netdev - Re: [PATCH v4 net-next 13/19] ionic: Add initial ethtool support

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Message-ID: <3e0e946b-dbe3-2a64-2a2b-7654038a5fb4@pensando.io>
Date:   Tue, 30 Jul 2019 14:51:27 -0700
From:   Shannon Nelson <snelson@...sando.io>
To:     Andrew Lunn <andrew@...n.ch>
Cc:     netdev@...r.kernel.org, davem@...emloft.net
Subject: Re: [PATCH v4 net-next 13/19] ionic: Add initial ethtool support

On 7/25/19 6:35 AM, Andrew Lunn wrote:
>> +static int ionic_get_module_eeprom(struct net_device *netdev,
>> +				   struct ethtool_eeprom *ee,
>> +				   u8 *data)
>> +{
>> +	struct lif *lif = netdev_priv(netdev);
>> +	struct ionic_dev *idev = &lif->ionic->idev;
>> +	struct xcvr_status *xcvr;
>> +	u32 len;
>> +
>> +	/* The NIC keeps the module prom up-to-date in the DMA space
>> +	 * so we can simply copy the module bytes into the data buffer.
>> +	 */
>> +	xcvr = &idev->port_info->status.xcvr;
>> +	len = min_t(u32, sizeof(xcvr->sprom), ee->len);
>> +	memcpy(data, xcvr->sprom, len);
>> +
>> +	return 0;
>> +}
> Is the firmware doing this DMA update atomically? The diagnostic
> values are u16s. Is there any chance we do this memcpy at the same
> time the DMA is active and we get a mix of old and new data?
>
> Often in cases like this you do the copy twice and ensure you get the
> same values each time. If not, keep repeating the copy until you do
> get the same values twice.

Regardless of how the structs are all aligned and our PCI block does 
large writes, I can see how an unoptimized memcpy() that is doing 
byte-by-byte copy rather than by words might result in a mangled value.  
I think this is the only buffer that may be susceptible to this.  Sure, 
doing a double copy should work here.

sln