linux-kernel - Re: [PATCH v2] drm/mxsfb: Fix runtime PM for unpowering lcdif block

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Date:   Thu, 02 Aug 2018 13:06:42 +0200
From:   Stefan Agner <stefan@...er.ch>
To:     Philipp Zabel <p.zabel@...gutronix.de>
Cc:     Leonard Crestez <leonard.crestez@....com>, marex@...x.de,
        shawnguo@...nel.org, Anson Huang <anson.huang@....com>,
        airlied@...ux.ie, linux-kernel@...r.kernel.org,
        dri-devel@...ts.freedesktop.org,
        Fabio Estevam <fabio.estevam@....com>,
        dl-linux-imx <linux-imx@....com>, kernel@...gutronix.de,
        Robert Chiras <robert.chiras@....com>, l.stach@...gutronix.de
Subject: Re: [PATCH v2] drm/mxsfb: Fix runtime PM for unpowering lcdif block

On 02.08.2018 12:17, Philipp Zabel wrote:
> On Tue, 2018-07-31 at 12:17 +0000, Leonard Crestez wrote:
>> On Tue, 2018-07-31 at 13:54 +0200, Philipp Zabel wrote:
>> > On Tue, 2018-07-17 at 13:48 +0300, Leonard Crestez wrote:
>> > > Adding lcdif nodes to a power domain currently does work, it results in
>> > > black/corrupted screens or hangs. While the driver does enable runtime
>> > > pm it does not deal correctly with the block being unpowered.
>> > >
>> > > Ensure power is on when required by adding pm_runtime_get/put_sync to
>> > > mxsfb_pipe_enable/disable.
>> > >
>> > > Since power is lost on suspend implement PM_SLEEP_OPS using
>> > > drm_mode_config_helper_suspend/resume.
>> > >
>> > > The mxsfb_plane_atomic_update function can get called before
>> > > mxsfb_pipe_enable while the block is not yet powered. When this happens
>> > > the write to LCDIF_NEXT_BUF is lost causing corrupt display on unblank
>> > > until a refresh.
>> >
>> > Why does this happen?
>>
>> I'm not sure what you're asking but register writes to unpowered or
>> unclocked blocks are not expected to "just work". Here the write is
>> ignored/lost but I think on imx8 it can even cause a bus error.
>>
>> The approach here is to only set the framebuffer address as part of
>> activating the display.
> 
> I wonder why atomic update is called at all while the pipe is not
> enabled.

It can be made to behave differently (see also my review).

However, the default seems also a bit unfortunate to me.

--
Stefan