linux-kernel - Re: [PATCH 5.10 055/101] net: ll_temac: Add memory-barriers for TX BD access

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Date:   Mon, 05 Jul 2021 09:42:01 +0200
From:   Esben Haabendal <esben@...nix.com>
To:     Pavel Machek <pavel@...x.de>
Cc:     Sasha Levin <sashal@...nel.org>, linux-kernel@...r.kernel.org,
        stable@...r.kernel.org, "David S . Miller" <davem@...emloft.net>
Subject: Re: [PATCH 5.10 055/101] net: ll_temac: Add memory-barriers for TX
 BD access

Pavel Machek <pavel@...x.de> writes:

> Hi!
>
>> Add a couple of memory-barriers to ensure correct ordering of read/write
>> access to TX BDs.
>
> So... this is dealing with CPU-to-device consistency, not CPU-to-CPU,
> right?

Actually, One of both.  When looping over the buffers, looking for CMPLT
bit in APP0 (in temac_start_xmit_done()), the challenge is CPU-to-device
consistency, as the CMPLT bit is set by device, and read by CPU.

But when we clear APP0 (and the other fields) in the same loop, it is
CPU-to-CPU, as APP0 is cleared by CPU and read by CPU.

>
>> +++ b/drivers/net/ethernet/xilinx/ll_temac_main.c
>> @@ -774,12 +774,15 @@ static void temac_start_xmit_done(struct net_device *ndev)
>>  	stat = be32_to_cpu(cur_p->app0);
>>  
>>  	while (stat & STS_CTRL_APP0_CMPLT) {
>> +		/* Make sure that the other fields are read after bd is
>> +		 * released by dma
>> +		 */
>> +		rmb();
>>  		dma_unmap_single(ndev->dev.parent,
>
> Full barrier, as expected.
>
>> @@ -788,6 +791,12 @@ static void temac_start_xmit_done(struct net_device *ndev)
>>  		ndev->stats.tx_packets++;
>>  		ndev->stats.tx_bytes += be32_to_cpu(cur_p->len);
>>  
>> +		/* app0 must be visible last, as it is used to flag
>> +		 * availability of the bd
>> +		 */
>> +		smp_mb();
>
> SMP-only barrier, but full barrier is needed here AFAICT.

I don't think that is needed.  See above.

/Esben