linux-kernel - Re: [PATCH 4/9] net: moxart: use correct accessors for DMA memory

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Date:	Thu, 28 Jan 2016 17:53:43 +0100
From:	Arnd Bergmann <arnd@...db.de>
To:	linux-arm-kernel@...ts.infradead.org
Cc:	David Laight <David.Laight@...lab.com>,
	"David S. Miller" <davem@...emloft.net>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	françois romieu <romieu@...zoreil.com>
Subject: Re: [PATCH 4/9] net: moxart: use correct accessors for DMA memory

On Thursday 28 January 2016 12:36:19 David Laight wrote:
> From: Arnd Bergmann
> > Sent: 27 January 2016 14:05
> > The moxart ethernet driver confuses coherent DMA buffers with
> > MMIO registers.
> > 
> > moxart_ether.c: In function 'moxart_mac_setup_desc_ring':
> > moxart_ether.c:146:428: error: passing argument 1 of '__fswab32' makes integer from pointer without a
> > cast [-Werror=int-conversion]
> > moxart_ether.c:74:39: warning: incorrect type in argument 3 (different address spaces)
> > moxart_ether.c:74:39:    expected void *cpu_addr
> > moxart_ether.c:74:39:    got void [noderef] <asn:2>*tx_desc_base
> > 
> > This leaves the basic logic alone and uses normal pointers for
> > the virtual address of the descriptor. As we cannot use readl/writel
> > to access them, we also introduce our own moxart_desc_read
> > moxart_desc_write helpers that perform the same endianess swap
> > as the original code, but without the extra barriers and address
> > space conversion.
> 
> I'm pretty sure you need to add some explicit barriers:
> 
> > @@ -354,8 +364,8 @@ static int moxart_mac_start_xmit(struct sk_buff *skb, struct net_device *ndev)
> >       txdes1 = TX_DESC1_LTS | TX_DESC1_FTS | (len & TX_DESC1_BUF_SIZE_MASK);
> >       if (tx_head == TX_DESC_NUM_MASK)
> >               txdes1 |= TX_DESC1_END;
> > -     writel(txdes1, desc + TX_REG_OFFSET_DESC1);
> > -     writel(TX_DESC0_DMA_OWN, desc + TX_REG_OFFSET_DESC0);
> > +     moxart_desc_write(txdes1, desc + TX_REG_OFFSET_DESC1);
> > +     moxart_desc_write(TX_DESC0_DMA_OWN, desc + TX_REG_OFFSET_DESC0);
> 
> Those last two writes must happen in that order.
> There may be others.

Makes sense. I looked at the ftmac100 driver, which is another driver
for the same hardware, and it's also missing barriers. We should
probably add them for both then.

I think for the SoC that uses this, a barrier() would be sufficient
because of the page flags that dma_alloc_coherent() uses on ARM for
non-coherent platforms, but to be on the safe side we need a full
rmb()/wmb(). Sending a version 2 now.

	Arnd