linux-kernel - Re: [PATCH v3 02/15] drm/rockchip: Set dma mask to 64 bit

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Message-ID: <4c2ebf98-7c0b-418a-a9de-ebe54acd21af@kwiboo.se>
Date: Thu, 17 Oct 2024 10:58:08 +0200
From: Jonas Karlman <jonas@...boo.se>
To: Andy Yan <andyshrk@....com>, Robin Murphy <robin.murphy@....com>
Cc: heiko@...ech.de, hjc@...k-chips.com, krzk+dt@...nel.org, robh@...nel.org,
 conor+dt@...nel.org, s.hauer@...gutronix.de, devicetree@...r.kernel.org,
 dri-devel@...ts.freedesktop.org, linux-arm-kernel@...ts.infradead.org,
 linux-kernel@...r.kernel.org, linux-rockchip@...ts.infradead.org,
 derek.foreman@...labora.com, minhuadotchen@...il.com,
 detlev.casanova@...labora.com, xxm@...k-chips.com,
 Andy Yan <andy.yan@...k-chips.com>
Subject: Re: [PATCH v3 02/15] drm/rockchip: Set dma mask to 64 bit

On 2024-10-17 09:06, Andy Yan wrote:
> 
> 
> Hi Robin,
> 
>  Thanks for your comment。
> 
> At 2024-10-17 01:38:23, "Robin Murphy" <robin.murphy@....com> wrote:
>> On 2024-09-20 9:20 am, Andy Yan wrote:
>>> From: Andy Yan <andy.yan@...k-chips.com>
>>>
>>> The vop mmu support translate physical address upper 4 GB to iova
>>> below 4 GB. So set dma mask to 64 bit to indicate we support address
>>>> 4GB.
>>>
>>> This can avoid warnging message like this on some boards with DDR
>>>> 4 GB:
>>>
>>> rockchip-drm display-subsystem: swiotlb buffer is full (sz: 266240 bytes), total 32768 (slots), used 130 (slots)
>>> rockchip-drm display-subsystem: swiotlb buffer is full (sz: 266240 bytes), total 32768 (slots), used 0 (slots)
>>> rockchip-drm display-subsystem: swiotlb buffer is full (sz: 266240 bytes), total 32768 (slots), used 130 (slots)
>>> rockchip-drm display-subsystem: swiotlb buffer is full (sz: 266240 bytes), total 32768 (slots), used 130 (slots)
>>> rockchip-drm display-subsystem: swiotlb buffer is full (sz: 266240 bytes), total 32768 (slots), used 0 (slots)
>>
>> There are several things wrong with this...
>>
>> AFAICS the VOP itself still only supports 32-bit addresses, so the VOP 
>> driver should only be setting a 32-bit DMA mask. The IOMMUs support 
>> either 32-bit or 40-bit addresses, and the IOMMU driver does set its DMA 
> Does that mean we can only use the dev of IOMMU ? If that is true， would you
> please give some inspiration on how to implement this? Or is there any other
> diver i can follow。Very sorry for that  I'm not familiar with memory management and the IOMMU。
> 
> 
>> mask appropriately. None of those numbers is 64, so that's clearly 
>> suspicious already. Plus it would seem the claim of the IOMMU being able 
>> to address >4GB isn't strictly true for RK3288 (which does supposedly 
>> support 8GB of RAM).
> 
> We can set DMA mask per device if we can find a right way to do it。

Removing the use of custom rockchip_drm_gem and use the common gem dma
fops should also allow import of framebuffers in >4GB address.

I played around with that [1] last year but never took it further
because it broke multiple VOPs/IOMMUSs on e.g. rk3288. Only IOMMU dte
address handling fixes for >4GB support was sent and got merged.

When I tested [1] on an RK3568 back then it was possible to import video
framebuffers located in >4GB memory and display them on screen without a
spam of "swiotlb buffer is full" lines.

Maybe there is some part of the current custom rockchip_drm_gem code
that can be adjusted to work closer to the common gem dma fops?, or
maybe fully drop rockchip_drm_gem in favor of common gem dma fops could
be an alternative solution?

[1] https://github.com/Kwiboo/linux-rockchip/commit/70695c8f868adec630592fef536364e59793de81

Regards,
Jonas

> 
>>
>> Furthermore, the "display-subsystem" doesn't even exist - it does not 
>> represent any actual DMA-capable hardware, so it should not have a DMA 
>> mask, and it should not be used for DMA API operations. Buffers for the 
>> VOP should be DMA-mapped for the VOP device itself. At the very least
>> the rockchip_gem_alloc_dma() path is clearly broken otherwise (I guess 
>> this patch possibly *would* make that brokenness apparent).
>>
>>> Signed-off-by: Andy Yan <andy.yan@...k-chips.com>
>>> Tested-by: Derek Foreman <derek.foreman@...labora.com>
>>> ---
>>>
>>> (no changes since v1)
>>>
>>>   drivers/gpu/drm/rockchip/rockchip_drm_drv.c | 4 +++-
>>>   1 file changed, 3 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/drivers/gpu/drm/rockchip/rockchip_drm_drv.c b/drivers/gpu/drm/rockchip/rockchip_drm_drv.c
>>> index 04ef7a2c3833..8bc2ff3b04bb 100644
>>> --- a/drivers/gpu/drm/rockchip/rockchip_drm_drv.c
>>> +++ b/drivers/gpu/drm/rockchip/rockchip_drm_drv.c
>>> @@ -445,7 +445,9 @@ static int rockchip_drm_platform_probe(struct platform_device *pdev)
>>>   		return ret;
>>>   	}
>>>   
>>> -	return 0;
>>> +	ret = dma_coerce_mask_and_coherent(dev, DMA_BIT_MASK(64));
>>
>> Finally as a general thing, please don't misuse 
>> dma_coerce_mask_and_coherent() in platform drivers, just use normal 
>> dma_set_mask_and_coherent(). The platform bus code has been initialising 
>> the dev->dma_mask pointer for years now, drivers should not be messing 
>> with it any more.
> 
> Got it ， thanks again。
> 
>>
>> Thanks,
>> Robin.
>>
>>> +
>>> +	return ret;
>>>   }
>>>   
>>>   static void rockchip_drm_platform_remove(struct platform_device *pdev)