Re: [PATCH v3 3/4] hw/arm: sabrelite: Connect the Ethernet PHY at address 6

[Guenter Roeck]

On 4/30/21 9:28 PM, Bin Meng wrote:
> On Fri, Apr 30, 2021 at 10:41 PM Guenter Roeck <linux@roeck-us.net> wrote:
>>
>> Hi,
>>
>> On Wed, Jan 06, 2021 at 02:35:03PM +0800, Bin Meng wrote:
>>> From: Bin Meng <bin.meng@windriver.com>
>>>
>>> At present, when booting U-Boot on QEMU sabrelite, we see:
>>>
>>>   Net:   Board Net Initialization Failed
>>>   No ethernet found.
>>>
>>> U-Boot scans PHY at address 4/5/6/7 (see board_eth_init() in the
>>> U-Boot source: board/boundary/nitrogen6x/nitrogen6x.c). On the real
>>> board, the Ethernet PHY is at address 6. Adjust this by updating the
>>> "fec-phy-num" property of the fsl_imx6 SoC object.
>>>
>>> With this change, U-Boot sees the PHY but complains MAC address:
>>>
>>>   Net:   using phy at 6
>>>   FEC [PRIME]
>>>   Error: FEC address not set.
>>>
>>> This is due to U-Boot tries to read the MAC address from the fuse,
>>> which QEMU does not have any valid content filled in. However this
>>> does not prevent the Ethernet from working in QEMU. We just need to
>>> set up the MAC address later in the U-Boot command shell, by:
>>>
>>>   => setenv ethaddr 00:11:22:33:44:55
>>>
>>
>> With this patch in place, the standard Ethernet interface no longer works 
>> when
>> booting sabrelite Linux images directly (without u-boot) using the following
>> qemu command.
>>         qemu-system-arm -M sabrelite -kernel arch/arm/boot/zImage
>>         ...
>>
>> The Ethernet interface still instantiates, but packet transfer to the host
>> no longer works. Reverting this patch fixes the problem for me.
>>
>> Is there a qemu command line parameter that is now necessary to instantiate
>> the Ethernet interface when booting Linux ?
> 
> Enabling "guest_errors" shows that Linux kernel fec driver is trying
> to read PHY at address 0, which is not what we want.
> 
> [imx.fec.phy]imx_phy_read: Bad phy num 0
> 
> The device tree blob of the sabrelite does not contain a node for the
> ethernet phy specifying phy address, so I suspect Linux kernel driver
> is using default phy address 0 instead.
> 
> Could you please test on a real hardware to see what happens?
> 

The problem is that qemu returns 0 when the OS tries to read from a
non-existing PHY. Linux expects it to return 0xffff, and believes that
a PHY is there if 0 is returned. This helps:

diff --git a/hw/net/imx_fec.c b/hw/net/imx_fec.c
index f03450c028..3c90c38e26 100644
--- a/hw/net/imx_fec.c
+++ b/hw/net/imx_fec.c
@@ -285,7 +285,7 @@ static uint32_t imx_phy_read(IMXFECState *s, int reg)
     if (phy != s->phy_num) {
         qemu_log_mask(LOG_GUEST_ERROR, "[%s.phy]%s: Bad phy num %u\n",
                       TYPE_IMX_FEC, __func__, phy);
-        return 0;
+        return 0xffff;
     }

Note that this is not really a guest error; any OS can and likely
will scan the MII bus for connected phy chips.

Guenter