Subject: Re: [PATCH] ARM: dts: bcm2711: fix soc's node dma-ranges

From: Florian Fainelli <[email-masked]>
Date: Fri, 6 Dec 2019 10:13:36 -0800

On 12/6/19 2:16 AM, Phil Elwell wrote:
> Hi Nicolas,
> 
> On 06/12/2019 00:08, Florian Fainelli wrote:
>> On 12/4/19 4:56 AM, Nicolas Saenz Julienne wrote:
>>> Raspberry Pi's firmware has a feature to select how much memory to
>>> reserve for its GPU called 'gpu_mem'. The possible values go from 16MB
>>> to 944MB, with a default of 64MB. This memory resides in the topmost
>>> part of the lower 1GB memory area and grows bigger expanding towards the
>>> begging of memory.
>>>
>>> It turns out that with low 'gpu_mem' values (16MB and 32MB) the size of
>>> the memory available to the system in the lower 1GB area can outgrow the
>>> interconnect's dma-range as its size was selected based on the maximum
>>> system memory available given the default gpu_mem configuration. This
>>> makes that memory slice unavailable for DMA. And may cause nasty kernel
>>> warnings if CMA happens to include it.
>>>
>>> Change soc's dma-ranges to really reflect it's HW limitation, which is
>>> being able to only DMA to the lower 1GB area.
>>>
>>> Fixes: 7dbe8c62ceeb ("ARM: dts: Add minimal Raspberry Pi 4 support")
>>> Signed-off-by: Nicolas Saenz Julienne <[email-masked]>
>>> ---
>>>
>>> NOTE: I'd appreciate if someone from the RPi foundation commented on
>>> this as it's something that I'll propose to be backported to their tree.
> 
> The 0x3c000000 size was a mistake that arose from c0000000 + 3c000000 =
> fc000000, but that is mixing apples and oranges (actually DMA addresses
> and host physical addresses). Please correct it as you are proposing.

Do you want to add an Acked-by or Reviewed-by tag to make this statement
official?
-- 
Florian

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