Thursday, March 31, 2022

[fedora-arm] Re: Change in kernel PPS GPIO handling?

Hi Stefan,

> >> At least the 32 bit issues on Raspberry Pi 4 are expected since the
> >> kernel config doesn't have ARM_LPAE enabled.
> >>
> > Okay, here is the explanation for the different behavior on Raspberry Pi
> > 400 and Raspberry Pi 4 B. The Raspberry Pi 400 has a newer BCM2711 SoC
> > (Stepping C0), which have less DMA restrictions for the emmc2 interface
> > (responsible for SD card access). For the Raspberry Pi 4 B there are
> > older boards which have Stepping B0 and all the new boards should have
> > Stepping C0 [1].
> >
> > Unfortunately there is no 100% reliable way to detect the stepping from
> > the kernel side. So currently the Raspberry Pi firmware patches the
> > dma-ranges in the firmware DT [2]. So in case U-Boot [3] or another
> > bootloader ignores this firmware DT and read a fresh DTB the right
> > dma-ranges get lost. Finally this results in unexpected behavior as soon
> > the emmc2 switches to DMA mode [4].
> >
> Okay, at least i found a fix [1] for the 64 bit boot issues (Original
> Fedora 35, Linux 5.14) with RPi 400/RPi 4 Stepping C0. This requires the
> DTB files to be updated and U-Boot to choose between the B0 and the C0
> variant of the Rpi 4 DTB file.

Thanks for the update on this, great news. In the case of the RPi-400
are they all the C0 stepping? I don't see the 400 in your patch.

Is there a documented way of detecting the stepping that can be used
in U-Boot I can use to create a patch there, or did you have a test
patch for this?

Do you plan to send this patch upstream, if so feel free to add me to
the cc and in the mean time I'll test it.

> Unfortunately this doesn't fix the SD card issues on 32 bit.

That's less of a problem, Fedora 36 will be the last version of Fedora
to support 32 bit and I've always suggested people run 64 bit Fedora
on their RPi4 and most people do tend to do that.

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