Re: [RFC PATCH v3 06/12] memory-provider: dmabuf devmem memory provider

From: Mina Almasry <almasrymina@google.com>
To: Jakub Kicinski <kuba@kernel.org>
Cc: netdev@vger.kernel.org, linux-kernel@vger.kernel.org,
	linux-arch@vger.kernel.org, linux-kselftest@vger.kernel.org,
	linux-media@vger.kernel.org, dri-devel@lists.freedesktop.org,
	linaro-mm-sig@lists.linaro.org,
	"David S. Miller" <davem@davemloft.net>,
	"Eric Dumazet" <edumazet@google.com>,
	"Paolo Abeni" <pabeni@redhat.com>,
	"Jesper Dangaard Brouer" <hawk@kernel.org>,
	"Ilias Apalodimas" <ilias.apalodimas@linaro.org>,
	"Arnd Bergmann" <arnd@arndb.de>,
	"David Ahern" <dsahern@kernel.org>,
	"Willem de Bruijn" <willemdebruijn.kernel@gmail.com>,
	"Shuah Khan" <shuah@kernel.org>,
	"Sumit Semwal" <sumit.semwal@linaro.org>,
	"Christian König" <christian.koenig@amd.com>,
	"Shakeel Butt" <shakeelb@google.com>,
	"Jeroen de Borst" <jeroendb@google.com>,
	"Praveen Kaligineedi" <pkaligineedi@google.com>,
	"Willem de Bruijn" <willemb@google.com>,
	"Kaiyuan Zhang" <kaiyuanz@google.com>
Subject: Re: [RFC PATCH v3 06/12] memory-provider: dmabuf devmem memory provider
Date: Sun, 12 Nov 2023 20:54:18 -0800	[thread overview]
Message-ID: <CAHS8izMGNLM18TF1RCDBfdOXXpqseePA4_27qmQt-FsrFzGfdQ@mail.gmail.com> (raw)
In-Reply-To: <20231110151622.2f45f618@kernel.org>

On Fri, Nov 10, 2023 at 3:16 PM Jakub Kicinski <kuba@kernel.org> wrote:
>
> On Sun,  5 Nov 2023 18:44:05 -0800 Mina Almasry wrote:
> > +static int mp_dmabuf_devmem_init(struct page_pool *pool)
> > +{
> > +     struct netdev_dmabuf_binding *binding = pool->mp_priv;
> > +
> > +     if (!binding)
> > +             return -EINVAL;
> > +
> > +     if (pool->p.flags & PP_FLAG_DMA_MAP ||
> > +         pool->p.flags & PP_FLAG_DMA_SYNC_DEV)
> > +             return -EOPNOTSUPP;
>
> This looks backwards, we should _force_ the driver to use the dma
> mapping built into the page pool APIs, to isolate the driver from
> how the DMA addr actually gets obtained. Right?
>
> Maybe seeing driver patches would illuminate.

The full tree with driver patches is here:

https://github.com/torvalds/linux/compare/master...mina:linux:tcpdevmem-v3

This is probably the most relevant patch, it implements POC page-pool
support into GVE + devmem support:

https://github.com/torvalds/linux/commit/3c27aa21eb3374f2f1677ece6258f046da234443

But, to answer your question, yes, this is a mistake. devmem doesn't
need to be mapped, which is why I disabled the flag. Actually what
should happen is like you said, we should enforce that PP_FLAG_DMA_MAP
is on, and have it be a no-op, so the driver doesn't try to map the
devmem on its own.

-- 
Thanks,
Mina