netdev - Re: [PATCH] net: thunderbolt: Stop using zero to mean no valid DMA mapping

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Message-Id: <20171125.235624.170651475497287701.davem@davemloft.net>
Date:   Sat, 25 Nov 2017 23:56:24 +0900 (KST)
From:   David Miller <davem@...emloft.net>
To:     mika.westerberg@...ux.intel.com
Cc:     michael.jamet@...el.com, yehezkel.bernat@...el.com,
        David.Laight@...LAB.COM, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] net: thunderbolt: Stop using zero to mean no valid DMA
 mapping

From: Mika Westerberg <mika.westerberg@...ux.intel.com>
Date: Fri, 24 Nov 2017 14:05:36 +0300

> Commit 86dabda426ac ("net: thunderbolt: Clear finished Tx frame bus
> address in tbnet_tx_callback()") fixed a DMA-API violation where the
> driver called dma_unmap_page() in tbnet_free_buffers() for a bus address
> that might already be unmapped. The fix was to zero out the bus address
> of a frame in tbnet_tx_callback().
> 
> However, as pointed out by David Miller, zero might well be valid
> mapping (at least in theory) so it is not good idea to use it here.
> 
> It turns out that we don't need the whole map/unmap dance for Tx buffers
> at all. Instead we can map the buffers when they are initially allocated
> and unmap them when the interface is brought down. In between we just
> DMA sync the buffers for the CPU or device as needed.
> 
> Signed-off-by: Mika Westerberg <mika.westerberg@...ux.intel.com>

Applied, thanks.