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

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Message-ID: <87zg76evh3.fsf@minerva.mail-host-address-is-not-set>
Date:   Mon, 17 Apr 2023 10:58:16 +0200
From:   Javier Martinez Canillas <javierm@...hat.com>
To:     Thomas Zimmermann <tzimmermann@...e.de>,
        Pierre Asselin <pa@...ix.com>
Cc:     jfalempe@...hat.com, daniel.vetter@...ll.ch,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        hdegoede@...hat.com, ardb@...nel.org
Subject: Re: [PATCH] firmware/sysfb: Fix wrong stride when bits-per-pixel is
 calculated

Thomas Zimmermann <tzimmermann@...e.de> writes:

> Hi,
>
> thanks a lot to both of you for this bug fix.
>
> Am 13.04.23 um 03:34 schrieb Pierre Asselin:
>>> (not tested)
>> 
>> Tested.  It fixes the regression on my laptop.
>> 
>>> diff --git a/drivers/firmware/sysfb_simplefb.c
>>> b/drivers/firmware/sysfb_simplefb.c
>>> index 82c64cb9f531..9f5299d54732 100644
>>> --- a/drivers/firmware/sysfb_simplefb.c
>>> +++ b/drivers/firmware/sysfb_simplefb.c
>>> @@ -56,10 +56,11 @@ __init bool sysfb_parse_mode(const struct screen_info
>>> *si,
>>>   	 * don't specify alpha channels.
>>>   	 */
>>>   	if (si->lfb_depth > 8) {
>>> -		bits_per_pixel = max(max3(si->red_size + si->red_pos,
>>> +		bits_per_pixel = max3(max3(si->red_size + si->red_pos,
>>>   					  si->green_size + si->green_pos,
>>>   					  si->blue_size + si->blue_pos),
>>> -				     si->rsvd_size + si->rsvd_pos);
>>> +				     si->rsvd_size + si->rsvd_pos,
>>> +				     si->lfb_depth);
>
> I'm OK with this change. There's a comment
>
>   "The best solution is to compute bits_per_pixel here and ignore 
> lfb_depth."
>
> I'd change this to
>
>   "The best solution is to compute bits_per_pixel here from the color 
> bits, the reserved bits and the reported color depth; whatever is highest."
>
> That will hopefully clarify the meaning of these max3() statements. They 
> are not obvious at first.
>

I'm OK with this as well but then should probably also apply my patch [1]
that computed the stride too. Since if we don't trust the lfb_depth and
calculate the BPP, then we shouldn't trust the reported line length too.

As Pierre reported in the thread [2], when the wrong BPP was calculated (and
wrong pixel format chosen), the line lenght didn't match the BPP * lfb_width.

He mentioned that it was like this:

 format=r8g8b8, mode=1024x768x24, linelength=4096

Instead of the expected:

 format=r8g8b8, mode=1024x768x24, linelength=3072

My patch in [1], fixed the linelength calculation so it was internally
consistent (but still wrong since the pixel format was really xr8g8b8).

In other words, I think that we should either:

a) Trust the lfb_linelength and lfb_width (we are already doing that since
   mode->stride and mode->width are set to those once the format matches).

   If we decided to trust those, then the bits-per-pixel could just be
   calculated as: bits_per_pixel = si->lfb_linelength * 8 / si->lfb_width

   which is what I do on my v2 patch [3].

b) Not trust lfb_linelength, since that would need to be recalculated after
   the BPP was calcualted. That's why I mentioned that we need Pierre's fix +
   my patch [1] that did:

   stride = DIV_ROUND_UP(si->lfb_width * bits_per_pixel, 8)

But calculating a BPP yet blindly using linelength doens't make sense to me.

[1]: https://lists.freedesktop.org/archives/dri-devel/2023-April/399963.html
[2]: https://lore.kernel.org/dri-devel/dfb4f25ca8dfb0d81d778d6315f104ad.squirrel@mail.panix.com/
[3]: https://lists.freedesktop.org/archives/dri-devel/2023-April/400088.html

-- 
Best regards,

Javier Martinez Canillas
Core Platforms
Red Hat