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Date:   Mon, 18 Mar 2019 15:28:58 -0400
From:   Jerome Glisse <jglisse@...hat.com>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Linux MM <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Felix Kuehling <Felix.Kuehling@....com>,
        Christian König <christian.koenig@....com>,
        Ralph Campbell <rcampbell@...dia.com>,
        John Hubbard <jhubbard@...dia.com>,
        Jason Gunthorpe <jgg@...lanox.com>
Subject: Re: [PATCH 00/10] HMM updates for 5.1

On Mon, Mar 18, 2019 at 12:18:38PM -0700, Dan Williams wrote:
> On Mon, Mar 18, 2019 at 11:55 AM Jerome Glisse <jglisse@...hat.com> wrote:
> >
> > On Mon, Mar 18, 2019 at 11:30:15AM -0700, Dan Williams wrote:
> > > On Mon, Mar 18, 2019 at 10:04 AM Jerome Glisse <jglisse@...hat.com> wrote:
> > > >
> > > > On Wed, Mar 13, 2019 at 09:10:04AM -0700, Andrew Morton wrote:
> > > > > On Tue, 12 Mar 2019 21:27:06 -0400 Jerome Glisse <jglisse@...hat.com> wrote:
> > > > >
> > > > > > Andrew you will not be pushing this patchset in 5.1 ?
> > > > >
> > > > > I'd like to.  It sounds like we're converging on a plan.
> > > > >
> > > > > It would be good to hear more from the driver developers who will be
> > > > > consuming these new features - links to patchsets, review feedback,
> > > > > etc.  Which individuals should we be asking?  Felix, Christian and
> > > > > Jason, perhaps?
> > > > >
> > > >
> > > > So i am guessing you will not send this to Linus ? Should i repost ?
> > > > This patchset has 2 sides, first side is just reworking the HMM API
> > > > to make something better in respect to process lifetime. AMD folks
> > > > did find that helpful [1]. This rework is also necessary to ease up
> > > > the convertion of ODP to HMM [2] and Jason already said that he is
> > > > interested in seing that happening [3]. By missing 5.1 it means now
> > > > that i can not push ODP to HMM in 5.2 and it will be postpone to 5.3
> > > > which is also postoning other work ...
> > > >
> > > > The second side is it adds 2 new helper dma map and dma unmap both
> > > > are gonna be use by ODP and latter by nouveau (after some other
> > > > nouveau changes are done). This new functions just do dma_map ie:
> > > >     hmm_dma_map() {
> > > >         existing_hmm_api()
> > > >         for_each_page() {
> > > >             dma_map_page()
> > > >         }
> > > >     }
> > > >
> > > > Do you want to see anymore justification than that ?
> > >
> > > Yes, why does hmm needs its own dma mapping apis? It seems to
> > > perpetuate the perception that hmm is something bolted onto the side
> > > of the core-mm rather than a native capability.
> >
> > Seriously ?
> 
> Yes.
> 
> > Kernel is fill with example where common code pattern that are not
> > device specific are turn into helpers and here this is exactly what
> > it is. A common pattern that all device driver will do which is turn
> > into a common helper.
> 
> Yes, but we also try not to introduce thin wrappers around existing
> apis. If the current dma api does not understand some hmm constraint
> I'm questioning why not teach the dma api that constraint and make it
> a native capability rather than asking the driver developer to
> understand the rules about when to use dma_map_page() vs
> hmm_dma_map().

There is nothing special here, existing_hmm_api() return an array of
page and the new helper just call dma_map_page for each entry in that
array. If it fails it undo everything so that error handling is share.

So i am not playing trick with DMA API i am just providing an helper
for a common pattern. Maybe the name confuse you but the pseudo should
be selft explanatory:
    Before
        mydriver_mirror_range() {
            err = existing_hmm_mirror_api(pages)
            if (err) {...}
            for_each_page(pages) {
                pas[i]= dma_map_page()
                if (dma_error(pas[i])) { ... }
            }
            // use pas[]
        }

    After
        mydriver_mirror_range() {
            err = hmm_range_dma_map(pas)
            if (err) { ... }
            // use pas[]
        }

So there is no rule of using one or the other. In the end it is the
same code. But instead of duplicating it in multiple drivers it is
share.

> 
> For example I don't think we want to end up with more headers like
> include/linux/pci-dma-compat.h.
> 
> > Moreover this allow to share the same error code handling accross
> > driver when mapping one page fails. So this avoid the needs to
> > duplicate same boiler plate code accross different drivers.
> >
> > Is code factorization not a good thing ? Should i duplicate every-
> > thing in every single driver ?
> 
> I did not ask for duplication, I asked why is it not more deeply integrated.

Because it is a common code pattern for HMM user not for DMA user.

> > If that's not enough, this will also allow to handle peer to peer
> > and i posted patches for that [1] and again this is to avoid
> > duplicating common code accross different drivers.
> 
> I went looking for the hmm_dma_map() patches on the list but could not
> find them, so I was reacting to the "This new functions just do
> dma_map", and wondered if that was the full extent of the
> justification.

They are here [1] patch 7 in this patch serie

Cheers,
Jérôme

[1] https://lkml.org/lkml/2019/1/29/1016

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