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SubjectRe: [PATCH 5.10 055/101] net: ll_temac: Add memory-barriers for TX BD access
Date
Pavel Machek <pavel@denx.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

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