linux-kernel - Re: [PATCH 1/2] regmap: Add DSI bus support

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Date:   Thu, 11 Jul 2019 15:41:59 +0100
From:   Mark Brown <broonie@...nel.org>
To:     Jeffrey Hugo <jeffrey.l.hugo@...il.com>
Cc:     Andrzej Hajda <a.hajda@...sung.com>,
        Laurent Pinchart <Laurent.pinchart@...asonboard.com>,
        Dave Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        Rob Clark <robdclark@...il.com>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        "open list:DRM PANEL DRIVERS" <dri-devel@...ts.freedesktop.org>,
        MSM <linux-arm-msm@...r.kernel.org>,
        lkml <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] regmap: Add DSI bus support

On Wed, Jul 10, 2019 at 12:08:34PM -0600, Jeffrey Hugo wrote:
> On Fri, Jul 5, 2019 at 7:06 PM Mark Brown <broonie@...nel.org> wrote:

> The addresses for these spec defined messages are 8-bit wide, so 256
> valid "destinations".  However, the payload is variable.  Most of the
> defined operations take an 8-bit payload, but there are a few that I
> see with 16-bit payloads.

Oh, good, variable register sizes, what a market leading idea :(
That basically doesn't work with regmap, you need to either
define one regmap per register size and attach them to the device
or use reg_read() and reg_write() and hide the complexity in
there.

> As the contents of the generic read/write messages are implementation
> defined, the answer to your question seems to be no - the spec does
> not define that the registers are 8-bit addressable, and 8-bit wide.

The code definitely ought to at least be more flexible then.
Right now it's very hard coded.

> I think perhaps the discussion needs to step back a bit, and decide
> how flexible do we want this regmap over DSI to be?  I think its
> usefulness comes from when a device can be configured via multiple
> interfaces, so I don't expect it to be useful for every DSI interface.
> It seems like the DSI panels use DSI directly to craft their
> configuration.  As a result, we are probably looking at just devices
> which use the generic read/write commands, but sadly the format for
> those is not universal per the spec.  From the implementations I've
> seen, I suspect 8-bit addressing of 8-bit wide registers to be the
> most common, but apparently there is an exception to that already in
> the one device that I care about.

It's relatively easy to add a bunch of special cases in - look at
how the I2C code handles it, keying off a combination of the
register configuration and the capabilities of the host
controller.  I guess for this it'd mainly be the register
configuration.  You might find the reg_read()/reg_write()
interface better than the raw buffer one for some of the formats,
it does let 

> Do we want to go forward with this regmap support just for the one TI
> device, and see what other usecases come out of it, and attempt to
> solve those as we go?

I have no strong opinions here, it looks fine from a framework
point of view though it's unclear to me if viewing it as a
register map meshes well with how the hardware is designed or not
- it seems plausible though.

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