Re: [PATCH v9 0/4] PCI: brcmstb: Configure appropriate HW CLKREQ# mode

Cyril Brulebois
2024-04-04T20:01:44Z

[-- Attachment #1: Type: text/plain, Size: 3351 bytes --]

Hi Jim,

Jim Quinlan <james.quinlan@broadcom.com> (2024-04-03):
> v9 -- v8 was setting an internal bus timeout to accomodate large L1 exit
> latencies. After meeting the PCIe HW team it was revealed that the
> HW default timeout value was set low for the purposes of HW debugging
> convenience; for nominal operation it needs to be set to a higher
> value independent of this submission's purpose. This is now a
> separate commit.
>
> -- With v8, Bjorne asked what was preventing a device from exceeding the
> time required for the above internal bus timeout. The answer to this
> is for us to set the endpoints' max latency {no-,}snoop value to
> something below this internal timeout value. If the endpoint
> respects this value as it should, it will not send an LTR request
> with a larger latency value and not put itself in a situation
> that requires more latency than is possible for the platform.
>
> Typically, ACPI or FW sets these max latency values. In most of our
> systems we do not have this happening so it is up to the RC driver to
> set these values in the endpoint devices. If the endpoints already
> have non-zero values that are lower than what we are setting, we let
> them be, as it is possible ACPI or FW set them and knows something
> that we do not.
>
> -- The "clkreq" commit has only been changed to remove the code that was
> setting the timeout value, as this code is now its own commit.

Given the bot's feedback, I took the liberty of running tests on your
patch series except with an extra “static” keyword. On my build system,
gcc 12 wasn't complaining about it but I didn't spend time trying to
find the right options, or trying a switch to clang to confirm the
before/after situation:

-void brcm_set_downstream_devs_ltr_max(struct brcm_pcie *pcie)
+static void brcm_set_downstream_devs_ltr_max(struct brcm_pcie *pcie)

Anyway, this is still:

Tested-by: Cyril Brulebois <cyril@debamax.com>

Test setup:
-----------

- using a $CM with the 20230111 EEPROM
- on the same CM4 IO Board
- with a $PCIE board (PCIe to multiple USB ports)
- and the same Samsung USB flash drive + Logitech keyboard.

where $CM is one of:

- CM4 Lite Rev 1.0
- CM4 8/32 Rev 1.0
- CM4 4/32 Rev 1.1

and $PCIE is one of:

- SupaHub PCE6U1C-R02, VER 006
- SupaHub PCE6U1C-R02, VER 006S
- Waveshare VIA VL805/806-based

Results:
--------

1. Given this is already v9, and given I don't see how this could have
possibly changed, I didn't build or tested an unpatched kernel,
which I would still expect to produce either a successful boot
*without* seeing the devices plugged on the PCIe-to-USB board or the
dreaded SError in most cases.

2. With a patched kernel (v6.7-562-g9f8413c4a66f2 + this series +
“static” in front of brcm_set_downstream_devs_ltr_max()), for all
$CM/$PCIE combinations, I'm getting a system that boots, sees the
flash drive, and gives decent read/write performance on it (plus a
functional keyboard).

Cheers,
--
Cyril Brulebois (kibi@debian.org) <https://debamax.com/>
D-I release manager -- Release team member -- Freelance Consultant

[-- Attachment #2: signature.asc --]
[-- Type: application/pgp-signature, Size: 833 bytes --]