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

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Date: Fri, 31 May 2024 11:30:02 +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 Fri, May 31, 2024 at 09:57:54AM +0800, Baolu Lu wrote:
> On 5/30/24 3:58 PM, Dmitry Baryshkov wrote:
> > 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;
> 
> Oh I see. msm_iommu_new() returning NULL indicates a no-IOMMU case,
> right? So perhaps we can make it explicit like below?
> 
>         if (!device_iommu_mapped(dev))
>                 return NULL;
> 
>         domain = iommu_paging_domain_alloc(dev);
>         if (IS_ERR(domain))
>                 return ERR_CAST(domain);

Yes, this should work, thank you.

-- 
With best wishes
Dmitry