Message-ID: <20250929-gregarious-worm-of-memory-c5354d@houat>
Date: Mon, 29 Sep 2025 15:00:04 +0200
From: Maxime Ripard <mripard@...nel.org>
To: Dmitry Baryshkov <dmitry.baryshkov@....qualcomm.com>
Cc: Daniel Stone <daniel@...ishbar.org>, 
	Andrzej Hajda <andrzej.hajda@...el.com>, Neil Armstrong <neil.armstrong@...aro.org>, 
	Robert Foss <rfoss@...nel.org>, Laurent Pinchart <Laurent.pinchart@...asonboard.com>, 
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	Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>, Thomas Zimmermann <tzimmermann@...e.de>, 
	David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>, 
	Sandy Huang <hjc@...k-chips.com>, Heiko Stübner <heiko@...ech.de>, 
	Andy Yan <andy.yan@...k-chips.com>, Chen-Yu Tsai <wens@...e.org>, 
	Samuel Holland <samuel@...lland.org>, Dave Stevenson <dave.stevenson@...pberrypi.com>, 
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	Liu Ying <victor.liu@....com>, Rob Clark <robin.clark@....qualcomm.com>, 
	Dmitry Baryshkov <lumag@...nel.org>, Abhinav Kumar <abhinav.kumar@...ux.dev>, 
	Jessica Zhang <jessica.zhang@....qualcomm.com>, Sean Paul <sean@...rly.run>, 
	Marijn Suijten <marijn.suijten@...ainline.org>, dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org, 
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Subject: Re: [PATCH v3 00/11] drm/connector: hdmi: limit infoframes per
 driver capabilities

