[U-Boot] [PATCH 2/2] net: add driver for Synopsys Ethernet QoS device

[Stephen Warren]

On 10/19/2016 12:29 PM, Joe Hershberger wrote:
> Hi Stephen,
>
> On Mon, Oct 17, 2016 at 1:32 PM, Stephen Warren <swarren at wwwdotorg.org> wrote:
>> On 10/13/2016 05:46 PM, Joe Hershberger wrote:
>>> On Fri, Oct 14, 2016 at 1:35 AM, Stephen Warren <swarren at wwwdotorg.org> wrote:
>>>> On 10/11/2016 04:48 PM, Joe Hershberger wrote:
>>>>> On Tue, Oct 4, 2016 at 12:13 AM, Stephen Warren <swarren at wwwdotorg.org> wrote:
>>>>>> On 09/23/2016 03:49 PM, Joe Hershberger wrote:
>>>>>>> On Mon, Sep 12, 2016 at 12:48 PM, Stephen Warren <swarren at wwwdotorg.org> wrote:
>>>>>>>> This driver supports the Synopsys Designware Ethernet QoS (Quality of
>>>>>>>> Service) a/k/a eqos IP block, which is a different design than the HW
>>>>>>>> supported by the existing designware.c driver. The IP supports many
>>>>>>>> options for bus type, clocking/reset structure, and feature list.
>>>>>>>> This
>>>>>>>> driver currently supports the specific configuration used in NVIDIA's
>>>>>>>> Tegra186 chip, but should be extensible to other combinations quite
>>>>>>>> easily, as explained in the source.
...
>>>>>>>> +static int eqos_start(struct udevice *dev)
...
>>>>>>>> +       /* Update the MAC address */
>>>>>>>> +       val = (plat->enetaddr[5] << 8) |
>>>>>>>> +               (plat->enetaddr[4]);
>>>>>>>> +       writel(val, eqos->regs + EQOS_MAC_ADDRESS0_HIGH);
>>>>>>>> +       val = (plat->enetaddr[3] << 24) |
>>>>>>>> +               (plat->enetaddr[2] << 16) |
>>>>>>>> +               (plat->enetaddr[1] << 8) |
>>>>>>>> +               (plat->enetaddr[0]);
>>>>>>>> +       writel(val, eqos->regs + EQOS_MAC_ADDRESS0_LOW);
>>>>>>>
>>>>>>> This should be implemented in write_hwaddr() op.
...
>>>> Anyway, I still don't believe using write_hwaddr() is correct for this HW.
>>>> It's marked optional in include/net.h; it would be implemented in cases
>>>> where the MAC address should be passed to subsequent SW in Ethernet
>>>> controller registers. That's not the case here. The master location for
>>>> the MAC address is in an unrelated EEPROM that all drivers must read.
>>>
>>> That sounds more like a NV storage location for a read_rom_hwaddr() op
>>> to get a default mac addr that can be overridden with the env.
>>
>> If the EQoS HW module contained the interface to this EEPROM, such that all
>> instances of the HW module always accessed the EEPROM in the same way and
>> the layout of data in the EEPROM was fixed by the HW module, then yes.
>>
>> However, the EqoS HW module doesn't define any mechanism for non-volatile
>> MAC address storage; only the runtime registers. So, we can't implement
>> read_rom_hwaddr() inside the EQoS driver unfortunately.
>
> OK.
>
>>> The
>>> write_hwaddr is about what the mac uses to filter for limiting packet
>>> ingress. One reason to support it as an op is so that when the env var
>>> for the mac address is changed, the mac filter in the hw is also
>>> updated.
>>
>> I believe that every time the Ethernet device is used, the start() op is
>> called first, followed by packet transfer, followed by the stop() op. If
>> start() always programs the MAC address, the driver will always end up using
>> the value requested by the user.
>
> That may be. I still don't understand the reluctance to implement it.

I don't want to implement it because it can't work.

write_hwaddr() is called before start() is called. At that point, clocks 
to the EQoS HW are not running and the EQoS HW is in reset, and hence it 
cannot accept any register accesses; attempting any accesses will hang 
the bus and CPU.

These are the possible solutions:

a) Don't implement write_hwaddr()

b) Make write_hwaddr() turn on the clock and clear the reset, program 
the register, then reset the device and assert the reset. Re-asserting 
reset is required so that setting the MAC address doesn't leave the 
clock running even when the device isn't in use.This is pointless since 
the written value will not last beyond the end of the function.

c) Make probe() start the clock and clear the reset. Then write_hwaddr() 
can successfully write the MAC address registers at any time. This would 
waste power running the clock to the device when it's not in use. Also, 
Simon Glass continually asks that U-Boot not initialize HW that the user 
hasn't attempted to use. I believe turning on the clocks in probe() 
violates this.

d) Modify the network core to only call write_hwaddr() between the 
device's start() and stop() functions. I haven't looked into this at 
all, but I imagine it will have a fairly significant impact across many 
parts of the network core and other drivers.

 > When the behavior of start is restructured, it will be one fewer place
 > to be forced to retrofit.

I believe that the extra work required to refactor write_hwaddr() will 
be miniscule compared to splitting probe(), start(), and stop() up into 
separate functions anyway.

...
>> a) The mainline kernel's EQoS driver.
>>
>> b) The Synopsis-supplied EQoS driver as used in NVIDIA's downstream Tegra
>> kernel.
>
> Is this different from a) for some reason?

Yes. Synopsis supplies a non-mainlined Linux driver for the EQoS HW. For 
whatever reason, we're using that in our downstream kernel.

>> In both cases, the driver retrieves the desired MAC address from sources
>> other than the EQoS registers (i.e. device tree, or a system-specific
>> user-space application which sets the MAC address before enabling the
>> interface), and unconditionally programs that value into the EQoS runtime
>> registers.
>>
>> I also talked to the only user of the mainline Linux EQoS driver, and he
>> also is of the opinion that we can't rely on transferring the MAC address
>> between U-Boot (or any FW/...) and Linux using the EQoS registers.
>
> I think you can choose to rely on it and make it work. It's done by
> others, but does not have to be if you choose to go another way.
>
> If you were to rely on it the way others do, then you would be able to
> isolate the knowledge about how to determine what the MAC is in a
> single location in U-Boot and be able to share that logic between
> U-Boot and Linux, since U-Boot is the only one that needs to know
> about the EEPROM (for instance).

The knowledge is already isolated; the mainline Linux kernel will take 
the MAC address from device tree. This is the standard mechanism (for 
systems that boot using DT) that works across all Ethernet devices.