Date:   Tue, 18 Jan 2022 15:06:00 +0100
From:   Thomas Zimmermann <tzimmermann@...e.de>
To:     Sven Schnelle <svens@...ckframe.org>
Cc:     Helge Deller <deller@....de>, linux-fbdev@...r.kernel.org,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] MAINTAINERS: Add Helge as fbdev maintainer

Hi

Am 17.01.22 um 19:47 schrieb Sven Schnelle:
> Hi Thomas,
> 
> Thomas Zimmermann <tzimmermann@...e.de> writes:
> 
>> Hi
>>
>> Am 14.01.22 um 19:11 schrieb Helge Deller:
>>> The fbdev layer is orphaned, but seems to need some care.
>>> So I'd like to step up as new maintainer.
>>> Signed-off-by: Helge Deller <deller@....de>
>>
>> First of all, thank you for stepping up to maintain the fbdev
>> codebase. It really needs someone actively looking after it.
>>
>> And now comes the BUT.
>>
>> I want to second everything said by Danial and Javier. In addition to
>> purely organizational topics (trees, PRs, etc), there are a number of
>> inherit problems with fbdev.
>>
>>   * It's 90s technology. Neither does it fit today's userspace, not
>>     hardware. If you have more than just the most trivial of graphical
>>     output fbdev isn't for you.
>>
>>   * There's no new development in fbdev and there are no new
>>     drivers. Everyone works on DRM, which is better in most
>>     regards. The consequence is that userspace is slowly loosing the
>>    ability to use fbdev.
> 
> That might be caused by the fact that no new drivers are accepted for

And that is caused by the fact that fbdev is nowhere up to todays 
requirements.

> fbdev. I wrote a driver for the HP Visualize FX5/10 cards end of last
> year which was rejected for inclusion into fbdev[1].

Yep, I was hoping for a reply.

> 
> Based on your recommendation i re-wrote the whole thing in DRM. This
> works but has several drawbacks:
> 
> - no modesetting. With fbdev, i can nicely switch resolutions with
>    fbset. That doesn't work, and i've been told that this is not supported[2]

I didn't say that we're not going to support it at all. It's just not 
supported at the momont. vmwgfx has modesetting code that can serve as 
starting point.

> 
> - It is *much* slower than fbset with hardware blitting. I would have to
>    dig out the numbers, but it's in the ratio of 1:15. The nice thing
>    with fbdev blitting is that i get an array of pixels and the
>    foreground/background colors all of these these pixels should have.
>    With the help of the hardware blitting, i can write 32 pixels at once
>    with every 32-bit transfer.

For comparison, how fast is fbdev with plain memcpy() and memset()?


> 
>    With DRM, the closest i could find was DRM_FORMAT_C8, which means one
>    byte per pixel. So i can put 4 pixels into one 32-bit transfer.

IIRC the hardware only supported 8-bit palette colors, so C8 was the 
correct choice. Otherwise, you can add new formats and add them to the 
console.

> 
>    fbdev also clears the lines with hardware blitting, which is much
>    faster than clearing it with memcpy.
> 
>    Based on your recommendation i also verified that pci coalescing is
>    enabled.
> 
>    These numbers are with DRM's unnatural scrolling behaviour - it seems
>    to scroll several (text)lines at once if it takes to much time. I
>    guess if DRM would scroll line by line it would be even slower.
> 
>    If DRM would add those things - hardware clearing of memory regions,
>    hw blitting for text with a FG/BG color and modesetting i wouldn't
>    care about fbdev at all. But right now, it's working way faster for me.

I admit that your hardware is at the edge of what DRM currently 
supports. But I've used some of the DRM stuff on Athlon XPs with PCI 
graphics. While the performance wasn't good, it was far from unusable.

I guess you used GEM SHMEM for memory buffers. fbdev and mmap with shmem 
pages use some of the same bits in struct page, so shmem cannot mmap 
it's pages directly. We have to use an additional shadow buffer. Any 
display update goes from the shadow buffer into the shmem buffer and 
into the videoram. That's two memcpys. This can be reduced to one 
memcpy, but we never had the requirement to do so.

There's also potential for reducing the amount of page 
mappings/unmappings with gem shmem.

And DRM supports shadow buffers, virtual screen sizes and damage 
handling in DRM. A sophisticated driver might be able to use shadow 
buffering, damage handling and hardware panning to reduce the amount of 
screen updates to a minimum.

Until these things are fixed, adding hardware blitting doesn't really 
make sense IMHO.

As with other things, we didn't have a requirement for all these 
optimizations so far. A usually good approach to improve the sitution is 
to get a basic driver merged and then address the problems one by one.

Best regards
Thomas



> 
> I also tested the speed on my Thinkpad X1 with Intel graphics, and there
> a dmesg with 919 lines one the text console took about 2s to display. In
> x11, i measure 22ms. This might be unfair because encoding might be
> different, but i cannot confirm the 'memcpy' is faster than hardware
> blitting' point. I think if that would be the case, no-one would care
> about 2D acceleration.
> 
> Don't get me wrong, i'm not saying there's no reason for DRM. I fully
> understand why it exists and think it's a good way to go. But for system
> where a (fast) local console is required without X11, fbdev might be the
> better choice at the moment.
> 
> Regards
> Sven
> 
> [1] https://lore.kernel.org/all/87ee7qvcc7.fsf@x1.stackframe.org/T/#m57cdea83608fc78bfc6c2e76eb037bf82017b302
> [2] https://lore.kernel.org/all/87ee7qvcc7.fsf@x1.stackframe.org/T/#m46a52815036a958f6a11d2f3f62e1340a09bd981
> 

-- 
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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