From: Peter Rosin <peda@axentia.se>
To: Boris Brezillon <bbrezillon@kernel.org>
Cc: Alexandre Belloni <alexandre.belloni@bootlin.com>,
David Airlie <airlied@linux.ie>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
"dri-devel@lists.freedesktop.org"
<dri-devel@lists.freedesktop.org>,
Boris Brezillon <boris.brezillon@bootlin.com>,
"linux-arm-kernel@lists.infradead.org"
<linux-arm-kernel@lists.infradead.org>
Subject: Re: [PATCH 4/4] drm/atmel-hlcdc: do not immediately disable planes, wait for next frame
Date: Fri, 11 Jan 2019 14:29:28 +0000 [thread overview]
Message-ID: <e35d6894-fd62-efd1-38f7-cc5cbdbbacbb@axentia.se> (raw)
In-Reply-To: <20190110202522.10206f2d@bbrezillon>
On 2019-01-10 20:25, Boris Brezillon wrote:
> On Thu, 10 Jan 2019 18:51:21 +0000
> Peter Rosin <peda@axentia.se> wrote:
>
>> On 2019-01-10 18:29, Boris Brezillon wrote:
>>> On Thu, 10 Jan 2019 15:10:48 +0000
>>> Peter Rosin <peda@axentia.se> wrote:
>>>
>>>> The A2Q and UPDATE bits have no effect in the channel disable registers.
>>>> However, since they are present, assume that the intention is to disable
>>>> planes, not immediately as indicated by the RST bit, but on the next
>>>> frame shift since that is what A2Q and UPDATE means in the channel enable
>>>> registers.
>>>>
>>>> Disabling the plane on the next frame shift is done with the EN bit,
>>>> so use that.
>>>
>>> It's been a long time, but I think I had a good reason for forcing a
>>> reset. IIRC, when you don't do that and the CRTC is disabled before the
>>> plane, the EN bit stays around, and next time you queue a plane update,
>>> you'll start with an invalid buf pointer.
>>
>> It might be possible to clear the EN bit in ...CHDR before enabling the
>> plane in ...CHER. Or is that too late?
>
> I think I tried that, but I'm not sure (BTW, this change was done in
> bd4248bb5e8b ("drm: atmel-hlcdc: reset layer A2Q and UPDATE bits when
That patch is a big fat NOP if you read the documentation. Those bits
are marked with a '-' for all LCDC channel disable registers, for all
supported chips IIUC. Are the effects of those bits mentioned in any
errata?
It would be good with a comment that the present undocumented disable
method is intentional. That would have kept me from assuming the whole
thing was just copy-paste junk from ..._enable that happened to work.
>> disabling it")). Anyway, I'm not even sure this is still needed now
>> that atomic updates have a wait_for_flip_done/vblank() in the commit
>> path.
The documentation for the RST bit states "Resets the layer immediately.
The frame is aborted." which sounds a bit scary and heavy-handed. But
again, I don't know what that actually means and what the effects are
but that was the reason for me wanting to avoid the RST bit.
Cheers,
Peter
>> But this patch is not overly
>> important, I just wanted to avoid the resulting "black hole" when the
>> plane DMA is disabled mid-frame. But disabling planes is probably not
>> something that happens frequently and will perhaps not be noticed at
>> all...
>
> Okay. Other patches look good to me, I'm just waiting for Nicolas
> feedback before applying them.
>
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next prev parent reply other threads:[~2019-01-11 14:29 UTC|newest]
Thread overview: 21+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-01-10 15:10 [PATCH 0/4] drm/atmel-hlcdc: fix plane clipping/rotation issues Peter Rosin
2019-01-10 15:10 ` [PATCH 1/4] drm/atmel-hlcdc: rotate planes counterclockwise Peter Rosin
2019-01-10 15:10 ` [PATCH 2/4] drm/atmel-hlcdc: do not swap w/h of the crtc when a plane is rotated Peter Rosin
2019-01-10 17:48 ` Boris Brezillon
2019-01-11 13:29 ` Peter Rosin
2019-01-11 14:14 ` Nicolas.Ferre
2019-01-10 15:10 ` [PATCH 3/4] drm/atmel-hlcdc: fix clipping of planes Peter Rosin
2019-01-10 15:10 ` [PATCH 4/4] drm/atmel-hlcdc: do not immediately disable planes, wait for next frame Peter Rosin
2019-01-10 17:29 ` Boris Brezillon
2019-01-10 18:51 ` Peter Rosin
2019-01-10 19:25 ` Boris Brezillon
2019-01-11 14:29 ` Peter Rosin [this message]
2019-01-16 14:45 ` Boris Brezillon
2019-01-10 17:45 ` [PATCH 0/4] drm/atmel-hlcdc: fix plane clipping/rotation issues Boris Brezillon
2019-01-10 20:16 ` Sam Ravnborg
2019-01-11 9:16 ` Peter Rosin
2019-01-10 21:24 ` Peter Rosin
2019-01-11 14:18 ` Nicolas.Ferre
2019-01-27 8:27 ` Boris Brezillon
2019-01-31 13:13 ` Peter Rosin
2019-01-31 13:21 ` Boris Brezillon
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