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Message-ID: <c9064cc6-2a9d-1094-53fb-cf56f9cbf191@redhat.com>
Date:   Thu, 3 Nov 2022 12:53:14 +0100
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Subject: Re: [PATCH] drm: rcar_du: DRM_RCAR_DU optionally depends on
 RCAR_MIPI_DSI
Content-Language: en-US
To:     Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>,
        Randy Dunlap <rdunlap@infradead.org>,
        linux-kernel@vger.kernel.org
Cc:     Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>,
        dri-devel@lists.freedesktop.org, linux-renesas-soc@vger.kernel.org,
        Tomi Valkeinen <tomi.valkeinen+renesas@ideasonboard.com>,
        LUU HOAI <hoai.luu.ub@renesas.com>
References: <20221018181828.19528-1-rdunlap@infradead.org>
 <a68a24a0-eda3-8711-21c1-334289eca0d3@infradead.org>
 <166747314442.3962897.9754510086268412956@Monstersaurus>
From:   Javier Martinez Canillas <javierm@redhat.com>
In-Reply-To: <166747314442.3962897.9754510086268412956@Monstersaurus>
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Hello Kieran,

On 11/3/22 11:59, Kieran Bingham wrote:
> Hi Randy,
> 
> Quoting Randy Dunlap (2022-11-03 06:06:45)
>> ping. I have verified (on linux-next-20221103) that this is still needed.
>> Thanks.
>>
>> On 10/18/22 11:18, Randy Dunlap wrote:
>>> When CONFIG_DRM_RCAR_DU=y and CONFIG_DRM_RCAR_MIPI_DSI=m, calls
>>> from the builtin driver to the mipi driver fail due to linker
>>> errors.
>>> Since the RCAR_MIPI_DSI driver is not always required, fix the
>>> build error by making DRM_RCAR_DU optionally depend on the
>>> RCAR_MIPI_DSI Kconfig symbol. This prevents the problematic
>>> kconfig combination without requiring that RCAR_MIPI_DSI always
>>> be enabled.
>>>
>>> aarch64-linux-ld: drivers/gpu/drm/rcar-du/rcar_du_crtc.o: in function `rcar_du_crtc_atomic_enable':
>>> rcar_du_crtc.c:(.text+0x3a18): undefined reference to `rcar_mipi_dsi_pclk_enable'
>>> aarch64-linux-ld: drivers/gpu/drm/rcar-du/rcar_du_crtc.o: in function `rcar_du_crtc_atomic_disable':
>>> rcar_du_crtc.c:(.text+0x47cc): undefined reference to `rcar_mipi_dsi_pclk_disable'
>>>
>>> Fixes: 957fe62d7d15 ("drm: rcar-du: Fix DSI enable & disable sequence")
>>> Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
>>> Cc: Tomi Valkeinen <tomi.valkeinen+renesas@ideasonboard.com>
>>> Cc: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
>>> Cc: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
>>> Cc: LUU HOAI <hoai.luu.ub@renesas.com>
>>> Cc: dri-devel@lists.freedesktop.org
>>> Cc: linux-renesas-soc@vger.kernel.org
>>> Cc: David Airlie <airlied@gmail.com>
>>> Cc: Daniel Vetter <daniel@ffwll.ch>
>>> ---
>>>  drivers/gpu/drm/rcar-du/Kconfig |    1 +
>>>  1 file changed, 1 insertion(+)
>>>
>>> diff -- a/drivers/gpu/drm/rcar-du/Kconfig b/drivers/gpu/drm/rcar-du/Kconfig
>>> --- a/drivers/gpu/drm/rcar-du/Kconfig
>>> +++ b/drivers/gpu/drm/rcar-du/Kconfig
>>> @@ -4,6 +4,7 @@ config DRM_RCAR_DU
>>>       depends on DRM && OF
>>>       depends on ARM || ARM64
>>>       depends on ARCH_RENESAS || COMPILE_TEST
>>> +     depends on DRM_RCAR_MIPI_DSI || DRM_RCAR_MIPI_DSI=n
> 
> Please forgive my ignorance, but I don't understand how this works.
> Could you explain what this is doing please?
> 
> I know you've explained above that it fixes it to optionally depend on
> DRM_RCAR_MIPI_DSI ... but it's not making sense to me.
> 
> To me - this is saying we depend on DRM_RCAR_MIPI_DSI being enabled, or
> not being enabled ... ? Which is like saying if (0 || 1) ?
> 
> I'm guessing I'm missing something obvious :-S
>

What this Kconfig expression is saying is that it depends on DRM_RCAR_MIPI_DSI=y
if DRM_RCAR_DU=y and DRM_RCAR_MIPI_DSI=m if DRM_RCAR_DU=m. But that the it can
also be satisfied if is not set DRM_RCAR_MIPI_DSI.

This is usually used to make sure that you don't end with a configuration where
DRM_RCAR_MIPI_DSI=y and DRM_RCAR_DU=m or DRM_RCAR_MIPI_DSI=m and DRM_RCAR_DU=y.

Randy, I think that it's more idiomatic though to it express as following:

depends on DRM_RCAR_MIPI_DSI || !DRM_RCAR_MIPI_DSI

-- 
Best regards,

Javier Martinez Canillas
Core Platforms
Red Hat