Re: [PATCH v4 5/5] drm/ofdrm: Support big-endian scanout buffers

From: Thomas Zimmermann <tzimmermann@suse.de>
To: Javier Martinez Canillas <javierm@redhat.com>,
	airlied@linux.ie, daniel@ffwll.ch, deller@gmx.de,
	maxime@cerno.tech, sam@ravnborg.org, msuchanek@suse.de,
	mpe@ellerman.id.au, benh@kernel.crashing.org, paulus@samba.org,
	geert@linux-m68k.org, mark.cave-ayland@ilande.co.uk
Cc: linux-fbdev@vger.kernel.org, linuxppc-dev@lists.ozlabs.org,
	dri-devel@lists.freedesktop.org
Subject: Re: [PATCH v4 5/5] drm/ofdrm: Support big-endian scanout buffers
Date: Tue, 11 Oct 2022 13:30:57 +0200	[thread overview]
Message-ID: <83071743-a7f2-f761-baa3-da688f26b5e3@suse.de> (raw)
In-Reply-To: <23333ff7-3ae1-494f-7abe-62da6698fd00@redhat.com>

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Hi

Am 11.10.22 um 09:46 schrieb Javier Martinez Canillas:
> Hello Thomas,
> 
> On 9/28/22 12:50, Thomas Zimmermann wrote:
>> All DRM formats assume little-endian byte order. On big-endian systems,
>> it is likely that the scanout buffer is in big endian as well. Update
> 
> You say it is likely, not always then? Does it depend on whether the Open
> Firmware is BE or LE ?

It's the endianess of the framebuffer. There's graphics hardware that 
can switch between the two or even support both at the same time 
(depending on the aperture range). I don't know the exact semantics when 
each is being used, but I suspect that it corresponds to host endianess.

> 
> [...]
> 
>> +static bool display_get_big_endian_of(struct drm_device *dev, struct device_node *of_node)
>> +{
>> +	bool big_endian;
>> +
>> +#ifdef __BIG_ENDIAN
>> +	big_endian = true;
>> +	if (of_get_property(of_node, "little-endian", NULL))
>> +		big_endian = false;
>> +#else
>> +	big_endian = false;
>> +	if (of_get_property(of_node, "big-endian", NULL))
>> +		big_endian = true;
>> +#endif
>> +
>> +	return big_endian;
>> +}
>> +
> 
> Ah, I see. The heuristic then is whether the build is BE or LE or if the Device
> Tree has an explicit node defining the endianess. The patch looks good to me:

Yes. I took this test from offb.

> 
> Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
> 

Thanks

Best regards
Thomas

-- 
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Maxfeldstr. 5, 90409 Nürnberg, Germany
(HRB 36809, AG Nürnberg)
Geschäftsführer: Ivo Totev

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