linux-kernel - Re: [PATCH v5 2/7] iommu: Decouple iommu

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Date:   Thu, 12 Oct 2023 20:57:40 +0800
From:   Baolu Lu <baolu.lu@...ux.intel.com>
To:     Jason Gunthorpe <jgg@...dia.com>,
        Robin Murphy <robin.murphy@....com>
Cc:     baolu.lu@...ux.intel.com, joro@...tes.org, will@...nel.org,
        iommu@...ts.linux.dev, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 2/7] iommu: Decouple iommu_present() from bus ops

On 2023/10/12 20:37, Jason Gunthorpe wrote:
> On Thu, Oct 12, 2023 at 12:40:01PM +0100, Robin Murphy wrote:
>> On 2023-10-12 07:05, Baolu Lu wrote:
>>> On 10/12/23 2:14 AM, 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. Since commit 57365a04c921 ("iommu: Move bus setup
>>>
>>> The iommu_present() is only used in below two drivers.
>>>
>>> $ git grep iommu_present
>>> drivers/gpu/drm/mediatek/mtk_drm_drv.c: if
>>> (!iommu_present(&platform_bus_type))
>>> drivers/gpu/drm/tegra/drm.c:    if (host1x_drm_wants_iommu(dev) &&
>>> iommu_present(&platform_bus_type)) {
>>>
>>> Both are platform drivers and have the device pointer passed in. Just
>>> out of curiosity, why not replacing them with device_iommu_mapped()
>>> instead? Sorry if I overlooked previous discussion.
>>
>> Yes, we've already gone round in circles on this several times, that's why
>> it's explicitly called out as "stubbornly difficult" in the commit message.
>> The Mediatek one is entirely redundant, but it seems I have yet to figure
>> out the right CC list to get anyone to care about that patch[1].

I see now. Thanks for the explanation.

> 
> Please just have Joerg take such a trivial patch, there is no reason
> we need to torture outselves because DRM side is not behaving well. :(

I was not object to the patch. Just want to make sure that I understand
the reason why device_iommu_mapped() can't be used in those two drivers.

It's fine to me. I will add my r-b.

Best regards,
baolu