[PATCH] ipq40xx: add PCIe magic hack to improve VRX518 compatibility

[Jan Hoffmann]

Am 02.02.23 um 11:54 schrieb Robert Marko:
> On Tue, 31 Jan 2023 at 23:52, Jan Hoffmann <jan at 3e8.eu> wrote:
>>
>> Hi Robert,
>>
>>
>> On 2023-01-30 at 00:08, Robert Marko wrote:
>>> Shouldn't it be possible for the modem driver itself to be fixed
>>> instead of faking
>>> the PCI details?
>>
>> This hack is definitely far from ideal, but I'm not sure if there is a
>> better way to fix this.
>>
>> Here are a few more details about the issue:
>>
>> On the affected devices (so far, three users reported it on the VRX518
>> thread in the forum [0]), the function call to turn on the "EMA"
>> hardware unit in the vrx518_tc driver [1] fails with the error mentioned
>> in the commit message.
>>
>> I don't have any details about it, but this EMA unit is part of the data
>> path (it is also referenced in the ltq-atm and ltq-ptm data path drivers
>> for older Lantiq modems). If the EMA unit is not running, then at least
>> the transmit data path doesn't work, and any packets the driver writes
>> to the TX ring are not actually sent out by the device.
>>
>> This is also reproducible on non-affected devices by calling tc_clkoff
>> instead of tc_clkon in the vrx518_tc driver (i.e. disabling the hardware
>> unit). The same issue also occurs on affected devices running vendor
>> firmware, if the "magic" in the PCIe driver is disabled in the device
>> tree. So this is not just a bug in the data path driver.
> 
> I get the issue, however, I am failing to see how faking the PCI ID for the
> root adaptor is magically solving that?
> If that works, why can't you just patch the driver to stop looking for
> the ancient
> Lantiq ID?

As far as I can see, the magic values don't appear anywhere in the DSL 
drivers. So it doesn't seem like there is an easy fix like that.

To me this looks like whatever access to these values is being done, 
seems to happen in hardware (or firmware). Maybe there are some 
revisions or variants of the modem that don't like to cooperate with 
non-Lantiq SoCs.

But in the end, I don't know with certainty what exactly is happening 
here, as about the only public information on these modems are the 
open-source drivers (including the magic hack in the PCIe driver which 
in its original form contains comments like "do some magic" without 
really explaining what it actually does).

Regards,
Jan


> 
> Regards,
> Robert
>>
>>
>> Regards,
>> Jan
>>
>>
>>>
>>> Especially considering that now modem support is not self-contained
>>> and will require
>>> patching the DWC Qualcomm PCI driver forever.
>>>
>>> Regards,
>>> Robert
>>
>>
>> [0]
>> https://forum.openwrt.org/t/adding-support-for-vrx518-and-maybe-vrx320/55160
>> [1]
>> https://gitlab.com/prpl-foundation/intel/vrx518_tc_drv/-/blob/ugw-8.5.2/dcdp/tc_main.c#L112