linux-kernel - Re: [PATCH 03/10] drm/sun4i: Remove obsolete references to PHYS

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Date:   Mon, 11 Apr 2022 21:56:31 -0500
From:   Samuel Holland <samuel@...lland.org>
To:     Jernej Škrabec <jernej.skrabec@...il.com>,
        Chen-Yu Tsai <wens@...e.org>,
        Maxime Ripard <mripard@...nel.org>
Cc:     Daniel Vetter <daniel@...ll.ch>, David Airlie <airlied@...ux.ie>,
        Krzysztof Kozlowski <krzk+dt@...nel.org>,
        Rob Herring <robh+dt@...nel.org>, devicetree@...r.kernel.org,
        dri-devel@...ts.freedesktop.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-sunxi@...ts.linux.dev
Subject: Re: [PATCH 03/10] drm/sun4i: Remove obsolete references to
 PHYS_OFFSET

On 4/11/22 10:37 AM, Jernej Škrabec wrote:
> Dne ponedeljek, 11. april 2022 ob 06:34:15 CEST je Samuel Holland napisal(a):
>> commit b4bdc4fbf8d0 ("soc: sunxi: Deal with the MBUS DMA offsets in a
>> central place") added a platform device notifier that sets the DMA
>> offset for all of the display engine frontend and backend devices.
>>
>> The code applying the offset to DMA buffer physical addresses was then
>> removed from the backend driver in commit 756668ba682e ("drm/sun4i:
>> backend: Remove the MBUS quirks"), but the code subtracting PHYS_OFFSET
>> was left in the frontend driver.
>>
>> As a result, the offset was applied twice in the frontend driver. This
>> likely went unnoticed because it only affects specific configurations
>> (scaling or certain pixel formats) where the frontend is used, on boards
>> with both one of these older SoCs and more than 1 GB of DRAM.
>>
>> In addition, the references to PHYS_OFFSET prevent compiling the driver
>> on architectures where PHYS_OFFSET is not defined.
>>
>> Fixes: b4bdc4fbf8d0 ("soc: sunxi: Deal with the MBUS DMA offsets in a central 
> place")
>> Signed-off-by: Samuel Holland <samuel@...lland.org>
> 
> Good catch! Actually, people complained about non-working display on 
> Cubietruck IIRC, which has 2 GB of RAM.
> 
> Did you test this on HW?

The only DE1 board I have is an A33 tablet with 512 MB of DRAM. So while I
boot-tested the patch, I am not able to verify if it has any real effect.

The reason for sending this is that folks ran in to compilation errors on
RISC-V. And I was surprised to still see references to PHYS_OFFSET, since
sunxi_mbus.c includes the frontend compatibles.

Regards,
Samuel

> Reviewed-by: Jernej Skrabec <jernej.skrabec@...il.com>
> 
> Best regards,
> Jernej