linux-kernel - Re: [PATCH 06/20] drm/msm: Use iommu_paging_domain

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Date: Thu, 30 May 2024 10:58:42 +0300
From: Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
To: Baolu Lu <baolu.lu@...ux.intel.com>
Cc: Joerg Roedel <joro@...tes.org>, Will Deacon <will@...nel.org>, 
	Robin Murphy <robin.murphy@....com>, Jason Gunthorpe <jgg@...pe.ca>, Kevin Tian <kevin.tian@...el.com>, 
	Yi Liu <yi.l.liu@...el.com>, David Airlie <airlied@...il.com>, Daniel Vetter <daniel@...ll.ch>, 
	Kalle Valo <kvalo@...nel.org>, Bjorn Andersson <andersson@...nel.org>, 
	Mathieu Poirier <mathieu.poirier@...aro.org>, Alex Williamson <alex.williamson@...hat.com>, mst@...hat.com, 
	Jason Wang <jasowang@...hat.com>, Thierry Reding <thierry.reding@...il.com>, 
	Jonathan Hunter <jonathanh@...dia.com>, Mikko Perttunen <mperttunen@...dia.com>, iommu@...ts.linux.dev, 
	dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 06/20] drm/msm: Use iommu_paging_domain_alloc()

On Thu, 30 May 2024 at 04:59, Baolu Lu <baolu.lu@...ux.intel.com> wrote:
>
> On 5/29/24 4:21 PM, Dmitry Baryshkov wrote:
> > On Wed, May 29, 2024 at 01:32:36PM +0800, Lu Baolu wrote:
> >> The domain allocated in msm_iommu_new() is for the @dev. Replace
> >> iommu_domain_alloc() with iommu_paging_domain_alloc() to make it explicit.
> >>
> >> Update msm_iommu_new() to always return ERR_PTR in failure cases instead
> >> of NULL.
> > Please don't mix unrelated changes, because ...
> >
> >> Signed-off-by: Lu Baolu<baolu.lu@...ux.intel.com>
> >> ---
> >>   drivers/gpu/drm/msm/msm_iommu.c | 8 ++++----
> >>   1 file changed, 4 insertions(+), 4 deletions(-)
> >>
> >> diff --git a/drivers/gpu/drm/msm/msm_iommu.c b/drivers/gpu/drm/msm/msm_iommu.c
> >> index d5512037c38b..f7e28d4b5f62 100644
> >> --- a/drivers/gpu/drm/msm/msm_iommu.c
> >> +++ b/drivers/gpu/drm/msm/msm_iommu.c
> >> @@ -407,9 +407,9 @@ struct msm_mmu *msm_iommu_new(struct device *dev, unsigned long quirks)
> >>      struct msm_iommu *iommu;
> >>      int ret;
> >>
> >> -    domain = iommu_domain_alloc(dev->bus);
> >> -    if (!domain)
> >> -            return NULL;
> >> +    domain = iommu_paging_domain_alloc(dev);
> >> +    if (IS_ERR(domain))
> >> +            return ERR_CAST(domain);
> >>
> >>      iommu_set_pgtable_quirks(domain, quirks);
> >>
> >> @@ -441,7 +441,7 @@ struct msm_mmu *msm_iommu_gpu_new(struct device *dev, struct msm_gpu *gpu, unsig
> >>      struct msm_mmu *mmu;
> >>
> >>      mmu = msm_iommu_new(dev, quirks);
> >> -    if (IS_ERR_OR_NULL(mmu))
> >> +    if (IS_ERR(mmu))
> >>              return mmu;
> > NAK, not having an IOMMU is a poor but legit usecase for some of devices
> > which don't have IOMMU support yet (for example because of the buggy
> > implementation for which we were not able to get all the hooks in).
> >
> > Please don't break compatibility for existing platforms.
>
> Sure. I will remove this line of change. Though I have no idea in which
> case msm_iommu_new() could return NULL after this patch.

So, even without this chunk you are going to break the no-IOMMU case.
Please don't. This will result in a regression report and a revert.

Instead please provide a way for the existing drivers to continue
working. For example, something like:

if (IS_ERR(mmu) && ERR_PTR(mmu) == -ENODEV))
    return NULL;



-- 
With best wishes
Dmitry