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Subject: Re: [PATCH 1/2] regmap: Add DSI bus support
To:     Rob Clark <robdclark@gmail.com>
Cc:     Mark Brown <broonie@kernel.org>,
        Jeffrey Hugo <jeffrey.l.hugo@gmail.com>,
        Laurent Pinchart <Laurent.pinchart@ideasonboard.com>,
        David Airlie <airlied@linux.ie>,
        Daniel Vetter <daniel@ffwll.ch>,
        Bjorn Andersson <bjorn.andersson@linaro.org>,
        dri-devel <dri-devel@lists.freedesktop.org>,
        linux-arm-msm <linux-arm-msm@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
From:   Andrzej Hajda <a.hajda@samsung.com>
Message-ID: <10b1313f-7a60-df04-a9e3-76649b74f2f0@samsung.com>
Date:   Fri, 12 Jul 2019 15:01:44 +0200
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On 11.07.2019 15:56, Rob Clark wrote:
> On Thu, Jul 11, 2019 at 6:11 AM Andrzej Hajda <a.hajda@samsung.com> wrote:
>> 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).
>>
> I assume it is not *just* this one bridge that can be programmed over
> either i2c or dsi, depending on how things are wired up on the board.
> It certainly would be nice for regmap to support this case, so we
> don't have to write two different bridge drivers for the same bridge.
> I wouldn't expect a panel that is only programmed via dsi to use this.


On the other side supporting DSI and I2C in one driver is simply matter
of writing proper accesors.


Regards

Andrzej


>
> BR,
> -R
>
>> [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.
>>