On Thu, Sep 25, 2025 at 05:16:07PM +0300, Dmitry Baryshkov wrote:
> On Thu, Sep 25, 2025 at 03:13:47PM +0200, Maxime Ripard wrote:
> > On Wed, Sep 10, 2025 at 06:26:56PM +0300, Dmitry Baryshkov wrote:
> > > On Wed, Sep 10, 2025 at 09:30:19AM +0200, Maxime Ripard wrote:
> > > > On Wed, Sep 03, 2025 at 03:03:43AM +0300, Dmitry Baryshkov wrote:
> > > > > On Tue, Sep 02, 2025 at 08:06:54PM +0200, Maxime Ripard wrote:
> > > > > > On Tue, Sep 02, 2025 at 06:45:44AM +0300, Dmitry Baryshkov wrote:
> > > > > > > On Mon, Sep 01, 2025 at 09:07:02AM +0200, Maxime Ripard wrote:
> > > > > > > > On Sun, Aug 31, 2025 at 01:29:13AM +0300, Dmitry Baryshkov wrote:
> > > > > > > > > On Sat, Aug 30, 2025 at 09:30:01AM +0200, Daniel Stone wrote:
> > > > > > > > > > Hi Dmitry,
> > > > > > > > > > 
> > > > > > > > > > On Sat, 30 Aug 2025 at 02:23, Dmitry Baryshkov
> > > > > > > > > > <dmitry.baryshkov@....qualcomm.com> wrote:
> > > > > > > > > > > It's not uncommon for the particular device to support only a subset of
> > > > > > > > > > > HDMI InfoFrames. It's not a big problem for the kernel, since we adopted
> > > > > > > > > > > a model of ignoring the unsupported Infoframes, but it's a bigger
> > > > > > > > > > > problem for the userspace: we end up having files in debugfs which do
> > > > > > > > > > > mot match what is being sent on the wire.
> > > > > > > > > > >
> > > > > > > > > > > Sort that out, making sure that all interfaces are consistent.
> > > > > > > > > > 
> > > > > > > > > > Thanks for the series, it's a really good cleanup.
> > > > > > > > > > 
> > > > > > > > > > I know that dw-hdmi-qp can support _any_ infoframe, by manually
> > > > > > > > > > packing it into the two GHDMI banks. So the supported set there is
> > > > > > > > > > 'all of the currently well-known ones, plus any two others, but only
> > > > > > > > > > two and not more'. I wonder if that has any effect on the interface
> > > > > > > > > > you were thinking about for userspace?
> > > > > > > > > 
> > > > > > > > > I was mostly concerned with the existing debugfs interface (as it is
> > > > > > > > > also used e.g. for edid-decode, etc).
> > > > > > > > > 
> > > > > > > > > It seems "everything + 2 spare" is more or less common (ADV7511, MSM
> > > > > > > > > HDMI also have those. I don't have at hand the proper datasheet for
> > > > > > > > > LT9611 (non-UXC one), but I think its InfoFrames are also more or less
> > > > > > > > > generic).  Maybe we should change debugfs integration to register the
> > > > > > > > > file when the frame is being enabled and removing it when it gets unset.
> > > > > > > > 
> > > > > > > > But, like, for what benefit?
> > > > > > > > 
> > > > > > > > It's a debugfs interface for userspace to consume. The current setup
> > > > > > > > works fine with edid-decode already. Why should we complicate the design
> > > > > > > > that much and create fun races like "I'm running edid-decode in parallel
> > > > > > > > to a modeset that would remove the file I just opened, what is the file
> > > > > > > > now?".
> > > > > > > 
> > > > > > > Aren't we trading that with the 'I'm running edid-decode in paralle with
> > > > > > > to a modeset and the file suddenly becomes empty'?
> > > > > > 
> > > > > > In that case, you know what the file is going to be: empty. And you went
> > > > > > from a racy, straightforward, design to a racy, complicated, design.
> > > > > > 
> > > > > > It was my question before, but I still don't really see what benefits it
> > > > > > would have, and why we need to care about it in the core, when it could
> > > > > > be dealt with in the drivers just fine on a case by case basis.
> > > > > 
> > > > > Actually it can not: debugfs files are registered from the core, not
> > > > > from the drivers. That's why I needed all the supported_infoframes
> > > > > (which later became software_infoframes).
> > > > 
> > > > That's one thing we can change then.
> > > > 
> > > > > Anyway, I'm fine with having empty files there.
> > > > > 
> > > > > > > > > Then in the long run we can add 'slots' and allocate some of the frames
> > > > > > > > > to the slots. E.g. ADV7511 would get 'software AVI', 'software SPD',
> > > > > > > > > 'auto AUDIO' + 2 generic slots (and MPEG InfoFrame which can probably be
> > > > > > > > > salvaged as another generic one)). MSM HDMI would get 'software AVI',
> > > > > > > > > 'software AUDIO' + 2 generic slots (+MPEG + obsucre HDMI which I don't
> > > > > > > > > want to use). Then the framework might be able to prioritize whether to
> > > > > > > > > use generic slots for important data (as DRM HDR, HDMI) or less important
> > > > > > > > > (SPD).
> > > > > > > > 
> > > > > > > > Why is it something for the framework to deal with? If you want to have
> > > > > > > > extra infoframes in there, just go ahead and create additional debugfs
> > > > > > > > files in your driver.
> > > > > > > > 
> > > > > > > > If you want to have the slot mechanism, check in your atomic_check that
> > > > > > > > only $NUM_SLOT at most infoframes are set.
> > > > > > > 
> > > > > > > The driver can only decide that 'we have VSI, SPD and DRM InfoFrames
> > > > > > > which is -ETOOMUCH for 2 generic slots'. The framework should be able to
> > > > > > > decide 'the device has 2 generic slots, we have HDR data, use VSI and
> > > > > > > DRM InfoFrames and disable SPD for now'.
> > > > > > 
> > > > > > I mean... the spec does? The spec says when a particular feature
> > > > > > requires to send a particular infoframe. If your device cannot support
> > > > > > to have more than two "features" enabled at the same time, so be it. It
> > > > > > something that should be checked in that driver atomic_check.
> > > > > 
> > > > > Sounds good to me. Let's have those checks in the drivers until we
> > > > > actually have seveal drivers performing generic frame allocation.
> > > > > 
> > > > > > Or just don't register the SPD debugfs file, ignore it, put a comment
> > > > > > there, and we're done too.
> > > > > 
> > > > > It's generic code.
> > > > > 
> > > > > > > But... We are not there yet and I don't have clear usecase (we support
> > > > > > > HDR neither on ADV7511 nor on MSM HDMI, after carefully reading the
> > > > > > > guide I realised that ADV7511 has normal audio infoframes). Maybe I
> > > > > > > should drop all the 'auto' features, simplifying this series and land
> > > > > > > [1] for LT9611UXC as I wanted origianlly.
> > > > > > > 
> > > > > > > [1] https://lore.kernel.org/dri-devel/20250803-lt9611uxc-hdmi-v1-2-cb9ce1793acf@oss.qualcomm.com/
> > > > > > 
> > > > > > Looking back at that series, I think it still has value to rely on the
> > > > > > HDMI infrastructure at the very least for the atomic_check sanitization.
> > > > > > 
> > > > > > But since you wouldn't use the generated infoframes, just skip the
> > > > > > debugfs files registration. You're not lying to userspace anymore, and
> > > > > > you get the benefits of the HDMI framework.
> > > > > 
> > > > > We create all infoframe files for all HDMI connectors.
> > > > 
> > > > Then we can provide a debugfs_init helper to register all of them, or
> > > > only some of them, and let the drivers figure it out.
> > > > 
> > > > Worst case scenario, debugfs files will not get created, which is a much
> > > > better outcome than having to put boilerplate in every driver that will
> > > > get inconsistent over time.
> > > 
> > > debugfs_init() for each infoframe or taking some kind of bitmask?
> > 
> > I meant turning hdmi_debugfs_add and create_hdmi_*_infoframe_file into
> > public helpers. That way, drivers that don't care can use the (renamed)
> > hdmi_debugfs_add, and drivers with different constraints can register
> > the relevant infoframes directly.
> 
> Doesn't that mean more boilerplate?

I don't think it would? In the general case, it wouldn't change
anything, and in special cases, then it's probably going to be different
from one driver to the next so there's not much we can do.

> In the end, LT9611UXC is a special case, for which I'm totally fine
> not to use HDMI helpers at this point: we don't control infoframes
> (hopefully that can change), we don't care about the TMDS clock, no
> CEC, etc.

Not using the helpers sound pretty reasonable here too.

> For all other usecases I'm fine with having atomic_check() unset all
> unsupported infoframes and having empty files in debugfs. Then we can
> evolve over the time, once we see a pattern. We had several drivers
> which had very limited infoframes support, but I think this now gets
> sorted over the time.

I never talked about atomic_check()? You were initially concerned that
the framework would expose data in debugfs that it's not using. Not
registering anything in debugfs solves that, but I'm not sure we need to
special case atomic_check.

Worst case scenario, we're going to generate infoframes the driver will
ignore.

Maxime

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