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

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Message-ID: <60b7cad3-8923-98b1-2c0f-71bfabd0b2e9@suse.de>
Date:   Mon, 17 Apr 2023 11:15:53 +0200
From:   Thomas Zimmermann <tzimmermann@...e.de>
To:     Javier Martinez Canillas <javierm@...hat.com>,
        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

Hi

Am 17.04.23 um 10:58 schrieb Javier Martinez Canillas:
> 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)

I'd rather keep the code as-is until we get an actual bug report.

For example, DRM framebuffer sizes are often multiples of 64. Creating a 
framebuffer of 800x600 will create a framebuffer with 
stride/pitch/linelength of 832.  I can imagine that some BIOSes out 
there do something similar with the system framebuffer. Messing with the 
stride would break them.

Best regards
Thomas

> 
> 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
> 

-- 
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Maxfeldstr. 5, 90409 Nürnberg, Germany
(HRB 36809, AG Nürnberg)
Geschäftsführer: Ivo Totev

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