Re: [PATCH net-next v7 1/6] page_pool: frag API support for 32-bit arch with 64-bit DMA

[Ilias Apalodimas]

Hi Jakub,

On Thu, 17 Aug 2023 at 19:15, Jakub Kicinski <kuba@xxxxxxxxxx> wrote:
>
> On Thu, 17 Aug 2023 16:57:16 +0300 Ilias Apalodimas wrote:
> > Why should we care about this?  Even an architecture that's 32-bit and
> > has a 64bit DMA should be allowed to split the pages internally if it
> > decides to do so.  The trick that drivers usually do is elevate the
> > page refcnt and deal with that internally.
>
> Can we assume the DMA mapping of page pool is page aligned? We should
> be, right?

Yes

> That means we're storing 12 bits of 0 at the lower end.
> So even with 32b of space we can easily store addresses for 32b+12b =>
> 16TB of memory. "Ought to be enough" to paraphrase Bill G, and the
> problem is only in our heads?

Do you mean moving the pp_frag_count there?  I was questioning the
need to have PP_FLAG_PAGE_SPLIT_IN_DRIVER overall.  With Yunshengs
patches such a platform would allocate a page, so why should we
prevent it from splitting it internally?

Thanks
/Ilias
>
> Before we go that way - Mina, are the dma-buf "chunks" you're working
> with going to be fragment-able? Or rather can driver and/or core take
> multiple references on a single buffer?