[lkml]   [2019]   [Dec]   [10]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
Messages in this thread
SubjectRe: [PATCH] pcie: Add quirk for the Arm Neoverse N1SDP platform
On Mon, Dec 09, 2019 at 04:06:38PM +0000, Andre Przywara wrote:
> From: Deepak Pandey <>
> The Arm N1SDP SoC suffers from some PCIe integration issues, most
> prominently config space accesses to not existing BDFs being answered
> with a bus abort, resulting in an SError.

Can we tease this apart a little more? Linux doesn't program all the
bits that control error signaling, so even on hardware that works
perfectly, much of this behavior is determined by what firmware did.
I wonder if Linux could be more careful about this.

"Bus abort" is not a term used in PCIe. IIUC, a config read to a
device that doesn't exist should terminate with an Unsupported Request
completion, e.g., see the implementation note in PCIe r5.0 sec 2.3.1.

The UR should be an uncorrectable non-fatal error (Table 6-5), and
Figures 6-2 and 6-3 show how it should be handled and when it should
be signaled as a system error. In case you don't have a copy of the
spec, I extracted those two figures and put them at [1].

Can you collect "lspci -vvxxx" output to see if we can correlate it
with those figures and the behavior you see?


> To mitigate this, the firmware scans the bus before boot (catching the
> SErrors) and creates a table with valid BDFs, which acts as a filter for
> Linux' config space accesses.
> Add code consulting the table as an ACPI PCIe quirk, also register the
> corresponding device tree based description of the host controller.
> Also fix the other two minor issues on the way, namely not being fully
> ECAM compliant and config space accesses being restricted to 32-bit
> accesses only.

As I'm sure you've noticed, controllers that support only 32-bit
config writes are not spec compliant and devices may not work
correctly. The comment in pci_generic_config_write32() explains why.

You may not trip over this problem frequently, but I wouldn't call it
a "minor" issue because when you *do* trip over it, you have no
indication that a register was corrupted.

Even ECAM compliance is not really minor -- if this controller were
fully compliant with the spec, you would need ZERO Linux changes to
support it. Every quirk like this means additional maintenance
burden, and it's not just a one-time thing. It means old kernels that
*should* "just work" on your system will not work unless somebody
backports the quirk.

> This allows the Arm Neoverse N1SDP board to boot Linux without crashing
> and to access *any* devices (there are no platform devices except UART).

 \ /
  Last update: 2019-12-10 15:42    [W:0.102 / U:11.268 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site