linux-kernel - Re: [PATCH v2 5/7] drm/sun4i: backend: Offset layer buffer address by DRAM starting address

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Message-ID: <20171017142608.lycfvzydn5xvnazh@flea.lan>
Date:   Tue, 17 Oct 2017 16:26:08 +0200
From:   Maxime Ripard <maxime.ripard@...e-electrons.com>
To:     Chen-Yu Tsai <wens@...e.org>
Cc:     David Airlie <airlied@...ux.ie>, dri-devel@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org, linux-sunxi@...glegroups.com
Subject: Re: [PATCH v2 5/7] drm/sun4i: backend: Offset layer buffer address
 by DRAM starting address

On Tue, Oct 17, 2017 at 12:23:47PM +0800, Chen-Yu Tsai wrote:
> The display backend, as well as other peripherals that have a DRAM
> clock gate and access DRAM directly, bypassing the system bus,
> address the DRAM starting from 0x0, while physical addresses the
> system uses starts from 0x40000000 (or 0x20000000 in A80's case).
> 
> This issue was witnessed on the Cubietruck, which has 2GB of RAM.
> 
> Devices with less RAM function normally due to the DRAM address
> wrapping around. CMA seems to always allocate its buffer at a
> very high address, close to the end of DRAM.
> 
> On a 1GB RAM device, the physical address would be something like
> 0x78000000. The DRAM address 0x78000000 would access the same DRAM
> region as 0x38000000 on a system, as the DRAM address would only
> span 0x0 ~ 0x3fffffff. The bit 0x40000000 is non-functional in this
> case.
> 
> However on the Cubietruck, the DRAM is 2GB. The physical address
> is 0x40000000 ~ 0xbfffffff. The buffer would be something like
> 0xb8000000. But the DRAM address span 0x0 ~ 0x7fffffff, meaning
> the buffer address wraps around to 0x38000000, which is wrong.
> The correct DRAM address for it should be 0x78000000.
> 
> Correct the address configured into the backend layer registers
> by PHYS_OFFSET to account for this.
> 
> Fixes: 9026e0d122ac ("drm: Add Allwinner A10 Display Engine support")
> Signed-off-by: Chen-Yu Tsai <wens@...e.org>

Applied, thanks!
Maxime

-- 
Maxime Ripard, Free Electrons
Embedded Linux and Kernel engineering
http://free-electrons.com

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