Re: [PATCH v3 5/7] drm/vc4: kms: Document the muxing corner cases

From: Maxime Ripard <maxime@cerno.tech>
To: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Mark Rutland <mark.rutland@arm.com>,
	devicetree@vger.kernel.org, Tim Gover <tim.gover@raspberrypi.com>,
	Dave Stevenson <dave.stevenson@raspberrypi.com>,
	David Airlie <airlied@linux.ie>,
	dri-devel@lists.freedesktop.org,
	Phil Elwell <phil@raspberrypi.com>,
	Rob Herring <robh+dt@kernel.org>,
	bcm-kernel-feedback-list@broadcom.com,
	linux-rpi-kernel@lists.infradead.org,
	Daniel Vetter <daniel.vetter@intel.com>,
	Frank Rowand <frowand.list@gmail.com>,
	Hoegeun Kwon <hoegeun.kwon@samsung.com>,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH v3 5/7] drm/vc4: kms: Document the muxing corner cases
Date: Thu, 19 Nov 2020 14:53:56 +0100	[thread overview]
Message-ID: <20201119135356.25tsxpbvjxopz6od@gilmour.lan> (raw)
In-Reply-To: <2a79055f-a2e7-913c-b566-91780f199016@suse.de>

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Hi Thomas,

Thanks again for your review

On Thu, Nov 19, 2020 at 09:11:58AM +0100, Thomas Zimmermann wrote:
> Hi,
> 
> A few suggestions below. But I'm not a native speaker.
> 
> Am 05.11.20 um 14:56 schrieb Maxime Ripard:
> > We've had a number of muxing corner-cases with specific ways to reproduce
> > them, so let's document them to make sure they aren't lost and introduce
> > regressions later on.
> > 
> > Reviewed-by: Hoegeun Kwon <hoegeun.kwon@samsung.com>
> > Tested-by: Hoegeun Kwon <hoegeun.kwon@samsung.com>
> > Signed-off-by: Maxime Ripard <maxime@cerno.tech>
> > ---
> >   drivers/gpu/drm/vc4/vc4_kms.c | 22 ++++++++++++++++++++++
> >   1 file changed, 22 insertions(+)
> > 
> > diff --git a/drivers/gpu/drm/vc4/vc4_kms.c b/drivers/gpu/drm/vc4/vc4_kms.c
> > index bb2efc5d2d01..499c6914fce4 100644
> > --- a/drivers/gpu/drm/vc4/vc4_kms.c
> > +++ b/drivers/gpu/drm/vc4/vc4_kms.c
> > @@ -662,6 +662,28 @@ static int vc4_load_tracker_obj_init(struct vc4_dev *vc4)
> >   	return drmm_add_action_or_reset(&vc4->base, vc4_load_tracker_obj_fini, NULL);
> >   }
> > +/*
> > + * The BCM2711 HVS has up to 7 output connected to the pixelvalves and
> 
> '7 outputs'
> 
> Is it 'pixelvalves' or 'pixel valves'?

The documentation uses both, but the driver uses the former all the
time.

> 
> > + * the TXP (and therefore all the CRTCs found on that platform).
> > + *
> > + * The naive (and our initial) implementation would just iterate over
> > + * all the active CRTCs, try to find a suitable FIFO, and then remove it
> > + * from the available FIFOs pool. However, there's a few corner cases
> 
> I'd write. 'and remove it from the pool of available FIFOs'. Sounds more
> natural to me.
> 
> 'there are a few'
> 
> > + * that need to be considered:
> > + *
> > + * - When running in a dual-display setup (so with two CRTCs involved),
> > + *   we can update the state of a single CRTC (for example by changing
> > + *   its mode using xrandr under X11) without affecting the other. In
> > + *   this case, the other CRTC wouldn't be in the state at all, so we
> > + *   need to consider all the running CRTCs in the DRM device to assign
> > + *   a FIFO, not just the one in the state.
> > + *
> > + * - Since we need the pixelvalve to be disabled and enabled back when
> > + *   the FIFO is changed, we should keep the FIFO assigned for as long
> > + *   as the CRTC is enabled, only considering it free again once that
> > + *   CRTC has been disabled. This can be tested by booting X11 on a
> > + *   single display, and changing the resolution down and then back up.
> > + */
> 
> With my suggestions considered,
> 
> Acked-by: Thomas Zimmermann <tzimmermann@suse.de>

I've applied it with your comments fixed. Thanks!
Maxime

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