Re: [PATCH] hw/net: npcm7xx_emc: set MAC in register space

[Patrick Venture <venture@google.com>]

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On Mon, Sep 26, 2022 at 8:45 AM Patrick Venture <venture@google.com> wrote:

>
>
> On Sat, Sep 24, 2022 at 2:10 AM Peter Maydell <peter.maydell@linaro.org>
> wrote:
>
>> On Sat, 24 Sept 2022 at 00:42, Patrick Venture <venture@google.com>
>> wrote:
>> > On Thu, Sep 22, 2022 at 8:21 PM Jason Wang <jasowang@redhat.com> wrote:
>> >>
>> >> On Thu, Sep 22, 2022 at 8:35 PM Peter Maydell <
>> peter.maydell@linaro.org> wrote:
>> >> > A question to which I don't know the answer: if the guest writes to
>> >> > the device to change the MAC address, should that persist across
>> >> > reset, or should reset revert the device to the original MAC address
>> >> > as specified by the user on the command line or whatever ? At the
>> >> > moment you have the former behaviour (and end up storing the MAC
>> >> > address in two places as a result -- it would be neater to either
>> >> > keep it in only one place, or else have emc->regs[] be the current
>> >> > programmed MAC address and emc->conf.macaddr the value to reset to).
>> >> >
>> >> > I'm not sure we're consistent between device models about that,
>> >> > eg the e1000 seems to reset to the initial MAC addr, but the
>> >> > imx_fec keeps the guest-set value over resets. Jason, is there
>> >> > a recommended "right way" to handle guest-programmable MAC addresses
>> >> > over device reset ?
>> >>
>> >> I think it depends on the NIC.
>> >>
>> >> E1000 has a EEPROM interface for providing the MAC address for the
>> >> ethernet controller before it can be accessed by the driver during
>> >> reset. For modern Intel NICs like E810, it has similar semantics but
>> >> using NVM instead of EEPROM. So the current e1000 behaviour seems to
>> >> be correct (treat the initiali MAC as the one stored in the EEPROM).
>> >>
>> >> I guess most NIC should behave the same as having a persistent storage
>> >> for MAC for the controller during reset, but I'm not sure this is the
>> >> case for imx_fec.
>>
>> > So the first time the NIC is realized, it should take the value from
>> > the command line.  Then later if the guest OS updates it, it should
>> > always on reset use that provided value?
>>
>> I think what Jason is suggesting is that that should depend on what
>> the real hardware does. On a physical board, if the guest sets the
>> MAC address, and then you power-cycle the hardware, does the MAC
>> that it set still persist after powercycle ? Does the guest writing
>> to these MAC registers correspond to writing to an EEPROM ?
>>
>
> Thanks, Peter - we've reached out to the vendor off-list to seek out some
> details, I'll update this with a v2 when I get an answer.
>

"No, The EMC driver reset the MAC address registers during boot
cycle/reset."

So in that case, we should disregard the value the user sets in reset and
use the value provided through Qemu.  Or, should we just not allow Qemu to
set the MAC address at all?


>
>>
>> thanks
>> -- PMM
>>
>

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