linux-kernel - Re: [PATCH v2 1/8] iommu: Decouple iommu

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Message-ID: <cb346a22-6548-f9d8-1d8a-f0d01cd0d0f7@arm.com>
Date:   Mon, 30 Jan 2023 18:21:54 +0000
From:   Robin Murphy <robin.murphy@....com>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     joro@...tes.org, will@...nel.org, iommu@...ts.linux.dev,
        linux-kernel@...r.kernel.org, hch@....de, baolu.lu@...ux.intel.com
Subject: Re: [PATCH v2 1/8] iommu: Decouple iommu_present() from bus ops

On 2023-01-30 17:31, Jason Gunthorpe wrote:
> On Thu, Jan 26, 2023 at 06:26:16PM +0000, Robin Murphy wrote:
>> Much as I'd like to remove iommu_present(), the final remaining users
>> are proving stubbornly difficult to clean up, so kick that can down
>> the road and just rework it to preserve the current behaviour without
>> depending on bus ops.
>>
>> Signed-off-by: Robin Murphy <robin.murphy@....com>
>> ---
> 
> Reviewed-by: Jason Gunthorpe <jgg@...dia.com>
> 
> This below might help. You might need to ask Thierry for help on the
> problematic stuff in tegra. Really we should finish the series here
> and organize things so that the domain allocation is defered until a
> client is present.

mrk_drm doesn't even allocate, it relies on the DMA API domain; it also 
only runs on DT systems where IOMMU dependency ordering has been 
enforced by core code for a very long time now. I sent a patch simply 
removing that check back at the time of the rest of my iommu_present() 
cleanup, and it just got resoundingly ignored :(

Thanks,
Robin.

> https://lore.kernel.org/linux-iommu/20220106022053.2406748-1-baolu.lu@linux.intel.com/
> 
> diff --git a/drivers/gpu/drm/mediatek/mtk_drm_drv.c b/drivers/gpu/drm/mediatek/mtk_drm_drv.c
> index cd5b18ef79512c..e5c257e41f4933 100644
> --- a/drivers/gpu/drm/mediatek/mtk_drm_drv.c
> +++ b/drivers/gpu/drm/mediatek/mtk_drm_drv.c
> @@ -353,9 +353,6 @@ static int mtk_drm_kms_init(struct drm_device *drm)
>   	if (drm_firmware_drivers_only())
>   		return -ENODEV;
>   
> -	if (!iommu_present(&platform_bus_type))
> -		return -EPROBE_DEFER;
> -
>   	pdev = of_find_device_by_node(private->mutex_node);
>   	if (!pdev) {
>   		dev_err(drm->dev, "Waiting for disp-mutex device %pOF\n",
> @@ -709,6 +706,9 @@ static int mtk_drm_probe(struct platform_device *pdev)
>   	int ret;
>   	int i;
>   
> +	if (!device_iommu_mapped(dev))
> +		return -EPROBE_DEFER;
> +
>   	private = devm_kzalloc(dev, sizeof(*private), GFP_KERNEL);
>   	if (!private)
>   		return -ENOMEM;