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Date:	Fri, 7 Mar 2014 00:41:05 +0100
From:	David Herrmann <>
To:	Bruno Prémont <>
Cc:	"" <>,
	Daniel Vetter <>,
	linux-kernel <>
Subject: Re: [RFC] drm: add kernel-log renderer


On Thu, Mar 6, 2014 at 10:56 PM, Bruno Prémont
<> wrote:
> Hi David,
> On Thu, 06 March 2014 David Herrmann <> wrote:
>> On modern linux user-space, the VT subsystem is no longer needed for
>> system consoles. Although most DEs will fail if VTs are disabled, there
>> are several gfx-systems that support this mode. Especially the lower
>> system stack has been extended to work without VTs.
>> However, there is one major drawback if VTs are disabled: You don't get
>> oops/panic-screens nor early boot-debugging. The VT subsystem registers a
>> console-driver, thus displays the kernel log and oops/panic screens in
>> those situations. This patch introduces a fallback for CONFIG_VT=n.
>> A new DRM-Log core is added. At its heart, DRM-Log maintains a log-buffer
>> of kernel-log messages. It registers a console-driver and pushes new
>> messages into this buffer whenever they appear. The size of the log-buffer
>> can be changed via drm_log_ensure_size(). Initially, a suitable buffer is
>> chosen, but whenever drivers register high-res CRTCs, they ought to
>> increase that buffer to guarantee there's always enough data to render the
>> whole screen.
>> This log-buffer is managed at the character-level, not pixel-level. It is
>> shared across all users, supports parallel, atomic readers and writers and
>> supports seamless resizing.
> Does it make sense to express drm_log_ensure_size() with pixel arguments
> and have the buffer below operated in characters? Shouldn't the pixel->char
> calculation be done at a higher level? (see also below regarding high-DPI)

I don't want to expose "character" sizes. Any external API should be
dealing with pixels and only pixels. There is no reason to let them
deal with glyphs and text.. Regarding DPI, see below.

>> Additionally to the log-buffer handling, a generic software renderer is
>> introduced. drm_log_draw() renders the current log-buffer into any
>> memory-mapped framebuffer you want. Note that it supports splitting lines
>> in case your framebuffer is smaller than the log-buffer, but also merging
>> continuation lines in case your framebuffer is wider. This allows
>> rendering an 80x25 log-buffer in full-width on high-res displays. On the
>> other hand, 800x250 log-buffers can be rendered without information loss
>> (apart from a shorter backlog) on low-res displays.
> Would it be possible to pass the dlog_font to drm_log_draw() so that
> DRM driver or high-level code could choose a larger font on high-DPI
> monitors (think retina-like)?
> Without that option kernel would need to be built specifically for a
> given DPI range and not be able to provide readable debug output on
> multiple CRTCs with (very) different DPIs.

First of all, I won't support any fancy high-DPI features, that's just
wrong for debugging stuff. What I can do (and what we do in fallback
consoles in user-space) is to add an "dpi" or "scale" argument to
drm_log_draw() which does _integer_ scaling of glyphs. This allows you
to render glyphs 2x or 3x .. as big as usual.
I don't think it makes sense to modify drm_log_ensure_size(). I mean,
the worst that can happen is that the *text*-backlog is twice as big
as required. But if you have a high-dpi display, you already require
like 10x as much space for each framebuffer than for the entire
log-buffer. The log-buffer requires 1 byte per *char*. The frambuffer
requires at least 8*16*4=512 bytes per char.

Anyhow, integer-scaling should be fairly easy to get done.

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