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Date:   Wed, 2 Feb 2022 17:00:05 +0100
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Subject: Re: [PATCH 1/4] drm: Add I2C connector type
To:     Pekka Paalanen <ppaalanen@gmail.com>
Cc:     Thomas Zimmermann <tzimmermann@suse.de>,
        Sam Ravnborg <sam@ravnborg.org>,
        Javier Martinez Canillas <javierm@redhat.com>,
        linux-fbdev@vger.kernel.org, David Airlie <airlied@linux.ie>,
        Daniel Vetter <daniel.vetter@ffwll.ch>,
        Emil Velikov <emil.l.velikov@gmail.com>,
        linux-kernel@vger.kernel.org, dri-devel@lists.freedesktop.org,
        Geert Uytterhoeven <geert@linux-m68k.org>,
        Maxime Ripard <maxime@cerno.tech>,
        Andy Shevchenko <andriy.shevchenko@linux.intel.com>
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From:   =?UTF-8?Q?Noralf_Tr=c3=b8nnes?= <noralf@tronnes.org>
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Den 02.02.2022 16.04, skrev Pekka Paalanen:
> On Wed, 2 Feb 2022 10:45:42 +0100
> Noralf Trønnes <noralf@tronnes.org> wrote:
> 
>> Den 02.02.2022 10.14, skrev Thomas Zimmermann:
>>> Hi Noralf,
>>>
>>> since you're here, I'll just hijack the discussion to ask something only
>>> semi-related.
>>>
>>> IIRC the gud driver doesn't update the display immediately during atomic
>>> commits. Instead, it instructs a helper thread to do the update. What's
>>> the rational behind this design? Is that something we should adopt for
>>> other drivers that operate over slow buses (USB, I2C, etc)? Would this
>>> be relevant for the ssd1307 driver?
>>>   
>>
>> Async flushing is only necessary on multi display setups where there's
>> only one rendering loop for all the displays. I saw what tiny/gm12u320.c
>> did and Hans gave me the rationale. The SPI drivers run flushing inline.
>> Info on the gud wiki:
>> https://github.com/notro/gud/wiki/Linux-Host-Driver#asynchronous-flushing
> 
> Hi,
> 
> please also consider that userspace may throttle to the KMS pageflip
> events. If the pageflip event is immediate from submitting a flip, that
> could mean userspace will be repainting in a busy-loop, like 1 kHz.
> However, I remember something about virtual KMS drivers doing exactly
> this, and there being something that tells userspace to throttle itself
> instead of depending on pageflip completions. I just forget how that is
> supposed to work, and I'm fairly sure that e.g. Weston does not behave
> well there.
> 
> Unfortunately, the pageflip event is also what synchronises FB usage.
> Once flipping in a new FB completed, the old FB is free for re-use.
> But, if the kernel is still copying out from the old FB, userspace may
> partially overwrite the contents, temporarily leading to an incomplete
> or too new image on screen. Do you have anything to prevent that?
> 

Unfortunately not. One solution would be to make a buffer copy during
the flip and do the USB transfer async but I haven't looked into that.
My plan is to wait and see what problems users report back before trying
to fix anything.

Noralf.