Hi Robin,
Am 25.05.21 um 22:09 schrieb Robin Murphy:
On 2021-05-25 14:05, Alex Deucher wrote:
On Tue, May 25, 2021 at 8:56 AM Peter Geis <pgwipeout@xxxxxxxxx> wrote:
On Tue, May 25, 2021 at 8:47 AM Alex Deucher <alexdeucher@xxxxxxxxx>
wrote:
On Tue, May 25, 2021 at 8:42 AM Peter Geis <pgwipeout@xxxxxxxxx>
wrote:
Good Evening,
I am stress testing the pcie controller on the rk3566-quartz64
prototype SBC.
This device has 1GB available at <0x3 0x00000000> for the PCIe
controller, which makes a dGPU theoretically possible.
While attempting to light off a HD7570 card I manage to get a modeset
console, but ring0 test fails and disables acceleration.
Note, we do not have UEFI, so all PCIe setup is from the Linux
kernel.
Any insight you can provide would be much appreciated.
Does your platform support PCIe cache coherency with the CPU? I.e.,
does the CPU allow cache snoops from PCIe devices? That is required
for the driver to operate.
Ah, most likely not.
This issue has come up already as the GIC isn't permitted to snoop on
the CPUs, so I doubt the PCIe controller can either.
Is there no way to work around this or is it dead in the water?
It's required by the pcie spec. You could potentially work around it
if you can allocate uncached memory for DMA, but I don't think that is
possible currently. Ideally we'd figure out some way to detect if a
particular platform supports cache snooping or not as well.
There's device_get_dma_attr(), although I don't think it will work
currently for PCI devices without an OF or ACPI node - we could
perhaps do with a PCI-specific wrapper which can walk up and defer to
the host bridge's firmware description as necessary.
The common DMA ops *do* correctly keep track of per-device coherency
internally, but drivers aren't supposed to be poking at that
information directly.
That sounds like you underestimate the problem. ARM has unfortunately
made the coherency for PCI an optional IP.
So we are talking about a hardware limitation which potentially can't be
fixed without replacing the hardware.
Christian.
Robin.
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