Re: [PATCH v5 1/2] drm/bridge: Add Cadence DSI driver

From: Tomi Valkeinen <tomi.valkeinen@ti.com>
To: Boris Brezillon <boris.brezillon@free-electrons.com>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>,
	"Menon, Nishanth" <nm@ti.com>, Pawel Moll <pawel.moll@arm.com>,
	Ian Campbell <ijc+devicetree@hellion.org.uk>,
	David Airlie <airlied@linux.ie>,
	Simon Hatliff <hatliff@cadence.com>,
	Mark Rutland <mark.rutland@arm.com>,
	dri-devel@lists.freedesktop.org,
	Alan Douglas <adouglas@cadence.com>,
	Rob Herring <robh+dt@kernel.org>, Jyri Sarha <jsarha@ti.com>,
	Suresh Punnoose <sureshp@cadence.com>,
	Kumar Gala <galak@codeaurora.org>,
	Maxime Ripard <maxime.ripard@free-electrons.com>,
	Richard Sproul <sproul@cadence.com>,
	devicetree@vger.kernel.org
Subject: Re: [PATCH v5 1/2] drm/bridge: Add Cadence DSI driver
Date: Tue, 30 Jan 2018 09:51:49 +0200	[thread overview]
Message-ID: <19cc13ff-d2eb-bc46-6ceb-c020ea3ddad3@ti.com> (raw)
In-Reply-To: <20180129173854.1f1ee8ce@bbrezillon>

On 29/01/18 18:38, Boris Brezillon wrote:

> Note that the TX byte clk should be configured to match the DPI pixel
> clock, which means we should refuse any config where the variation is
> too big to be recovered. Anyway, we still don't have a way to configure
> the PLL rate (which is driving the TX byte clk), so there's not much I
> can do about that right now.

We could have the code to check the allowed difference in place.

>> Another thing is that the mode->crtc_clock is in kHz, I wonder if that
>> rounding can cause miscalculations in the above code.
> 
> Do we really have modes exposing pixel clks not aligned on a Khz? I

Well, I think the clocks in the logical video modes are aligned, but the
actual clock from the display controller or the PHY most likely is not
aligned.

> know the display controller can adjust the timings, but then, the
> variation caused by the Khz approx should not be that big (999Khz /
> 10+MHz < 1/10000), and anyway, that's what the DRM framework
> manipulates...

Depends on how strict the HW is about the allowed difference between the
crtc and the DSI IP. It maybe that that 1 Hz is one too many... But that
can be avoided by just making sure that the check rounds the values
properly, or maybe just reduces the allowed range by 1kHz on both ends.

So I don't think it's an issue, but it is something to keep in mind.

 Tomi

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