Re: [PATCH 3/5] drm: fix drm_mode_addfb() on big endian machines.

From: Daniel Vetter <daniel@ffwll.ch>
To: Gerd Hoffmann <kraxel@redhat.com>
Cc: dri-devel@lists.freedesktop.org, David Airlie <airlied@linux.ie>,
	open list <linux-kernel@vger.kernel.org>,
	Sean Paul <sean@poorly.run>
Subject: Re: [PATCH 3/5] drm: fix drm_mode_addfb() on big endian machines.
Date: Mon, 3 Sep 2018 18:45:58 +0200	[thread overview]
Message-ID: <20180903164558.GL21634@phenom.ffwll.local> (raw)
In-Reply-To: <20180903105756.24912-4-kraxel@redhat.com>

On Mon, Sep 03, 2018 at 12:57:54PM +0200, Gerd Hoffmann wrote:
> Userspace on big endian machhines typically expects the ADDFB ioctl
> returns a big endian framebuffer.  drm_mode_addfb() will call
> drm_mode_addfb2() unconditionally with little endian DRM_FORMAT_*
> values though, which is wrong.  This patch fixes that.
> 
> Drivers (both kernel and xorg) have quirks in place to deal with the
> broken drm_mode_addfb() behavior.  Because of this we can't just change
> drm_mode_addfb() behavior for everybody without breaking things.  So add
> a new driver feature flag DRIVER_PREFER_HOST_BYTE_ORDER, so drivers can
> opt-in.
> 
> Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
> ---
>  include/drm/drm_drv.h             |  1 +
>  drivers/gpu/drm/drm_framebuffer.c | 11 +++++++++++
>  2 files changed, 12 insertions(+)
> 
> diff --git a/include/drm/drm_drv.h b/include/drm/drm_drv.h
> index 46a8009784..9cf12596cd 100644
> --- a/include/drm/drm_drv.h
> +++ b/include/drm/drm_drv.h
> @@ -57,6 +57,7 @@ struct drm_printer;
>  #define DRIVER_KMS_LEGACY_CONTEXT	0x20000
>  #define DRIVER_SYNCOBJ                  0x40000
>  #define DRIVER_PREFER_XBGR_30BPP        0x80000
> +#define DRIVER_PREFER_HOST_BYTE_ORDER   0x100000

Hm, not a huge fan of using driver_flags for random little quirks. I think
a boolean in sturct drm_mode_config would be much better. Bonus if you
also move the 30bpp hack over to that. Something like
mode_config.quirk_addfb_host_byte_order and
mode_config.quirk_addfb_prefer_xbgr_30bpp or whatever. That has the upside
of giving us a really nice place to put a huge comment about what this is
supposed to do.

I think otherwise this looks overall rather reasonable. I think the only
other driver that ever cared about big endian was radeon/amdgpu. Would be
good to get at least an ack from amd folks, or a "meh, stopped caring".
-Daniel

>  
>  /**
>   * struct drm_driver - DRM driver structure
> diff --git a/drivers/gpu/drm/drm_framebuffer.c b/drivers/gpu/drm/drm_framebuffer.c
> index 88758096d5..ccbda8a2e9 100644
> --- a/drivers/gpu/drm/drm_framebuffer.c
> +++ b/drivers/gpu/drm/drm_framebuffer.c
> @@ -124,6 +124,17 @@ int drm_mode_addfb(struct drm_device *dev, struct drm_mode_fb_cmd *or,
>  	    dev->driver->driver_features & DRIVER_PREFER_XBGR_30BPP)
>  		r.pixel_format = DRM_FORMAT_XBGR2101010;
>  
> +	if (dev->driver->driver_features & DRIVER_PREFER_HOST_BYTE_ORDER) {
> +		if (r.pixel_format == DRM_FORMAT_XRGB8888)
> +			r.pixel_format = DRM_FORMAT_HOST_XRGB8888;
> +		if (r.pixel_format == DRM_FORMAT_ARGB8888)
> +			r.pixel_format = DRM_FORMAT_HOST_ARGB8888;
> +		if (r.pixel_format == DRM_FORMAT_RGB565)
> +			r.pixel_format = DRM_FORMAT_HOST_RGB565;
> +		if (r.pixel_format == DRM_FORMAT_XRGB1555)
> +			r.pixel_format = DRM_FORMAT_HOST_XRGB1555;
> +	}
> +
>  	ret = drm_mode_addfb2(dev, &r, file_priv);
>  	if (ret)
>  		return ret;
> -- 
> 2.9.3
> 
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-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch