linux-kernel - RE: [PATCH 1/3] iommu: Add Visconti5 IOMMU driver

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Date:   Mon, 20 Jun 2022 05:49:13 +0000
From:   <nobuhiro1.iwamatsu@...hiba.co.jp>
To:     <jgg@...dia.com>, <baolu.lu@...ux.intel.com>
CC:     <joro@...tes.org>, <will@...nel.org>, <robh+dt@...nel.org>,
        <devicetree@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <iommu@...ts.linux-foundation.org>, <yuji2.ishikawa@...hiba.co.jp>,
        <linux-arm-kernel@...ts.infradead.org>
Subject: RE: [PATCH 1/3] iommu: Add Visconti5 IOMMU driver

Hi,

Thanks for your review.

> -----Original Message-----
> From: Jason Gunthorpe <jgg@...dia.com>
> Sent: Thursday, May 26, 2022 3:27 AM
> To: Baolu Lu <baolu.lu@...ux.intel.com>
> Cc: iwamatsu nobuhiro(岩松 信洋 □ＳＷＣ◯ＡＣＴ)
> <nobuhiro1.iwamatsu@...hiba.co.jp>; Joerg Roedel <joro@...tes.org>; Will
> Deacon <will@...nel.org>; Rob Herring <robh+dt@...nel.org>;
> devicetree@...r.kernel.org; linux-kernel@...r.kernel.org;
> iommu@...ts.linux-foundation.org; ishikawa yuji(石川 悠司 ○ＲＤＣ□ＡＩＴ
> Ｃ○ＥＡ開) <yuji2.ishikawa@...hiba.co.jp>;
> linux-arm-kernel@...ts.infradead.org
> Subject: Re: [PATCH 1/3] iommu: Add Visconti5 IOMMU driver
> 
> On Wed, May 25, 2022 at 02:26:37PM +0800, Baolu Lu wrote:
> > On 2022/5/25 09:31, Nobuhiro Iwamatsu wrote:
> > > +static const struct iommu_ops visconti_atu_ops = {
> > > +	.domain_alloc = visconti_atu_domain_alloc,
> > > +	.probe_device = visconti_atu_probe_device,
> > > +	.release_device = visconti_atu_release_device,
> > > +	.device_group = generic_device_group,
> > > +	.of_xlate = visconti_atu_of_xlate,
> > > +	.pgsize_bitmap = ATU_IOMMU_PGSIZE_BITMAP,
> > > +	.default_domain_ops = &(const struct iommu_domain_ops) {
> > > +		.attach_dev = visconti_atu_attach_device,
> > > +		.detach_dev = visconti_atu_detach_device,
> >
> > The detach_dev callback is about to be deprecated. The new drivers
> > should implement the default domain and blocking domain instead.
> 
> Yes please, new drivers need to use default_domains.
> 
> It is very strange that visconti_atu_detach_device() does nothing.  It is not
> required that a domain is fully unmapped before being destructed, I think
> detach should set ATU_AT_EN to 0.

I see, I rethink implementation.

> 
> What behavior does the HW have when ATU_AT_ENTRY_EN == 0? If DMA is
> rejected then this driver should have a IOMMU_DOMAIN_BLOCKING and
> return that from ops->def_domain_type().

If ATU_AT_ENTRY_EN is 0, nothing happens. It does not work with IOMMU,
it works with the memory space set in device tree.
Also, I investigate about IOMMU_DOMAIN_BLOCKING.

> 
> Attaching a the blocking domain should set ATU_AT_ENTRY_EN = 0
> 
> Also, if I surmise how this works properly, it is not following the iommu API to
> halt all DMA during map/unmap operations. Should at least document this and
> explain why it is OK..

I see, I will check DMA during map and unmap operations.

> 
> I'm feeling like these "special" drivers need some kind of handshake with their
> only users because they don't work with things like VFIO..

Since the devices that utilize this IOMMU function are fixed, I do not think that a special handshake is required.
Could you you tell me where you thought you needed a handshake?

Best regards,
  Nobuhiro

> 
> Jason