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

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Date:   Thu, 11 Jul 2019 15:11:56 +0200
From:   Andrzej Hajda <a.hajda@...sung.com>
To:     Mark Brown <broonie@...nel.org>,
        Jeffrey Hugo <jeffrey.l.hugo@...il.com>
Cc:     Laurent.pinchart@...asonboard.com, airlied@...ux.ie,
        daniel@...ll.ch, robdclark@...il.com, bjorn.andersson@...aro.org,
        dri-devel@...ts.freedesktop.org, linux-arm-msm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] regmap: Add DSI bus support

On 06.07.2019 03:06, Mark Brown wrote:
> On Wed, Jul 03, 2019 at 02:45:12PM -0700, Jeffrey Hugo wrote:
>> Add basic support with a simple implementation that utilizes the generic
>> read/write commands to allow device registers to be configured.
> This looks good to me but I really don't know anything about DSI,
> I'd appreciate some review from other people who do.  I take it
> there's some spec thing in DSI that says registers and bytes must
> both be 8 bit?

I am little bit confused about regmap usage here. On the one hand it
nicely fits to this specific driver, probably because it already uses
regmap_i2c.

On the other it will be unusable for almost all current DSI drivers and
probably for most new drivers. Why?

1. DSI protocol defines actually more than 30 types of transactions[1],
but this patchset implements only few of them (dsi generic write/read
family). Is it possible to implement multiple types of transactions in
regmap?

2. There is already some set of helpers which uses dsi bus, rewriting it
on regmap is possible or driver could use of regmap and direct access
together, the question is if it is really necessary.

3. DSI devices are no MFDs so regmap abstraction has no big value added
(correct me, if there are other significant benefits).

[1]:
https://elixir.bootlin.com/linux/latest/source/include/video/mipi_display.h#L15

Regards

Andrzej

>
> A couple of minor comments, no need to resend just for these:
>
>> +       payload[0] = (char)reg;
>> +       payload[1] = (char)val;
> Do you need the casts?
>
>> +	ret = mipi_dsi_generic_write(dsi, payload, 2);
>> +	return ret < 0 ? ret : 0;
> Please just write an if statement, it helps with legibility.
>
>> +struct regmap *__regmap_init_dsi(struct mipi_dsi_device *dsi,
>> +				 const struct regmap_config *config,
>> +				 struct lock_class_key *lock_key,
>> +				 const char *lock_name)
>> +{
>> +	return __regmap_init(&dsi->dev, &dsi_bus, &dsi->dev, config,
>> +			     lock_key, lock_name);
>> +}
>> +EXPORT_SYMBOL_GPL(__regmap_init_dsi);
> Perhaps validate that the config is OK (mainly the register/value
> sizes)?  Though I'm not sure it's worth it so perhaps not - up to
> you.