linux-kernel - Re: [PATCH net-next 3/3] net: stmmac: Introducing support for Page Pool

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Date:   Thu, 4 Jul 2019 15:49:41 +0300
From:   Ilias Apalodimas <ilias.apalodimas@...aro.org>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Jesper Dangaard Brouer <brouer@...hat.com>,
        Jose Abreu <Jose.Abreu@...opsys.com>,
        Joao Pinto <Joao.Pinto@...opsys.com>,
        Alexandre Torgue <alexandre.torgue@...com>,
        Maxime Ripard <maxime.ripard@...tlin.com>,
        Networking <netdev@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "David S . Miller" <davem@...emloft.net>,
        Chen-Yu Tsai <wens@...e.org>,
        Maxime Coquelin <mcoquelin.stm32@...il.com>,
        Giuseppe Cavallaro <peppe.cavallaro@...com>,
        linux-stm32@...md-mailman.stormreply.com,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH net-next 3/3] net: stmmac: Introducing support for Page
 Pool

On Thu, Jul 04, 2019 at 02:14:28PM +0200, Arnd Bergmann wrote:
> On Thu, Jul 4, 2019 at 12:31 PM Ilias Apalodimas
> <ilias.apalodimas@...aro.org> wrote:
> > > On Wed,  3 Jul 2019 12:37:50 +0200
> > > Jose Abreu <Jose.Abreu@...opsys.com> wrote:
> 
> > 1. page pool allocs packet. The API doesn't sync but i *think* you don't have to
> > explicitly since the CPU won't touch that buffer until the NAPI handler kicks
> > in. On the napi handler you need to dma_sync_single_for_cpu() and process the
> > packet.
> 
> > So bvottom line i *think* we can skip the dma_sync_single_for_device() on the
> > initial allocation *only*. If am terribly wrong please let me know :)
> 
> I think you have to do a sync_single_for_device /somewhere/ before the
> buffer is given to the device. On a non-cache-coherent machine with
> a write-back cache, there may be dirty cache lines that get written back
> after the device DMA's data into it (e.g. from a previous memset
> from before the buffer got freed), so you absolutely need to flush any
> dirty cache lines on it first.
Ok my bad here i forgot to add "when coherency is there", since the driver
i had in mind runs on such a device (i think this is configurable though so i'll
add the sync explicitly to make sure we won't break any configurations).

In general you are right, thanks for the explanation!
> You may also need to invalidate the cache lines in the following
> sync_single_for_cpu() to eliminate clean cache lines with stale data
> that got there when speculatively reading between the cache-invalidate
> and the DMA.
> 
>        Arnd


Thanks!
/Ilias