Re: [PATCH v2 3/3] drm/etnaviv: use a 32 bit mask as coherent DMA mask

[Robin Murphy]

Sorry I missed this earlier...

On 2021-09-07 17:49, Michael Walle wrote:
> The STLB and the first command buffer (which is used to set up the TLBs)
> has a 32 bit size restriction in hardware. There seems to be no way to
> specify addresses larger than 32 bit. Keep it simple and restict the
> addresses to the lower 4 GiB range for all coherent DMA memory
> allocations.
>
> Please note, that platform_device_alloc() will initialize dev->dma_mask
> to point to pdev->platform_dma_mask, thus dma_set_mask() will work as
> expected.
>
> While at it, move the dma_mask setup code to the of_dma_configure() to
> keep all the DMA setup code next to each other.
>
> Suggested-by: Lucas Stach <l.stach@xxxxxxxxxxxxxx>
> Signed-off-by: Michael Walle <michael@xxxxxxxx>
> ---
>   drivers/gpu/drm/etnaviv/etnaviv_drv.c | 20 ++++++++++++++++++--
>   1 file changed, 18 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/gpu/drm/etnaviv/etnaviv_drv.c b/drivers/gpu/drm/etnaviv/etnaviv_drv.c
> index 54eb653ca295..0b756ecb1bc2 100644
> --- a/drivers/gpu/drm/etnaviv/etnaviv_drv.c
> +++ b/drivers/gpu/drm/etnaviv/etnaviv_drv.c
> @@ -613,6 +613,24 @@ static int etnaviv_pdev_probe(struct platform_device *pdev)
>   			component_match_add(dev, &match, compare_str, names[i]);
>   	}
>   
> +	/*
> +	 * PTA and MTLB can have 40 bit base addresses, but
> +	 * unfortunately, an entry in the MTLB can only point to a
> +	 * 32 bit base address of a STLB. Moreover, to initialize the
> +	 * MMU we need a command buffer with a 32 bit address because
> +	 * without an MMU there is only an indentity mapping between
> +	 * the internal 32 bit addresses and the bus addresses.
> +	 *
> +	 * To make things easy, we set the dma_coherent_mask to 32
> +	 * bit to make sure we are allocating the command buffers and
> +	 * TLBs in the lower 4 GiB address space.
> +	 */
> +	if (dma_set_mask(&pdev->dev, DMA_BIT_MASK(40)) ||
> +	    dma_set_coherent_mask(&pdev->dev, DMA_BIT_MASK(32))) {

Since AFAICS you're not changing the default dma_mask pointer to point 
to some storage other than the coherent mask, the dma_set_mask() call 
effectively does nothing and both masks will end up reading back as 32 bits.

Robin.

> +		dev_dbg(&pdev->dev, "No suitable DMA available\n");
> +		return -ENODEV;
> +	}
> +
>   	/*
>   	 * Apply the same DMA configuration to the virtual etnaviv
>   	 * device as the GPU we found. This assumes that all Vivante
> @@ -671,8 +689,6 @@ static int __init etnaviv_init(void)
>   			of_node_put(np);
>   			goto unregister_platform_driver;
>   		}
> -		pdev->dev.coherent_dma_mask = DMA_BIT_MASK(40);
> -		pdev->dev.dma_mask = &pdev->dev.coherent_dma_mask;
>   
>   		ret = platform_device_add(pdev);
>   		if (ret) {