[PATCH v2 1/2] drm/mipi-dsi: add (LPM) Low Power Mode transfer support

[Thierry Reding]

On Mon, Jul 28, 2014 at 06:09:58PM +0200, Andrzej Hajda wrote:
> On 07/28/2014 04:00 AM, Inki Dae wrote:
> > This patch adds below two flags for LPM transfer, and it attaches LPM flags
> > to a msg in accordance with master's mode_flags set by LCD Panel driver.
> > 
> > MIPI_DSI_MODE_CMD_LPM
> > - If this flag is set by Panel driver, MIPI-DSI controller will tranfer
> > command data to Panel device in Low Power Mode.
> 
> What do you mean by command data? It could be:
> - all transfer in command mode of operations,
> - transfer initialized by the driver by writing to DSIM registers.
> 
> > 
> > MIPI_DSI_MODE_VIDEO_LPM
> > - If this flag is set by Panel driver, MIPI-DSI controller will tranfer
> > image data to Panel device in Low Power Mode.
> 
> What is the meaning of this flag in case of command mode of operation?
> 
> Maybe it would be better to create flags based on source of data/FIFOs:
> - commands send by SFR registers,
> - commands generated from data sent from Display Controller.

I have no idea what SFR is. But it sounds like you're talking about two
ways to generate command packets here. We have something similar on
Tegra, where it's called "host-driven command mode" and "DC-driven
command mode".

In host-driven command mode the driver needs to explicitly program some
of the DSI controller registers to send command packets. This is
essentially a FIFO register and a control register to trigger
transmission and poll for completion.

DC-driven command mode is typically used to update contents of a remote
framebuffer (what MIPI calls "Type 1 Display Architecture"). This is
done by programming a different set of registers that cause the DSI
controller to take data output by the display controller and wrap it
into DCS commands (e.g. write_memory_start and write_memory_continue).

I think that low power mode is more often used for command transmission
(in host-driven mode). I'm not sure how much sense it really makes to
transmit video data in low power mode. It also seems like low power mode
is what all peripherals need to support (if they can do command mode).
Hence I'd like to propose the attached patch that makes all command
messages use low power mode.

The .transfer() function was really designed with initialization
commands in mind, so it doesn't deal with mixing video data and commands
anyway and for initialization low-power mode should be fast enough. The
downside is that it may not be optimal for some peripherals, but it
gives us a good solution for the general case since it should support
all devices.

If we absolutely must have faster initialization, or if we come across a
device that can only initialize in high speed mode, then I think we
should introduce a new flag to allow DSI host controllers to optimize in
those cases.

Note that this is based on some of my local patches, so it won't apply
as-is. But if anybody wants to give this a go it should be easy to apply
manually as well.

Thierry
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