesn't look too hard, but trust me, I would
fumble it.)

I'll back out the original patch first.
Stand by.

--PA