linux-kernel - Re: [PATCH] drm: rcar-du: Fix R-Car Gen3 crash when VSP is disabled

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Date:   Wed, 26 Oct 2016 18:13:23 +0900
From:   Magnus Damm <magnus.damm@...il.com>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:     DRI Development <dri-devel@...ts.freedesktop.org>,
        Linux-Renesas <linux-renesas-soc@...r.kernel.org>,
        David Airlie <airlied@...ux.ie>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Laurent Pinchart <laurent.pinchart@...asonboard.com>
Subject: Re: [PATCH] drm: rcar-du: Fix R-Car Gen3 crash when VSP is disabled

Hi Geert,

On Wed, Oct 26, 2016 at 4:31 PM, Geert Uytterhoeven
<geert@...ux-m68k.org> wrote:
> On Wed, Oct 26, 2016 at 5:31 AM, Magnus Damm <magnus.damm@...il.com> wrote:
>> From: Magnus Damm <damm+renesas@...nsource.se>
>>
>> For the DU to operate on R-Car Gen3 hardware a combination of DU
>> and VSP devices are required. Since the DU driver also supports
>> earlier generations hardware the VSP portion is enabled via Kconfig.
>>
>> The arm64 defconfig is as of v4.9-rc1 having the DU driver enabled
>> as a module, however this is not enough to support R-Car Gen3. In
>> the current case of CONFIG_DRM_RCAR_VSP=n then the kernel crashes
>> when loading the module. This patch is fixing that particular case.
>>
>> In more detail, the crash triggers in drm_atomic_get_plane_state()
>> when __drm_atomic_helper_set_config() passes NULL as crtc->primary.
>>
>> This patch corrects this issue by failing to load the DU driver on
>> R-Car Gen3 when VSP is not available.
>>
>> Signed-off-by: Magnus Damm <damm+renesas@...nsource.se>
>> ---
>>
>>  drivers/gpu/drm/rcar-du/rcar_du_vsp.h |    2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> --- 0001/drivers/gpu/drm/rcar-du/rcar_du_vsp.h
>> +++ work/drivers/gpu/drm/rcar-du/rcar_du_vsp.h  2016-10-26 00:01:12.920607110 +0900
>> @@ -70,7 +70,7 @@ void rcar_du_vsp_disable(struct rcar_du_
>>  void rcar_du_vsp_atomic_begin(struct rcar_du_crtc *crtc);
>>  void rcar_du_vsp_atomic_flush(struct rcar_du_crtc *crtc);
>>  #else
>> -static inline int rcar_du_vsp_init(struct rcar_du_vsp *vsp) { return 0; };
>> +static inline int rcar_du_vsp_init(struct rcar_du_vsp *vsp) { return -ENXIO; };
>
> -ENODEV sounds more appropriate

Ok, however -ENXIO is the same error code as the DU driver currently
returns when dealing with the VSP and not finding DT nodes.

To avoid dealing with this again I would prefer skipping per-driver
Kconfig options entirely, for instance something like this:

[PATCH/RFC] Simplify Gen3 DU and VSP Kconfig bits
https://www.spinics.net/lists/linux-sh/msg45978.html

>>  static inline void rcar_du_vsp_enable(struct rcar_du_crtc *crtc) { };
>>  static inline void rcar_du_vsp_disable(struct rcar_du_crtc *crtc) { };
>>  static inline void rcar_du_vsp_atomic_begin(struct rcar_du_crtc *crtc) { };
>
> Alternatively,  DRM_RCAR_DU can force DRM_RCAR_VSP to y if ARCH_R8A7795
> or ARCH_R8A7796 is enabled.

The DU driver has symbol dependencies on VSP and FCP driver code
(V4L2), so we need to take the state of driver support for those
modules into consideration as well instead of only depending on
SoC-specific Kconfig symbols.

On Wed, Oct 26, 2016 at 4:31 PM, Geert Uytterhoeven
<geert@...ux-m68k.org> wrote:
>On Wed, Oct 26, 2016 at 5:31 AM, Magnus Damm <magnus.damm@...il.com> wrote:
>> The arm64 defconfig is as of v4.9-rc1 having the DU driver enabled
>> as a module, however this is not enough to support R-Car Gen3. In
>
> Nope, arm64 defconfig on v4.9-rc1:
> # CONFIG_DRM_RCAR_DU is not set

Right, you are correct. Sorry for the noise. It seems that I was using
renesas-drivers-2016-10-18-v4.9-rc1 - not mainline v4.9-rc1.

Without this fix the driver still crashes, but not due to defconfig in v4.9-rc1.

Thanks,

/ magnus