netdev - Re: [PATCH 08/14] net: ks8851: Use 16-bit read of RXFC register

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Date:   Tue, 24 Mar 2020 13:50:53 +0100
From:   Marek Vasut <marex@...x.de>
To:     Andrew Lunn <andrew@...n.ch>
Cc:     netdev@...r.kernel.org, "David S . Miller" <davem@...emloft.net>,
        Lukas Wunner <lukas@...ner.de>, Petr Stetiar <ynezz@...e.cz>,
        YueHaibing <yuehaibing@...wei.com>
Subject: Re: [PATCH 08/14] net: ks8851: Use 16-bit read of RXFC register

On 3/24/20 2:50 AM, Andrew Lunn wrote:
>> @@ -470,7 +455,7 @@ static void ks8851_rx_pkts(struct ks8851_net *ks)
>>  	unsigned rxstat;
>>  	u8 *rxpkt;
>>  
>> -	rxfc = ks8851_rdreg8(ks, KS_RXFC);
>> +	rxfc = (ks8851_rdreg16(ks, KS_RXFCTR) >> 8) & 0xff;
> 
> The datasheet says:
> 
> 2. When software driver reads back Receive Frame Count (RXFCTR)
> Register; the KSZ8851 will update both Receive Frame Header Status and
> Byte Count Registers (RXFHSR/RXFHBCR)
> 
> Are you sure there is no side affect here?

Yes, look at the RXFC register 0x9c itself. It's a 16bit register, 0x9c
is the LSByte and 0x9d is the MSByte.

What happened here before was readout of register 0x9d, MSByte of RXFC,
which triggers the update of RXFHSR/RXFHBCR. What happens now is the
readout of the whole RXFC as 16bit value, which also triggers the update.